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[/] [or1k/] [trunk/] [uclinux/] [uClinux-2.0.x/] [fs/] [sysv/] [INTRO] - Blame information for rev 1765

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Line No. Rev Author Line
1 199 simons
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   June 1992
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    SystemV FS    Paul B. Monday  March-June 1993
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    Coherent FS   B. Haible  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)
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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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* 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  
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