53 lines
2.2 KiB
ReStructuredText
53 lines
2.2 KiB
ReStructuredText
.. SPDX-License-Identifier: GPL-2.0
|
|
|
|
Orphan file
|
|
-----------
|
|
|
|
In unix there can inodes that are unlinked from directory hierarchy but that
|
|
are still alive because they are open. In case of crash the filesystem has to
|
|
clean up these inodes as otherwise they (and the blocks referenced from them)
|
|
would leak. Similarly if we truncate or extend the file, we need not be able
|
|
to perform the operation in a single journalling transaction. In such case we
|
|
track the inode as orphan so that in case of crash extra blocks allocated to
|
|
the file get truncated.
|
|
|
|
Traditionally ext4 tracks orphan inodes in a form of single linked list where
|
|
superblock contains the inode number of the last orphan inode (s\_last\_orphan
|
|
field) and then each inode contains inode number of the previously orphaned
|
|
inode (we overload i\_dtime inode field for this). However this filesystem
|
|
global single linked list is a scalability bottleneck for workloads that result
|
|
in heavy creation of orphan inodes. When orphan file feature
|
|
(COMPAT\_ORPHAN\_FILE) is enabled, the filesystem has a special inode
|
|
(referenced from the superblock through s\_orphan_file_inum) with several
|
|
blocks. Each of these blocks has a structure:
|
|
|
|
.. list-table::
|
|
:widths: 8 8 24 40
|
|
:header-rows: 1
|
|
|
|
* - Offset
|
|
- Type
|
|
- Name
|
|
- Description
|
|
* - 0x0
|
|
- Array of \_\_le32 entries
|
|
- Orphan inode entries
|
|
- Each \_\_le32 entry is either empty (0) or it contains inode number of
|
|
an orphan inode.
|
|
* - blocksize - 8
|
|
- \_\_le32
|
|
- ob\_magic
|
|
- Magic value stored in orphan block tail (0x0b10ca04)
|
|
* - blocksize - 4
|
|
- \_\_le32
|
|
- ob\_checksum
|
|
- Checksum of the orphan block.
|
|
|
|
When a filesystem with orphan file feature is writeably mounted, we set
|
|
RO\_COMPAT\_ORPHAN\_PRESENT feature in the superblock to indicate there may
|
|
be valid orphan entries. In case we see this feature when mounting the
|
|
filesystem, we read the whole orphan file and process all orphan inodes found
|
|
there as usual. When cleanly unmounting the filesystem we remove the
|
|
RO\_COMPAT\_ORPHAN\_PRESENT feature to avoid unnecessary scanning of the orphan
|
|
file and also make the filesystem fully compatible with older kernels.
|