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Diffstat (limited to 'static/v10/man5/fs.5')
| -rw-r--r-- | static/v10/man5/fs.5 | 147 |
1 files changed, 147 insertions, 0 deletions
diff --git a/static/v10/man5/fs.5 b/static/v10/man5/fs.5 new file mode 100644 index 00000000..0d52b711 --- /dev/null +++ b/static/v10/man5/fs.5 @@ -0,0 +1,147 @@ +.pa 1 +.he 'FILE SYSTEM (V)'3/15/72'FILE SYSTEM (V)' +.ti 0 +NAME format of file system +.sp +.ti 0 +DESCRIPTION +.br +.in 8 +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\u.\d((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 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 +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: +.sp +.nf +.in +3 +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 +.li +... +20-21 eighth indirect block or contents block +22-25 creation time +26-29 modification time +30-31 unused +.fi +.sp +.ti -3 +The flags are as follows: +.sp +.nf +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 +.sp +.in -3 +.fi +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 (flag 010000 is 0), +then b_ must be less than 8, and the physical block +number corresponding to b_ is the b_th 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 indirect block +and the word there found is the physical address 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 contained all zero words. + +.in 16 +.ti 0 +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. +.sp |