[linux-block.git] / Documentation / filesystems / sysv-fs.rst

.. 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".
Commit	Line	Data
826a613d MCC	1	.. SPDX-License-Identifier: GPL-2.0
	2
	3	==================
	4	SystemV Filesystem
	5	==================
	6
1da177e4 LT	7	It implements all of
	8	- Xenix FS,
	9	- SystemV/386 FS,
	10	- Coherent FS.
	11
1da177e4	12	To install:
826a613d	13
1da177e4 LT	14	* Answer the 'System V and Coherent filesystem support' question with 'y'
1da177e4 LT	15	when configuring the kernel.
826a613d MCC	16	* To mount a disk or a partition, use::
826a613d MCC	17
1da177e4	18	mount [-r] -t sysv device mountpoint
826a613d MCC	19
	20	The file system type names::
	21
1da177e4 LT	22	-t sysv
	23	-t xenix
	24	-t coherent
826a613d	25
1da177e4 LT	26	may be used interchangeably, but the last two will eventually disappear.
	27
	28	Bugs in the present implementation:
826a613d	29
1da177e4	30	- Coherent FS:
826a613d	31
1da177e4 LT	32	- The "free list interleave" n:m is currently ignored.
1da177e4 LT	33	- Only file systems with no filesystem name and no pack name are recognized.
826a613d MCC	34	(See Coherent "man mkfs" for a description of these features.)
826a613d MCC	35
1da177e4	36	- SystemV Release 2 FS:
826a613d	37
1da177e4 LT	38	The superblock is only searched in the blocks 9, 15, 18, which
	39	corresponds to the beginning of track 1 on floppy disks. No support
	40	for this FS on hard disk yet.
	41
	42
3982cd99 AB	43	These filesystems are rather similar. Here is a comparison with Minix FS:
	44
	45	* Linux fdisk reports on partitions
826a613d	46
3982cd99 AB	47	- Minix FS 0x81 Linux/Minix
	48	- Xenix FS ??
	49	- SystemV FS ??
	50	- Coherent FS 0x08 AIX bootable
	51
	52	* Size of a block or zone (data allocation unit on disk)
826a613d	53
3982cd99 AB	54	- Minix FS 1024
	55	- Xenix FS 1024 (also 512 ??)
	56	- SystemV FS 1024 (also 512 and 2048)
	57	- Coherent FS 512
	58
	59	* General layout: all have one boot block, one super block and
	60	separate areas for inodes and for directories/data.
	61	On SystemV Release 2 FS (e.g. Microport) the first track is reserved and
	62	all the block numbers (including the super block) are offset by one track.
	63
	64	* Byte ordering of "short" (16 bit entities) on disk:
826a613d	65
3982cd99 AB	66	- Minix FS little endian 0 1
	67	- Xenix FS little endian 0 1
	68	- SystemV FS little endian 0 1
	69	- Coherent FS little endian 0 1
826a613d	70
3982cd99 AB	71	Of course, this affects only the file system, not the data of files on it!
	72
	73	* Byte ordering of "long" (32 bit entities) on disk:
826a613d	74
3982cd99 AB	75	- Minix FS little endian 0 1 2 3
	76	- Xenix FS little endian 0 1 2 3
	77	- SystemV FS little endian 0 1 2 3
	78	- Coherent FS PDP-11 2 3 0 1
826a613d	79
3982cd99 AB	80	Of course, this affects only the file system, not the data of files on it!
	81
	82	* Inode on disk: "short", 0 means non-existent, the root dir ino is:
826a613d MCC	83
	84	================================= ==
	85	Minix FS 1
	86	Xenix FS, SystemV FS, Coherent FS 2
	87	================================= ==
3982cd99 AB	88
3982cd99 AB	89	* Maximum number of hard links to a file:
826a613d MCC	90
	91	=========== =========
	92	Minix FS 250
	93	Xenix FS ??
	94	SystemV FS ??
	95	Coherent FS >=10000
	96	=========== =========
3982cd99 AB	97
3982cd99 AB	98	* Free inode management:
826a613d MCC	99
	100	- Minix FS
	101	a bitmap
3982cd99 AB	102	- Xenix FS, SystemV FS, Coherent FS
	103	There is a cache of a certain number of free inodes in the super-block.
	104	When it is exhausted, new free inodes are found using a linear search.
	105
	106	* Free block management:
826a613d MCC	107
	108	- Minix FS
	109	a bitmap
3982cd99 AB	110	- Xenix FS, SystemV FS, Coherent FS
	111	Free blocks are organized in a "free list". Maybe a misleading term,
	112	since it is not true that every free block contains a pointer to
	113	the next free block. Rather, the free blocks are organized in chunks
	114	of limited size, and every now and then a free block contains pointers
	115	to the free blocks pertaining to the next chunk; the first of these
	116	contains pointers and so on. The list terminates with a "block number"
	117	0 on Xenix FS and SystemV FS, with a block zeroed out on Coherent FS.
	118
	119	* Super-block location:
826a613d MCC	120
	121	=========== ==========================
	122	Minix FS block 1 = bytes 1024..2047
	123	Xenix FS block 1 = bytes 1024..2047
	124	SystemV FS bytes 512..1023
	125	Coherent FS block 1 = bytes 512..1023
	126	=========== ==========================
3982cd99 AB	127
3982cd99 AB	128	* Super-block layout:
826a613d MCC	129
	130	- Minix FS::
	131
3982cd99 AB	132	unsigned short s_ninodes;
	133	unsigned short s_nzones;
	134	unsigned short s_imap_blocks;
	135	unsigned short s_zmap_blocks;
	136	unsigned short s_firstdatazone;
	137	unsigned short s_log_zone_size;
	138	unsigned long s_max_size;
	139	unsigned short s_magic;
826a613d MCC	140
	141	- Xenix FS, SystemV FS, Coherent FS::
	142
3982cd99 AB	143	unsigned short s_firstdatazone;
	144	unsigned long s_nzones;
	145	unsigned short s_fzone_count;
	146	unsigned long s_fzones[NICFREE];
	147	unsigned short s_finode_count;
	148	unsigned short s_finodes[NICINOD];
	149	char s_flock;
	150	char s_ilock;
	151	char s_modified;
	152	char s_rdonly;
	153	unsigned long s_time;
	154	short s_dinfo[4]; -- SystemV FS only
	155	unsigned long s_free_zones;
	156	unsigned short s_free_inodes;
	157	short s_dinfo[4]; -- Xenix FS only
	158	unsigned short s_interleave_m,s_interleave_n; -- Coherent FS only
	159	char s_fname[6];
	160	char s_fpack[6];
826a613d	161
3982cd99	162	then they differ considerably:
826a613d MCC	163
	164	Xenix FS::
	165
3982cd99 AB	166	char s_clean;
	167	char s_fill[371];
	168	long s_magic;
	169	long s_type;
826a613d MCC	170
	171	SystemV FS::
	172
3982cd99 AB	173	long s_fill[12 or 14];
	174	long s_state;
	175	long s_magic;
	176	long s_type;
826a613d MCC	177
	178	Coherent FS::
	179
3982cd99	180	unsigned long s_unique;
826a613d	181
3982cd99 AB	182	Note that Coherent FS has no magic.
	183
	184	* Inode layout:
826a613d MCC	185
	186	- Minix FS::
	187
3982cd99 AB	188	unsigned short i_mode;
	189	unsigned short i_uid;
	190	unsigned long i_size;
	191	unsigned long i_time;
	192	unsigned char i_gid;
	193	unsigned char i_nlinks;
	194	unsigned short i_zone[7+1+1];
826a613d MCC	195
	196	- Xenix FS, SystemV FS, Coherent FS::
	197
3982cd99 AB	198	unsigned short i_mode;
	199	unsigned short i_nlink;
	200	unsigned short i_uid;
	201	unsigned short i_gid;
	202	unsigned long i_size;
	203	unsigned char i_zone[3*(10+1+1+1)];
	204	unsigned long i_atime;
	205	unsigned long i_mtime;
	206	unsigned long i_ctime;
	207
826a613d	208
3982cd99	209	* Regular file data blocks are organized as
3982cd99	210
826a613d MCC	211	- Minix FS:
	212
	213	- 7 direct blocks
	214	- 1 indirect block (pointers to blocks)
	215	- 1 double-indirect block (pointer to pointers to blocks)
	216
	217	- Xenix FS, SystemV FS, Coherent FS:
	218
	219	- 10 direct blocks
	220	- 1 indirect block (pointers to blocks)
	221	- 1 double-indirect block (pointer to pointers to blocks)
	222	- 1 triple-indirect block (pointer to pointers to pointers to blocks)
	223
	224
	225	=========== ========== ================
	226	Inode size inodes per block
	227	=========== ========== ================
	228	Minix FS 32 32
	229	Xenix FS 64 16
	230	SystemV FS 64 16
	231	Coherent FS 64 8
	232	=========== ========== ================
3982cd99 AB	233
3982cd99 AB	234	* Directory entry on disk
826a613d MCC	235
	236	- Minix FS::
	237
3982cd99 AB	238	unsigned short inode;
3982cd99 AB	239	char name[14/30];
826a613d MCC	240
	241	- Xenix FS, SystemV FS, Coherent FS::
	242
3982cd99 AB	243	unsigned short inode;
	244	char name[14];
	245
826a613d MCC	246	=========== ============== =====================
	247	Dir entry size dir entries per block
	248	=========== ============== =====================
	249	Minix FS 16/32 64/32
	250	Xenix FS 16 64
	251	SystemV FS 16 64
	252	Coherent FS 16 32
	253	=========== ============== =====================
3982cd99 AB	254
3982cd99 AB	255	* How to implement symbolic links such that the host fsck doesn't scream:
826a613d	256
3982cd99 AB	257	- Minix FS normal
	258	- Xenix FS kludge: as regular files with chmod 1000
	259	- SystemV FS ??
	260	- Coherent FS kludge: as regular files with chmod 1000
1da177e4	261
1da177e4	262
3982cd99 AB	263	Notation: We often speak of a "block" but mean a zone (the allocation unit)
3982cd99 AB	264	and not the disk driver's notion of "block".