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