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Diffstat (limited to 'fs/sysv/INTRO')
| -rw-r--r-- | fs/sysv/INTRO | 182 |
1 files changed, 182 insertions, 0 deletions
diff --git a/fs/sysv/INTRO b/fs/sysv/INTRO new file mode 100644 index 00000000000..de4e4d17cac --- /dev/null +++ b/fs/sysv/INTRO @@ -0,0 +1,182 @@ +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> |