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) 
 
 - 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 - 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”.