FILSYS(5) File Formats Manual FILSYS(5)
filsys, flblk, ino - format of file system volume
Every file system storage volume (e.g. RF disk, RK disk, RP disk, DEC‐
tape reel) has a common format for certain vital information. Every
such volume is divided into a certain number of 512-byte blocks. Block
0 is unused and is available to contain a bootstrap program, pack
label, or other information.
Block 1 is the super block. The layout of the super block as defined
by the include file <sys/filsys.h> is:
S_isize is the address of the first block after the i-list, which
starts just after the super-block, in block 2. Thus is i-list is
s_isize-2 blocks long. S_fsize is the address of the first block not
potentially available for allocation to a file. These numbers are used
by the system to check for bad block addresses; if an `impossible'
block address 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 cor‐
rupted free list.
The free list for each volume is maintained as follows. The s_free
array contains, in s_free, ... , s_free[s_nfree-1], up to NICFREE
free block numbers. NICFREE is a configuration constant. S_free is
the block address of the head of a chain of blocks constituting the
free list. The layout of each block of the free chain as defined in
the include file <sys/fblk.h> is:
The fields df_nfree and df_free in a free block are used exactly like
s_nfree and s_free in the super block. To allocate a block: decrement
s_nfree, and the new block number is s_free[s_nfree]. If the new block
address is 0, there are no blocks left, so give an error. If s_nfree
became 0, read the new block into s_nfree and s_free. To free a block,
check if s_nfree is NICFREE; 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 address and increment s_nfree.
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 inodes (up to NICINOD) into the s_inode array, then
try again. To free an i-node, provided s_ninode is less than NICINODE,
place its number into s_inode[s_ninode] and increment s_ninode. If
s_ninode is already NICINODE, don't bother to enter the freed i-node
into any table. This list of i-nodes is only to speed up the alloca‐
tion process; the information as to whether the inode is really free or
not is maintained in the inode itself.
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 write-protection indicator; its disk value is also immate‐
S_time is the last time the super-block of the file system was changed.
During a reboot, s_time of the super-block for the root file system is
used to set the system's idea of the time.
The fields s_tfree, s_tinode, s_fname and s_fpack are not currently
I-numbers begin at 1, and the storage for i-nodes begins in block 2.
I-nodes are 64 bytes long, so 8 of them fit into a block. I-node 2 is
reserved for the root directory of the file system, but no other i-num‐
ber has a built-in meaning. Each i-node represents one file. The for‐
mat of an i-node as given in the include file <sys/ino.h> is:
Di_mode tells the kind of file; it is encoded identically to the
st_mode field of stat(2). Di_nlink is the number of directory entries
(links) that refer to this i-node. Di_uid and di_gid are the owner's
user and group IDs. Size is the number of bytes in the file. Di_atime
and di_mtime are the times of last access and modification of the file
contents (read, write or create) (see times(2)); Di_ctime records the
time of last modification to the inode or to the file, and is used to
determine whether it should be dumped.
Special files are recognized by their modes and not by i-number. A
block-type special file is one which can potentially be mounted as a
file system; a character-type special file cannot, though it is not
necessarily character-oriented. For special files, the di_addr field
is occupied by the device code (see types(5)). The device codes of
block and character special files overlap.
Disk addresses of plain files and directories are kept in the array
di_addr packed into 3 bytes each. The first 10 addresses specify
device blocks directly. The last 3 addresses are singly, doubly, and
triply indirect and point to blocks of 128 block pointers. Pointers in
indirect blocks have the type daddr_t (see types(5)).
For block b in a file to exist, it is not necessary that all blocks
less than b exist. A zero block number either in the address words of
the i-node or in an indirect block indicates that the corresponding
block has never been allocated. Such a missing block reads as if it
contained all zero words.
icheck(1), dcheck(1), dir(5), mount(1), stat(2), types(5)