FILE SYSTEM (V) 3/15/72 FILE SYSTEM (V)
NAME format of file system
DESCRIPTION
Every file system storage volume (e.g. RF disk, RK disk,
DECtape reel) has a common format for certain vital in-
formation.
Every such volume is divided into a certain number of 256
word (512 byte) blocks. Blocks 0 and 1 are collectively
known as the super-block for the device; they define its
extent and contain an i-node map and a free-storage map.
The first word contains the number of bytes in the free-
storage map; it is always even. It is followed by the
map. There is one bit for each block on the device; the
bit is "1" if the block is free. Thus if the number of
free-map bytes is n, the blocks on the device are num-
bered 0 through 8n-1. The free-map count is followed by
the free map itself. The bit for block k of the device
is in byte k/8 of the map; it is offset k(mod 8) bits
from the right. Notice that bits exist for the su-
perblock and the i-list, even though they are never allo-
cated or freed.
After the free map is a word containing the byte count
for the i-node map. It too is always even. I-numbers
below 41(10) are reserved for special files, and are nev-
er allocated; the first bit in the i-node free map refers
to i-number 41. Therefore the byte number in the i-node
map for i-node i is (i-41)/8. It is offset (i-41) (mod
8) bits from the right; unlike the free map, a "0" bit
indicates an available i-node.
I-numbers begin at 1, and the storage for i-nodes begins
at block 2. Also, i-nodes are 32 bytes long, so 16 of
them fit into a block. Therefore, i-node i is located in
block (i+31)/16 of the file system, and begins
.
32 ((i+31)(mod 16)) bytes from its start.
There is always one file system which is always mounted;
in standard UNIX it resides on the RF disk. This device
is also used for swapping. On the primary file system
device, there are several pieces of information following
that previously discussed. There are two words with the
calendar time (measured since 00:00 Jan 1, 1972); two
words with the time spent executing in the system; two
words with the time spent waiting for I/O on the RF and
RK disks; two words with the time spent executing in a
user's core; one byte with the count of errors on the RF
disk; and one byte with the count of errors on the RK
disk. All the times are measured in sixtieths of a sec-
ond.
I-node 41(10) is reserved for the root directory of the
file system. No i-numbers other than this one and those
from 1 to 40 (which represent special files) have a
built-in meaning. Each i-node represents one file. The
format of an i-node is as follows, where the left column
represents the offset from the beginning of the i-node:
0-1 flags (see below)
2 number of links
3 user ID of owner
4-5 size in bytes
6-7 first indirect block or contents block
...
20-21 eighth indirect block or contents block
22-25 creation time
26-29 modification time
30-31 unused
The flags are as follows:
100000 i-node is allocated
040000 directory
020000 file has been modified (always on)
010000 large file
000040 set user ID on execution
000020 executable
000010 read, owner
000004 write, owner
000002 read, non-owner
000001 write, non-owner
The allocated bit (flag 100000) is believed even if the
i-node map says the i-node is free; thus corruption of
the map may cause i-nodes to become unallocatable, but
will not cause active nodes to be reused.
Byte number n of a file is accessed as follows: n is di-
vided by 512 to find its logical block number (say b) in
the file. If the file is small (flag 010000 is 0), then
b must be less than 8, and the physical block number cor-
responding to b is the bth entry in the address portion
of the i-node.
Even if the file is large, b will be less than 128
(128*512 = 2^16). The first number in the i-node address
portion gives the physical block number of the indirect
block. b is doubled to give a byte offset in the indi-
rect block and the word there found is the physical ad-
dress of the block corresponding to b.
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 con-
tained all zero words.
BUGS Two blocks are not enough to handle the i- and
free-storage maps for an RP02 disk pack, which
contains around 10 million words.