fs (s5) -- format of s5 file system volume


   #include <sys/types.h>
   #include <sys/param.h>
   #include <sys/fs/s5filsys.h>


Every file system storage volume has a common format for certain vital information. Every such volume is divided into a certain number of 512-byte long sectors. Sector 0 is unused and is available to contain a bootstrap program or other information.

Sector 1 is the super-block. The format of a super-block is:

   struct    filsys
     ushort  s_isize;          /* size in blocks of i-list */
     daddr_t s_fsize;          /* size in blocks of entire volume */
     short   s_nfree;          /* number of addresses in s_free */
     daddr_t s_free[NICFREE];  /* free block list */
     short   s_ninode;         /* number of i-nodes in s_inode */
     o_ino_t s_inode[NICINOD]; /* free i-node list */
     char    s_flock;          /* lock during free list */
                               /* manipulation */
     char    s_ilock;          /* lock during i-list manipulation */
     char    s_fmod;           /* super block modified flag */
     char    s_ronly;          /* mounted read-only flag */
     time_t  s_time;           /* last super block update */
     short   s_dinfo[4];       /* device information */
     daddr_t s_tfree;          /* total free blocks*/
     o_ino_t s_tinode;         /* total free i-nodes */
     char    s_fname[6];       /* file system name */
     char    s_fpack[6];       /* file system pack name */
     long    s_fill[12];       /* ADJUST to make */
                               /* sizeof filsys be 512 */
     long    s_state;          /* file system state */
     long    s_magic;          /* magic number to denote new file
                               /* system */
     long    s_type;           /* type of new file system */

#define FsMAGIC 0xfd187e20 /* s_magic number */

#define Fs1b 1 /* 512-byte block */ #define Fs2b 2 /* 1024-byte block */ #define Fs4b 3 /* 2048-byte block */

#define FsOKAY 0x7c269d38 /* s_state: clean */ #define FsACTIVE 0x5e72d81a /* s_state: active */ #define FsBAD 0xcb096f43 /* s_state: bad root */ #define FsBADBLK 0xbadbc14b /* s_state: bad block corrupted it */

s_type indicates the file system type. Currently, three types of file systems are supported: the original 512-byte logical block, the 1024-byte logical block, and the 2048-byte logical block. s_magic is used to distinguish the s5 file system from other FSTypes. The s_type field is used to determine the blocksize of the file system; 512-bytes, 1K, or 2K. The operating system takes care of all conversions from logical block numbers to physical sector numbers.

s_state is unique for each file system and indicates the state of the file system. The numerical value of the ``file system state'' is computed as the sum of s_state and s_time and will ordinarily be one of FsOKAY, FsACTIVE, or FsBAD. A cleanly unmounted, undamaged file system is indicated by the FsOKAY state. After a file system had been mounted for update, the state changes to FsACTIVE. The state reverts to FsOKAY after a file system has been unmounted. A special case is used for the root file system. If it appears damaged at boot time, it is mounted but marked FsBAD.

s_isize is the address of the first data block after the i-list; the i-list starts just after the super-block, namely in block 2; thus the i-list is s_isize-2 blocks long. s_fsize is the first block not potentially available for allocation to a file. These numbers are used by the system to check for bad block numbers; if an ``impossible'' block number is allocated from the free list or is freed, a diagnostic is written on the on-line console. Moreover, the free array is cleared, so as to prevent further allocation from a presumably corrupted free list.

The free list for each volume is maintained as follows. The s_free array contains, in s_free[1], ..., s_free[s_nfree-1], up to 49 numbers of free blocks. s_free[0] is the block number of the head of a chain of blocks constituting the free list. The first long in each free-chain block is the number (up to 50) of free-block numbers listed in the next 50 longs of this chain member. The first of these 50 blocks is the link to the next member of the chain. To allocate a block: decrement s_nfree, and the new block is s_free[s_nfree]. If the new block number is 0, there are no blocks left, so give an error. If s_nfree became 0, read in the block named by the new block number, replace s_nfree by its first word, and copy the block numbers in the next 50 longs into the s_free array. To free a block, check if s_nfree is 50; if so, copy s_nfree and the s_free array into it, write it out, and set s_nfree to 0. In any event set s_free[s_nfree] to the freed block's number and increment s_nfree.

s_tfree is the total free blocks available in the file system.

s_ninode is the number of free i-numbers in the s_inode array. To allocate an i-node: if s_ninode is greater than 0, decrement it and return s_inode[s_ninode]. If it was 0, read the i-list and place the numbers of all free i-nodes (up to 100) into the s_inode array, then try again. To free an i-node, provided s_ninode is less than 100, place its number into s_inode[s_ninode] and increment s_ninode. If s_ninode is already 100, do not bother to enter the freed i-node into any table. This list of i-nodes is only to speed up the allocation process; the information as to whether the i-node is really free or not is maintained in the i-node itself.

s_tinode is the total free i-nodes available in the file system.

s_flock and s_ilock are flags maintained in the core copy of the file system while it is mounted and their values on disk are immaterial. The value of s_fmod on disk is likewise immaterial; it is used as a flag to indicate that the super-block has changed and should be copied to the disk during the next periodic update of file system information.

s_ronly is a read-only flag to indicate write-protection.

s_time is the last time the super-block of the file system was changed, and is the number of seconds that have elapsed since 00:00 Jan. 1, 1970 (UTC). During a reboot, the s_time of the super-block for the root file system is used to set the system's idea of the time.

s_fname is the name of the file system and s_fpack is the name of the pack.

I-numbers begin at 1, and the storage for i-nodes begins in block 2. Also, i-nodes are 64 bytes long. I-node 1 is reserved for future use. I-node 2 is reserved for the root directory of the file system, but no other i-number has a built-in meaning. Each i-node represents one file. For the format of an i-node and its flags, see inode_s5(4).


generic fsck(1M), generic fsdb(1M), inode_s5(4), generic mkfs(1M), mount(2)
© 2004 The SCO Group, Inc. All rights reserved.
UnixWare 7 Release 7.1.4 - 25 April 2004