filsys, flblk, ino - format of file system volume
Every file system storage volume (e.g. RF disk, RK disk, RP disk, DECtape reel) has a com-
mon 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 pro-
gram, pack label, or other information.
Block 1 is the super block. The layout of the super block as defined by the include file
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
corrupted 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 con-
figuration constant. S_free is the block address of the head of a chain of blocks con-
stituting 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 NICIN-
ODE, don't 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 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 immaterial.
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
The fields s_tfree, s_tinode, s_fname and s_fpack are not currently maintained.
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-number has a built-in meaning. Each i-node represents one
file. The format 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. Point-
ers 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 indi-
cates 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)