.. 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".