NAME

fs (s5) − format of s5 file system volume

SYNOPSIS

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

DESCRIPTION

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:

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 indicates the state of the file system.  A cleanly unmounted, not damaged file system is indicated by the FsOKAY state.  After a file system has been mounted for update, the state changes to FsACTIVE.  A special case is used for the root file system.  If the root file system appears damaged at boot time, it is mounted but marked FsBAD.  Lastly, after a file system has been unmounted, the state reverts to FsOKAY. 

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(4). 

SEE ALSO

mount(2).  fsck(1M), fsdb(1M), mkfs(1M), s5-specific inode(4)

  —  s5