| .. SPDX-License-Identifier: GPL-2.0 |
| |
| ================== |
| SystemV Filesystem |
| ================== |
| |
| It implements all of |
| - Xenix FS, |
| - SystemV/386 FS, |
| - Coherent FS. |
| |
| To install: |
| |
| * Answer the 'System V and Coherent filesystem support' question with 'y' |
| when configuring the kernel. |
| * To mount a disk or a partition, use:: |
| |
| mount [-r] -t sysv device mountpoint |
| |
| The file system type names:: |
| |
| -t sysv |
| -t xenix |
| -t coherent |
| |
| may be used interchangeably, but the last two will eventually disappear. |
| |
| Bugs in the present implementation: |
| |
| - Coherent FS: |
| |
| - The "free list interleave" n:m is currently ignored. |
| - Only file systems with no filesystem name and no pack name are recognized. |
| (See Coherent "man mkfs" for a description of these features.) |
| |
| - SystemV Release 2 FS: |
| |
| The superblock is only searched in the blocks 9, 15, 18, which |
| corresponds to the beginning of track 1 on floppy disks. No support |
| for this FS on hard disk yet. |
| |
| |
| 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". |
