832 lines
23 KiB
ReStructuredText
832 lines
23 KiB
ReStructuredText
.. SPDX-License-Identifier: GPL-2.0
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Super Block
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-----------
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The superblock records various information about the enclosing
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filesystem, such as block counts, inode counts, supported features,
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maintenance information, and more.
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If the sparse_super feature flag is set, redundant copies of the
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superblock and group descriptors are kept only in the groups whose group
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number is either 0 or a power of 3, 5, or 7. If the flag is not set,
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redundant copies are kept in all groups.
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The superblock checksum is calculated against the superblock structure,
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which includes the FS UUID.
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The ext4 superblock is laid out as follows in
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``struct ext4_super_block``:
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.. list-table::
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:widths: 8 8 24 40
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:header-rows: 1
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* - Offset
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- Size
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- Name
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- Description
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* - 0x0
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- __le32
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- s_inodes_count
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- Total inode count.
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* - 0x4
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- __le32
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- s_blocks_count_lo
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- Total block count.
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* - 0x8
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- __le32
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- s_r_blocks_count_lo
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- This number of blocks can only be allocated by the super-user.
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* - 0xC
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- __le32
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- s_free_blocks_count_lo
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- Free block count.
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* - 0x10
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- __le32
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- s_free_inodes_count
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- Free inode count.
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* - 0x14
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- __le32
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- s_first_data_block
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- First data block. This must be at least 1 for 1k-block filesystems and
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is typically 0 for all other block sizes.
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* - 0x18
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- __le32
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- s_log_block_size
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- Block size is 2 ^ (10 + s_log_block_size).
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* - 0x1C
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- __le32
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- s_log_cluster_size
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- Cluster size is 2 ^ (10 + s_log_cluster_size) blocks if bigalloc is
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enabled. Otherwise s_log_cluster_size must equal s_log_block_size.
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* - 0x20
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- __le32
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- s_blocks_per_group
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- Blocks per group.
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* - 0x24
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- __le32
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- s_clusters_per_group
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- Clusters per group, if bigalloc is enabled. Otherwise
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s_clusters_per_group must equal s_blocks_per_group.
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* - 0x28
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- __le32
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- s_inodes_per_group
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- Inodes per group.
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* - 0x2C
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- __le32
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- s_mtime
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- Mount time, in seconds since the epoch.
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* - 0x30
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- __le32
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- s_wtime
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- Write time, in seconds since the epoch.
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* - 0x34
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- __le16
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- s_mnt_count
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- Number of mounts since the last fsck.
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* - 0x36
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- __le16
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- s_max_mnt_count
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- Number of mounts beyond which a fsck is needed.
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* - 0x38
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- __le16
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- s_magic
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- Magic signature, 0xEF53
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* - 0x3A
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- __le16
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- s_state
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- File system state. See super_state_ for more info.
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* - 0x3C
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- __le16
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- s_errors
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- Behaviour when detecting errors. See super_errors_ for more info.
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* - 0x3E
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- __le16
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- s_minor_rev_level
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- Minor revision level.
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* - 0x40
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- __le32
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- s_lastcheck
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- Time of last check, in seconds since the epoch.
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* - 0x44
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- __le32
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- s_checkinterval
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- Maximum time between checks, in seconds.
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* - 0x48
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- __le32
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- s_creator_os
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- Creator OS. See the table super_creator_ for more info.
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* - 0x4C
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- __le32
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- s_rev_level
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- Revision level. See the table super_revision_ for more info.
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* - 0x50
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- __le16
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- s_def_resuid
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- Default uid for reserved blocks.
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* - 0x52
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- __le16
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- s_def_resgid
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- Default gid for reserved blocks.
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* -
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-
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-
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- These fields are for EXT4_DYNAMIC_REV superblocks only.
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Note: the difference between the compatible feature set and the
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incompatible feature set is that if there is a bit set in the
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incompatible feature set that the kernel doesn't know about, it should
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refuse to mount the filesystem.
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e2fsck's requirements are more strict; if it doesn't know
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about a feature in either the compatible or incompatible feature set, it
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must abort and not try to meddle with things it doesn't understand...
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* - 0x54
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- __le32
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- s_first_ino
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- First non-reserved inode.
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* - 0x58
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- __le16
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- s_inode_size
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- Size of inode structure, in bytes.
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* - 0x5A
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- __le16
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- s_block_group_nr
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- Block group # of this superblock.
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* - 0x5C
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- __le32
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- s_feature_compat
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- Compatible feature set flags. Kernel can still read/write this fs even
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if it doesn't understand a flag; fsck should not do that. See the
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super_compat_ table for more info.
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* - 0x60
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- __le32
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- s_feature_incompat
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- Incompatible feature set. If the kernel or fsck doesn't understand one
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of these bits, it should stop. See the super_incompat_ table for more
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info.
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* - 0x64
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- __le32
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- s_feature_ro_compat
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- Readonly-compatible feature set. If the kernel doesn't understand one of
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these bits, it can still mount read-only. See the super_rocompat_ table
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for more info.
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* - 0x68
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- __u8
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- s_uuid[16]
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- 128-bit UUID for volume.
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* - 0x78
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- char
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- s_volume_name[16]
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- Volume label.
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* - 0x88
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- char
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- s_last_mounted[64]
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- Directory where filesystem was last mounted.
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* - 0xC8
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- __le32
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- s_algorithm_usage_bitmap
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- For compression (Not used in e2fsprogs/Linux)
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* -
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-
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-
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- Performance hints. Directory preallocation should only happen if the
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EXT4_FEATURE_COMPAT_DIR_PREALLOC flag is on.
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* - 0xCC
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- __u8
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- s_prealloc_blocks
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- #. of blocks to try to preallocate for ... files? (Not used in
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e2fsprogs/Linux)
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* - 0xCD
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- __u8
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- s_prealloc_dir_blocks
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- #. of blocks to preallocate for directories. (Not used in
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e2fsprogs/Linux)
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* - 0xCE
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- __le16
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- s_reserved_gdt_blocks
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- Number of reserved GDT entries for future filesystem expansion.
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* -
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-
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-
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- Journalling support is valid only if EXT4_FEATURE_COMPAT_HAS_JOURNAL is
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set.
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* - 0xD0
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- __u8
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- s_journal_uuid[16]
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- UUID of journal superblock
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* - 0xE0
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- __le32
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- s_journal_inum
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- inode number of journal file.
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* - 0xE4
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- __le32
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- s_journal_dev
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- Device number of journal file, if the external journal feature flag is
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set.
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* - 0xE8
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- __le32
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- s_last_orphan
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- Start of list of orphaned inodes to delete.
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* - 0xEC
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- __le32
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- s_hash_seed[4]
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- HTREE hash seed.
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* - 0xFC
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- __u8
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- s_def_hash_version
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- Default hash algorithm to use for directory hashes. See super_def_hash_
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for more info.
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* - 0xFD
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- __u8
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- s_jnl_backup_type
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- If this value is 0 or EXT3_JNL_BACKUP_BLOCKS (1), then the
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``s_jnl_blocks`` field contains a duplicate copy of the inode's
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``i_block[]`` array and ``i_size``.
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* - 0xFE
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- __le16
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- s_desc_size
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- Size of group descriptors, in bytes, if the 64bit incompat feature flag
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is set.
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* - 0x100
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- __le32
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- s_default_mount_opts
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- Default mount options. See the super_mountopts_ table for more info.
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* - 0x104
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- __le32
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- s_first_meta_bg
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- First metablock block group, if the meta_bg feature is enabled.
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* - 0x108
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- __le32
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- s_mkfs_time
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- When the filesystem was created, in seconds since the epoch.
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* - 0x10C
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- __le32
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- s_jnl_blocks[17]
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- Backup copy of the journal inode's ``i_block[]`` array in the first 15
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elements and i_size_high and i_size in the 16th and 17th elements,
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respectively.
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* -
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-
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-
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- 64bit support is valid only if EXT4_FEATURE_COMPAT_64BIT is set.
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* - 0x150
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- __le32
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- s_blocks_count_hi
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- High 32-bits of the block count.
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* - 0x154
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- __le32
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- s_r_blocks_count_hi
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- High 32-bits of the reserved block count.
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* - 0x158
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- __le32
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- s_free_blocks_count_hi
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- High 32-bits of the free block count.
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* - 0x15C
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- __le16
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- s_min_extra_isize
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- All inodes have at least # bytes.
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* - 0x15E
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- __le16
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- s_want_extra_isize
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- New inodes should reserve # bytes.
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* - 0x160
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- __le32
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- s_flags
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- Miscellaneous flags. See the super_flags_ table for more info.
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* - 0x164
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- __le16
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- s_raid_stride
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- RAID stride. This is the number of logical blocks read from or written
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to the disk before moving to the next disk. This affects the placement
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of filesystem metadata, which will hopefully make RAID storage faster.
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* - 0x166
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- __le16
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- s_mmp_interval
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- #. seconds to wait in multi-mount prevention (MMP) checking. In theory,
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MMP is a mechanism to record in the superblock which host and device
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have mounted the filesystem, in order to prevent multiple mounts. This
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feature does not seem to be implemented...
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* - 0x168
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- __le64
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- s_mmp_block
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- Block # for multi-mount protection data.
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* - 0x170
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- __le32
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- s_raid_stripe_width
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- RAID stripe width. This is the number of logical blocks read from or
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written to the disk before coming back to the current disk. This is used
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by the block allocator to try to reduce the number of read-modify-write
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operations in a RAID5/6.
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* - 0x174
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- __u8
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- s_log_groups_per_flex
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- Size of a flexible block group is 2 ^ ``s_log_groups_per_flex``.
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* - 0x175
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- __u8
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- s_checksum_type
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- Metadata checksum algorithm type. The only valid value is 1 (crc32c).
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* - 0x176
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- __le16
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- s_reserved_pad
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-
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* - 0x178
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- __le64
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- s_kbytes_written
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- Number of KiB written to this filesystem over its lifetime.
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* - 0x180
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- __le32
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- s_snapshot_inum
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- inode number of active snapshot. (Not used in e2fsprogs/Linux.)
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* - 0x184
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- __le32
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- s_snapshot_id
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- Sequential ID of active snapshot. (Not used in e2fsprogs/Linux.)
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* - 0x188
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- __le64
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- s_snapshot_r_blocks_count
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- Number of blocks reserved for active snapshot's future use. (Not used in
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e2fsprogs/Linux.)
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* - 0x190
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- __le32
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- s_snapshot_list
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- inode number of the head of the on-disk snapshot list. (Not used in
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e2fsprogs/Linux.)
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* - 0x194
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- __le32
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- s_error_count
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- Number of errors seen.
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* - 0x198
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- __le32
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- s_first_error_time
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- First time an error happened, in seconds since the epoch.
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* - 0x19C
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- __le32
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- s_first_error_ino
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- inode involved in first error.
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* - 0x1A0
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- __le64
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- s_first_error_block
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- Number of block involved of first error.
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* - 0x1A8
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- __u8
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- s_first_error_func[32]
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- Name of function where the error happened.
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* - 0x1C8
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- __le32
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- s_first_error_line
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- Line number where error happened.
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* - 0x1CC
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- __le32
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- s_last_error_time
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- Time of most recent error, in seconds since the epoch.
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* - 0x1D0
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- __le32
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- s_last_error_ino
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- inode involved in most recent error.
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* - 0x1D4
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- __le32
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- s_last_error_line
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- Line number where most recent error happened.
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* - 0x1D8
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- __le64
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- s_last_error_block
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- Number of block involved in most recent error.
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* - 0x1E0
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- __u8
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- s_last_error_func[32]
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- Name of function where the most recent error happened.
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* - 0x200
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- __u8
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- s_mount_opts[64]
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- ASCIIZ string of mount options.
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* - 0x240
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- __le32
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- s_usr_quota_inum
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- Inode number of user `quota <quota>`__ file.
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* - 0x244
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- __le32
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- s_grp_quota_inum
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- Inode number of group `quota <quota>`__ file.
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* - 0x248
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- __le32
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- s_overhead_blocks
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- Overhead blocks/clusters in fs. (Huh? This field is always zero, which
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means that the kernel calculates it dynamically.)
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* - 0x24C
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- __le32
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- s_backup_bgs[2]
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- Block groups containing superblock backups (if sparse_super2)
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* - 0x254
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- __u8
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- s_encrypt_algos[4]
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- Encryption algorithms in use. There can be up to four algorithms in use
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at any time; valid algorithm codes are given in the super_encrypt_ table
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below.
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* - 0x258
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- __u8
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- s_encrypt_pw_salt[16]
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- Salt for the string2key algorithm for encryption.
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* - 0x268
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- __le32
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- s_lpf_ino
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- Inode number of lost+found
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* - 0x26C
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- __le32
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- s_prj_quota_inum
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- Inode that tracks project quotas.
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* - 0x270
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- __le32
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- s_checksum_seed
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- Checksum seed used for metadata_csum calculations. This value is
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crc32c(~0, $orig_fs_uuid).
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* - 0x274
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- __u8
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- s_wtime_hi
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- Upper 8 bits of the s_wtime field.
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* - 0x275
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- __u8
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- s_mtime_hi
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- Upper 8 bits of the s_mtime field.
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* - 0x276
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- __u8
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- s_mkfs_time_hi
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- Upper 8 bits of the s_mkfs_time field.
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* - 0x277
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- __u8
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- s_lastcheck_hi
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- Upper 8 bits of the s_lastcheck field.
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* - 0x278
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- __u8
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- s_first_error_time_hi
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- Upper 8 bits of the s_first_error_time field.
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* - 0x279
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- __u8
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- s_last_error_time_hi
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- Upper 8 bits of the s_last_error_time field.
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* - 0x27A
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- __u8
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- s_pad[2]
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- Zero padding.
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* - 0x27C
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- __le16
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- s_encoding
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- Filename charset encoding.
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* - 0x27E
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- __le16
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- s_encoding_flags
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- Filename charset encoding flags.
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* - 0x280
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- __le32
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- s_orphan_file_inum
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- Orphan file inode number.
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* - 0x284
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- __le32
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- s_reserved[94]
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- Padding to the end of the block.
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* - 0x3FC
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- __le32
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- s_checksum
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- Superblock checksum.
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.. _super_state:
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The superblock state is some combination of the following:
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.. list-table::
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:widths: 8 72
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:header-rows: 1
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* - Value
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- Description
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* - 0x0001
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- Cleanly umounted
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* - 0x0002
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- Errors detected
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* - 0x0004
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- Orphans being recovered
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.. _super_errors:
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The superblock error policy is one of the following:
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.. list-table::
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:widths: 8 72
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:header-rows: 1
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* - Value
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- Description
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* - 1
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- Continue
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* - 2
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- Remount read-only
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* - 3
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- Panic
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.. _super_creator:
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The filesystem creator is one of the following:
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.. list-table::
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:widths: 8 72
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:header-rows: 1
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* - Value
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- Description
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* - 0
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- Linux
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* - 1
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- Hurd
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* - 2
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- Masix
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* - 3
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- FreeBSD
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* - 4
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- Lites
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.. _super_revision:
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The superblock revision is one of the following:
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.. list-table::
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:widths: 8 72
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:header-rows: 1
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* - Value
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- Description
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* - 0
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- Original format
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* - 1
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- v2 format w/ dynamic inode sizes
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Note that ``EXT4_DYNAMIC_REV`` refers to a revision 1 or newer filesystem.
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.. _super_compat:
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The superblock compatible features field is a combination of any of the
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following:
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.. list-table::
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:widths: 16 64
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:header-rows: 1
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* - Value
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- Description
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* - 0x1
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- Directory preallocation (COMPAT_DIR_PREALLOC).
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* - 0x2
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- “imagic inodes”. Not clear from the code what this does
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(COMPAT_IMAGIC_INODES).
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* - 0x4
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- Has a journal (COMPAT_HAS_JOURNAL).
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* - 0x8
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- Supports extended attributes (COMPAT_EXT_ATTR).
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* - 0x10
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- Has reserved GDT blocks for filesystem expansion
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(COMPAT_RESIZE_INODE). Requires RO_COMPAT_SPARSE_SUPER.
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* - 0x20
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- Has directory indices (COMPAT_DIR_INDEX).
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* - 0x40
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- “Lazy BG”. Not in Linux kernel, seems to have been for uninitialized
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block groups? (COMPAT_LAZY_BG)
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* - 0x80
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- “Exclude inode”. Not used. (COMPAT_EXCLUDE_INODE).
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* - 0x100
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- “Exclude bitmap”. Seems to be used to indicate the presence of
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snapshot-related exclude bitmaps? Not defined in kernel or used in
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e2fsprogs (COMPAT_EXCLUDE_BITMAP).
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* - 0x200
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- Sparse Super Block, v2. If this flag is set, the SB field s_backup_bgs
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points to the two block groups that contain backup superblocks
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(COMPAT_SPARSE_SUPER2).
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* - 0x400
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- Fast commits supported. Although fast commits blocks are
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backward incompatible, fast commit blocks are not always
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present in the journal. If fast commit blocks are present in
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the journal, JBD2 incompat feature
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(JBD2_FEATURE_INCOMPAT_FAST_COMMIT) gets
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set (COMPAT_FAST_COMMIT).
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* - 0x1000
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- Orphan file allocated. This is the special file for more efficient
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tracking of unlinked but still open inodes. When there may be any
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entries in the file, we additionally set proper rocompat feature
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(RO_COMPAT_ORPHAN_PRESENT).
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.. _super_incompat:
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The superblock incompatible features field is a combination of any of the
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following:
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.. list-table::
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:widths: 16 64
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:header-rows: 1
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* - Value
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- Description
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* - 0x1
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- Compression (INCOMPAT_COMPRESSION).
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* - 0x2
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- Directory entries record the file type. See ext4_dir_entry_2 below
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(INCOMPAT_FILETYPE).
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* - 0x4
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- Filesystem needs recovery (INCOMPAT_RECOVER).
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* - 0x8
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- Filesystem has a separate journal device (INCOMPAT_JOURNAL_DEV).
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* - 0x10
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- Meta block groups. See the earlier discussion of this feature
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(INCOMPAT_META_BG).
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* - 0x40
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- Files in this filesystem use extents (INCOMPAT_EXTENTS).
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* - 0x80
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- Enable a filesystem size of 2^64 blocks (INCOMPAT_64BIT).
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* - 0x100
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- Multiple mount protection (INCOMPAT_MMP).
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* - 0x200
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- Flexible block groups. See the earlier discussion of this feature
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(INCOMPAT_FLEX_BG).
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* - 0x400
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- Inodes can be used to store large extended attribute values
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(INCOMPAT_EA_INODE).
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* - 0x1000
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- Data in directory entry (INCOMPAT_DIRDATA). (Not implemented?)
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* - 0x2000
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- Metadata checksum seed is stored in the superblock. This feature enables
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the administrator to change the UUID of a metadata_csum filesystem
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while the filesystem is mounted; without it, the checksum definition
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requires all metadata blocks to be rewritten (INCOMPAT_CSUM_SEED).
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* - 0x4000
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- Large directory >2GB or 3-level htree (INCOMPAT_LARGEDIR). Prior to
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this feature, directories could not be larger than 4GiB and could not
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have an htree more than 2 levels deep. If this feature is enabled,
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directories can be larger than 4GiB and have a maximum htree depth of 3.
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* - 0x8000
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- Data in inode (INCOMPAT_INLINE_DATA).
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* - 0x10000
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- Encrypted inodes are present on the filesystem. (INCOMPAT_ENCRYPT).
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.. _super_rocompat:
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The superblock read-only compatible features field is a combination of any of
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the following:
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.. list-table::
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:widths: 16 64
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:header-rows: 1
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* - Value
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- Description
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* - 0x1
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- Sparse superblocks. See the earlier discussion of this feature
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(RO_COMPAT_SPARSE_SUPER).
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* - 0x2
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- This filesystem has been used to store a file greater than 2GiB
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(RO_COMPAT_LARGE_FILE).
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* - 0x4
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- Not used in kernel or e2fsprogs (RO_COMPAT_BTREE_DIR).
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* - 0x8
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- This filesystem has files whose sizes are represented in units of
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logical blocks, not 512-byte sectors. This implies a very large file
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indeed! (RO_COMPAT_HUGE_FILE)
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* - 0x10
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- Group descriptors have checksums. In addition to detecting corruption,
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this is useful for lazy formatting with uninitialized groups
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(RO_COMPAT_GDT_CSUM).
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* - 0x20
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- Indicates that the old ext3 32,000 subdirectory limit no longer applies
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(RO_COMPAT_DIR_NLINK). A directory's i_links_count will be set to 1
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if it is incremented past 64,999.
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* - 0x40
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- Indicates that large inodes exist on this filesystem
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(RO_COMPAT_EXTRA_ISIZE).
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* - 0x80
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- This filesystem has a snapshot (RO_COMPAT_HAS_SNAPSHOT).
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* - 0x100
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- `Quota <Quota>`__ (RO_COMPAT_QUOTA).
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* - 0x200
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- This filesystem supports “bigalloc”, which means that file extents are
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tracked in units of clusters (of blocks) instead of blocks
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(RO_COMPAT_BIGALLOC).
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* - 0x400
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- This filesystem supports metadata checksumming.
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(RO_COMPAT_METADATA_CSUM; implies RO_COMPAT_GDT_CSUM, though
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GDT_CSUM must not be set)
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* - 0x800
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- Filesystem supports replicas. This feature is neither in the kernel nor
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e2fsprogs. (RO_COMPAT_REPLICA)
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* - 0x1000
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- Read-only filesystem image; the kernel will not mount this image
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read-write and most tools will refuse to write to the image.
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(RO_COMPAT_READONLY)
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* - 0x2000
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- Filesystem tracks project quotas. (RO_COMPAT_PROJECT)
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* - 0x8000
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- Verity inodes may be present on the filesystem. (RO_COMPAT_VERITY)
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* - 0x10000
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- Indicates orphan file may have valid orphan entries and thus we need
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to clean them up when mounting the filesystem
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(RO_COMPAT_ORPHAN_PRESENT).
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.. _super_def_hash:
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The ``s_def_hash_version`` field is one of the following:
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.. list-table::
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:widths: 8 72
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:header-rows: 1
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|
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* - Value
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- Description
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* - 0x0
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- Legacy.
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* - 0x1
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- Half MD4.
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* - 0x2
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- Tea.
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* - 0x3
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- Legacy, unsigned.
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* - 0x4
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- Half MD4, unsigned.
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* - 0x5
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- Tea, unsigned.
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.. _super_mountopts:
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The ``s_default_mount_opts`` field is any combination of the following:
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.. list-table::
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|
:widths: 8 72
|
|
:header-rows: 1
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|
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* - Value
|
|
- Description
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* - 0x0001
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- Print debugging info upon (re)mount. (EXT4_DEFM_DEBUG)
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* - 0x0002
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- New files take the gid of the containing directory (instead of the fsgid
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of the current process). (EXT4_DEFM_BSDGROUPS)
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* - 0x0004
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- Support userspace-provided extended attributes. (EXT4_DEFM_XATTR_USER)
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* - 0x0008
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- Support POSIX access control lists (ACLs). (EXT4_DEFM_ACL)
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* - 0x0010
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- Do not support 32-bit UIDs. (EXT4_DEFM_UID16)
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* - 0x0020
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- All data and metadata are commited to the journal.
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(EXT4_DEFM_JMODE_DATA)
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* - 0x0040
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- All data are flushed to the disk before metadata are committed to the
|
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journal. (EXT4_DEFM_JMODE_ORDERED)
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* - 0x0060
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- Data ordering is not preserved; data may be written after the metadata
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has been written. (EXT4_DEFM_JMODE_WBACK)
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* - 0x0100
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- Disable write flushes. (EXT4_DEFM_NOBARRIER)
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* - 0x0200
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- Track which blocks in a filesystem are metadata and therefore should not
|
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be used as data blocks. This option will be enabled by default on 3.18,
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hopefully. (EXT4_DEFM_BLOCK_VALIDITY)
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* - 0x0400
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- Enable DISCARD support, where the storage device is told about blocks
|
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becoming unused. (EXT4_DEFM_DISCARD)
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* - 0x0800
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- Disable delayed allocation. (EXT4_DEFM_NODELALLOC)
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.. _super_flags:
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The ``s_flags`` field is any combination of the following:
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|
|
.. list-table::
|
|
:widths: 8 72
|
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:header-rows: 1
|
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|
|
* - Value
|
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- Description
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* - 0x0001
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- Signed directory hash in use.
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* - 0x0002
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- Unsigned directory hash in use.
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* - 0x0004
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- To test development code.
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.. _super_encrypt:
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|
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The ``s_encrypt_algos`` list can contain any of the following:
|
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|
|
.. list-table::
|
|
:widths: 8 72
|
|
:header-rows: 1
|
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|
|
* - Value
|
|
- Description
|
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* - 0
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- Invalid algorithm (ENCRYPTION_MODE_INVALID).
|
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* - 1
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- 256-bit AES in XTS mode (ENCRYPTION_MODE_AES_256_XTS).
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* - 2
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- 256-bit AES in GCM mode (ENCRYPTION_MODE_AES_256_GCM).
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* - 3
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- 256-bit AES in CBC mode (ENCRYPTION_MODE_AES_256_CBC).
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Total size of the superblock is 1024 bytes.
|