Commit 826a613d authored by Mauro Carvalho Chehab's avatar Mauro Carvalho Chehab Committed by Jonathan Corbet
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docs: filesystems: convert sysv-fs.txt to ReST 



- Add a SPDX header;
- Add a document title;
- Some whitespace fixes and new line breaks;
- Mark literal blocks as such;
- Add table markups;
- Add it to filesystems/index.rst.

Signed-off-by: default avatarMauro Carvalho Chehab <mchehab+huawei@kernel.org>
Link: https://lore.kernel.org/r/5b96a6efba95773af439ab25a7dbe4d0edf8c867.1581955849.git.mchehab+huawei@kernel.org


Signed-off-by: default avatarJonathan Corbet <corbet@lwn.net>
parent 86beb976

Documentation/filesystems/index.rst

+1 −0
Original line number Diff line number Diff line
@@ -88,5 +88,6 @@ Documentation for filesystem implementations. 
   romfs

   squashfs

   sysfs

   sysv-fs

   virtiofs

   vfat

Documentation/filesystems/sysv-fs.txt→Documentation/filesystems/sysv-fs.rst

+110 −43
Original line number Diff line number Diff line 
.. 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

* To mount a disk or a partition, use::



    mount [-r] -t sysv device mountpoint

  The file system type names



  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.
@@ -28,12 +43,14 @@ Bugs in the present implementation: 
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)
@@ -45,37 +62,51 @@ These filesystems are rather similar. Here is a comparison with Minix FS: 
  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



  =================================  ==

  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



  ===========  =========

  Minix FS     250

  Xenix FS     ??

  SystemV FS   ??

  Coherent FS  >=10000

  ===========  =========



* Free inode management:

  - Minix FS                             a bitmap



  - 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



  - 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
@@ -86,13 +117,18 @@ These filesystems are rather similar. Here is a comparison with Minix FS: 
      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



  ===========  ==========================

  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



  - Minix FS::



                    unsigned short s_ninodes;

                    unsigned short s_nzones;

                    unsigned short s_imap_blocks;
@@ -101,7 +137,9 @@ These filesystems are rather similar. Here is a comparison with Minix FS: 
                    unsigned short s_log_zone_size;

                    unsigned long s_max_size;

                    unsigned short s_magic;

  - Xenix FS, SystemV FS, Coherent FS



  - Xenix FS, SystemV FS, Coherent FS::



                    unsigned short s_firstdatazone;

                    unsigned long  s_nzones;

                    unsigned short s_fzone_count;
@@ -120,23 +158,33 @@ These filesystems are rather similar. Here is a comparison with Minix FS: 
                    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



        Xenix FS::



                    char           s_clean;

                    char           s_fill[371];

                    long           s_magic;

                    long           s_type;

        SystemV FS



        SystemV FS::



                    long           s_fill[12 or 14];

                    long           s_state;

                    long           s_magic;

                    long           s_type;

        Coherent FS



        Coherent FS::



                    unsigned long  s_unique;



    Note that Coherent FS has no magic.



* Inode layout:

  - Minix FS



  - Minix FS::



                    unsigned short i_mode;

                    unsigned short i_uid;

                    unsigned long  i_size;
@@ -144,7 +192,9 @@ These filesystems are rather similar. Here is a comparison with Minix FS: 
                    unsigned char  i_gid;

                    unsigned char  i_nlinks;

                    unsigned short i_zone[7+1+1];

  - Xenix FS, SystemV FS, Coherent FS



  - Xenix FS, SystemV FS, Coherent FS::



                    unsigned short i_mode;

                    unsigned short i_nlink;

                    unsigned short i_uid;
@@ -155,38 +205,55 @@ These filesystems are rather similar. Here is a comparison with Minix FS: 
                    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

  - 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



  - Minix FS::



                    unsigned short inode;

                    char name[14/30];

  - Xenix FS, SystemV FS, Coherent FS



  - 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

  ===========    ==============    =====================

                 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   ??

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