183 lines
7.0 KiB
Plaintext
183 lines
7.0 KiB
Plaintext
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This is the implementation of the SystemV/Coherent filesystem for Linux.
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It grew out of separate filesystem implementations
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Xenix FS Doug Evans <dje@cygnus.com> June 1992
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SystemV FS Paul B. Monday <pmonday@eecs.wsu.edu> March-June 1993
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Coherent FS B. Haible <haible@ma2s2.mathematik.uni-karlsruhe.de> June 1993
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and was merged together in July 1993.
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These filesystems are rather similar. Here is a comparison with Minix FS:
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* Linux fdisk reports on partitions
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- Minix FS 0x81 Linux/Minix
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- Xenix FS ??
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- SystemV FS ??
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- Coherent FS 0x08 AIX bootable
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* Size of a block or zone (data allocation unit on disk)
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- Minix FS 1024
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- Xenix FS 1024 (also 512 ??)
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- SystemV FS 1024 (also 512 and 2048)
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- Coherent FS 512
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* General layout: all have one boot block, one super block and
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separate areas for inodes and for directories/data.
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On SystemV Release 2 FS (e.g. Microport) the first track is reserved and
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all the block numbers (including the super block) are offset by one track.
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* Byte ordering of "short" (16 bit entities) on disk:
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- Minix FS little endian 0 1
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- Xenix FS little endian 0 1
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- SystemV FS little endian 0 1
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- Coherent FS little endian 0 1
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Of course, this affects only the file system, not the data of files on it!
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* Byte ordering of "long" (32 bit entities) on disk:
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- Minix FS little endian 0 1 2 3
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- Xenix FS little endian 0 1 2 3
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- SystemV FS little endian 0 1 2 3
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- Coherent FS PDP-11 2 3 0 1
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Of course, this affects only the file system, not the data of files on it!
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* Inode on disk: "short", 0 means non-existent, the root dir ino is:
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- Minix FS 1
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- Xenix FS, SystemV FS, Coherent FS 2
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* Maximum number of hard links to a file:
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- Minix FS 250
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- Xenix FS ??
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- SystemV FS ??
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- Coherent FS >=10000
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* Free inode management:
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- Minix FS a bitmap
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- Xenix FS, SystemV FS, Coherent FS
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There is a cache of a certain number of free inodes in the super-block.
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When it is exhausted, new free inodes are found using a linear search.
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* Free block management:
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- Minix FS a bitmap
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- Xenix FS, SystemV FS, Coherent FS
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Free blocks are organized in a "free list". Maybe a misleading term,
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since it is not true that every free block contains a pointer to
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the next free block. Rather, the free blocks are organized in chunks
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of limited size, and every now and then a free block contains pointers
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to the free blocks pertaining to the next chunk; the first of these
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contains pointers and so on. The list terminates with a "block number"
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0 on Xenix FS and SystemV FS, with a block zeroed out on Coherent FS.
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* Super-block location:
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- Minix FS block 1 = bytes 1024..2047
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- Xenix FS block 1 = bytes 1024..2047
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- SystemV FS bytes 512..1023
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- Coherent FS block 1 = bytes 512..1023
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* Super-block layout:
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- Minix FS
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unsigned short s_ninodes;
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unsigned short s_nzones;
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unsigned short s_imap_blocks;
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unsigned short s_zmap_blocks;
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unsigned short s_firstdatazone;
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unsigned short s_log_zone_size;
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unsigned long s_max_size;
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unsigned short s_magic;
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- Xenix FS, SystemV FS, Coherent FS
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unsigned short s_firstdatazone;
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unsigned long s_nzones;
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unsigned short s_fzone_count;
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unsigned long s_fzones[NICFREE];
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unsigned short s_finode_count;
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unsigned short s_finodes[NICINOD];
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char s_flock;
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char s_ilock;
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char s_modified;
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char s_rdonly;
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unsigned long s_time;
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short s_dinfo[4]; -- SystemV FS only
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unsigned long s_free_zones;
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unsigned short s_free_inodes;
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short s_dinfo[4]; -- Xenix FS only
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unsigned short s_interleave_m,s_interleave_n; -- Coherent FS only
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char s_fname[6];
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char s_fpack[6];
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then they differ considerably:
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Xenix FS
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char s_clean;
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char s_fill[371];
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long s_magic;
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long s_type;
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SystemV FS
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long s_fill[12 or 14];
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long s_state;
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long s_magic;
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long s_type;
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Coherent FS
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unsigned long s_unique;
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Note that Coherent FS has no magic.
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* Inode layout:
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- Minix FS
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unsigned short i_mode;
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unsigned short i_uid;
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unsigned long i_size;
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unsigned long i_time;
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unsigned char i_gid;
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unsigned char i_nlinks;
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unsigned short i_zone[7+1+1];
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- Xenix FS, SystemV FS, Coherent FS
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unsigned short i_mode;
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unsigned short i_nlink;
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unsigned short i_uid;
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unsigned short i_gid;
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unsigned long i_size;
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unsigned char i_zone[3*(10+1+1+1)];
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unsigned long i_atime;
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unsigned long i_mtime;
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unsigned long i_ctime;
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* Regular file data blocks are organized as
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- Minix FS
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7 direct blocks
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1 indirect block (pointers to blocks)
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1 double-indirect block (pointer to pointers to blocks)
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- Xenix FS, SystemV FS, Coherent FS
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10 direct blocks
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1 indirect block (pointers to blocks)
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1 double-indirect block (pointer to pointers to blocks)
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1 triple-indirect block (pointer to pointers to pointers to blocks)
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* Inode size, inodes per block
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- Minix FS 32 32
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- Xenix FS 64 16
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- SystemV FS 64 16
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- Coherent FS 64 8
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* Directory entry on disk
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- Minix FS
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unsigned short inode;
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char name[14/30];
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- Xenix FS, SystemV FS, Coherent FS
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unsigned short inode;
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char name[14];
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* Dir entry size, dir entries per block
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- Minix FS 16/32 64/32
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- Xenix FS 16 64
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- SystemV FS 16 64
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- Coherent FS 16 32
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* How to implement symbolic links such that the host fsck doesn't scream:
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- Minix FS normal
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- Xenix FS kludge: as regular files with chmod 1000
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- SystemV FS ??
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- Coherent FS kludge: as regular files with chmod 1000
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Notation: We often speak of a "block" but mean a zone (the allocation unit)
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and not the disk driver's notion of "block".
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Bruno Haible <haible@ma2s2.mathematik.uni-karlsruhe.de>
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