V1/man/man5/filesys.5
11/3/71 FILE SYSTEM (V)
NAME format of file system
SYNOPSIS
DESCRIPTION Every file system storage volume (e.g. RF disk, RK disk,
DECtape reel) has a common format for certain vital
information.
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 numbered 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 superblock and the i--list, even though they
are never allocated 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 never
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. The swap areas are at the high
addresses on the device. It would be convenient if these
addresses did not appear in the free list, but in fact this
is not so. Therefore a certain number of blocks at the top
of the device appear in the free map, are not marked free,
yet do not appear within any file. These are the blocks
that show up missing in a check of the RE' disk.
Again on the primary file system device, there
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are several pieces of information following that previously
discussed. They contain basically the information typed by
the tm command; namely, the times spent since a cold boot
in various categories, and a count of I/O errors. In
particular, there are two words with the calendar time
(measured since 00:00 Jan 1, 1971); 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 second.
I--node 41(10) is reserved for the root directory of the
file system. No i--numbers other than this one and those
from I 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
divided by 512 to find its logical block number (say b) in
the file. If the file is small
11/3/71 FILE SYSTEM (V)
(flag 010000 is 0), then b must be less than 8, and the
physical block number corresponding to b is the bth entry
in the address portion of the i--node.
If the file is large, b is divided by 256 to yield a number
which must be less than 8 (or the file is too large for
UNIX to handle). The corresponding slot in the i--node
address portion gives the physical block number of an
indirect block. The residue mod 256 of b is multiplied by
two (to give a byte offset in the indirect block) and the
word found there is the physical address of the block
corresponding to b.
If block b in a file exists, 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.
FILES
SEE ALSO format of directories
DIAGNOSTICS
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.
OWNER