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120 lines
3.6 KiB
Groff
120 lines
3.6 KiB
Groff
.oh '11/3/70''FILE SYSTEM(V)'
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.ta 18 21 24
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NAME format of file system
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.sp
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SYNOPSIS --
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.sp
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DESCRIPTION The RB09 fixed head disk has 8,000 64-word
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blocks on each of its two surfaces. Unix uses
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the second surface (physical blocks 8,000 to
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15,999) and numbers them from logical block 0 to
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logical block 7,999. The following discussion
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refers to logical blocks.
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.sp
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The Unix filesystem is divided into 8,000 blocks
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of 64 words per block. Block 0 is the "sysdata"
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block. The first word points to the first block
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of the free-storage map. Each block in the
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free-storage map is structured as follows: the
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first word is the block number of the next block
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in the free-storage map, or zero if this is the
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end of the free-storage map. The next nine words
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hold free block numbers, or zero (no block
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number).
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.sp
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Block 1 on the filesystem is unused. (?)
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.sp
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I-numbers begin at 1, and the storage for
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i-nodes begins at block 2. Blocks 2 to 711
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contain the i-nodes, with five consecutive
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i-nodes per block. Certain i-nodes are reserved
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for special files, directories and devices:
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.sp
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1 The core file written by "sys save" or a
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bad system call
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2 The "dd" directory directory
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3 The "system" directory
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6 The "ttyin" special file
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7 The "keyboard" (graphic-2) special file
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8 The "pptin" (paper tape reader) special
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file
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10 The "ttyout" special file
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11 The "display" (graphic-2) special file
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12 The "pptout" (paper tape punch) special
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file
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.sp
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There is only one file system which is always
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mounted; it resides on the RB disk. This device
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is also used for swapping. The swap areas are at
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the high blocks on this device: blocks 6,400 to
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7,999. These blocks do not appear in the free
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list.
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.sp
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Each i-node represents one file. The format of
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an i-node is as follows, where the left column
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represents the offset in words from the
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beginning of the i-node:
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.sp
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0 flags (see below)
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1 first indirect block or contents block
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...
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7 seventh indirect block or contents block
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8 user ID of owner
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9 link count
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10 size in words
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11 unique value assigned at creation
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.sp
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The flags are as follows:
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.sp
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400000 i-node is allocated
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200000 large file
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?????? allocated bit (always on)
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000040 special file
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000020 directory
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000010 read, owner
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000004 write, owner
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000002 read, non-owner
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000001 write, non-owner
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.sp
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The allocated bit (flag ??????) is believed even
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if the i-node map says the i-node is free; thus
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corruption of the map may cause i-nodes to
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become unallocatable, but will not cause active
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nodes to be reused.
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.sp
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Word number n of a file is accessed as follows:
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n is divided by 64 to find its logical block
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number (say b) in the file. If the file is small
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(flag 200000 is 0), then b must be less than 7,
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and the physical block number corresponding to b
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is the bth entry in the address portion of the
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i-node.
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.sp
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If the file is large, b is divided by 64 to
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yield a number which must be less than 8 (or
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the file is too large for UNIX to handle).
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The corresponding slot in the i-node address
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portion gives the physical block number of an
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indirect block. The residue mod 64 gives a
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word offset in the indirect block, and the
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word found there is the physical address of
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the block corresponding to b.
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.sp
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If block b in a file exists, it is not necessary
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that all blocks less than b exist. A zero block
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number either in the address words of the i-node
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or in an indirect block indicates that the
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corresponding block has never been allocated.
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Such a missing block reads as if it contained
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all zero words.
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.sp
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FILES --
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.sp
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SEE ALSO format of directories
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.sp
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DIAGNOSTICS --
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.sp
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BUGS --
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.sp
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OWNER ken
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