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
may be used interchangeably, but the last two will eventually disappear.-t sysv -t xenix -t coherent
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
then they differ considerably: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];
Note that Coherent FS has no magic.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;
- Minix FS
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;
- Minix FS
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)
- Minix FS
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];
- Minix FS
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”.
