09-08-2011
question about unix file system
Hi,
The file system unix use a multilevel indexes access to disk, 12 direct blocks, 1 single indirect block, 1 double indirect block, 1 triple indirect block:
Assuming a:
block = 512 bytes,
pointer = 4 byte,
and there is a file of 200 blocks,
how many disk access is needed to read the block number 13 with direct disk access?
And how many disk access is needed to read this file sequentially form first block to 150?
I think that is needed 1 disk access to read the block 13 and 150 disk accesses to read the file sequentially.
Is it right?
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icheck(8) System Manager's Manual icheck(8)
NAME
icheck - File system storage consistency check
SYNOPSIS
/usr/sbin/icheck [-b numbers] [file system]
DESCRIPTION
The icheck command is obsoleted for normal consistency checking by fsck.
The icheck command examines a file system, builds a bit map of used blocks, and compares this bit map against the free map maintained on
the file system. If the file system is not specified, a set of default file systems is checked. The normal output of icheck includes a
report of the following items: The total number of files and the numbers of regular, directory, block special, character special, and fifo
files. The total number of blocks in use and the numbers of single-, double-, and triple-indirect blocks and directory blocks. The number
of free blocks. The number of blocks missing; that is, not in any file or in any free map.
A list of block numbers follows the -b option; whenever any of the named blocks turn up in a file, a diagnostic is produced.
The icheck command is faster if the raw version of the special file is used since it reads the i-list many blocks at a time.
NOTES
Since icheck is inherently two-pass in nature, extraneous diagnostics may be produced if applied to active file systems. It believes even
preposterous super-blocks and consequently causes a core dump.
DIAGNOSTICS
For duplicate blocks and bad blocks which lie outside the file system, icheck announces the difficulty, the i-number, and the kind of block
involved. If a read error is encountered, the block number of the bad block is printed and icheck considers it to contain zero.
FILES
Specifies the command path.
RELATED INFORMATION
Commands: clri(8), fsck(8), ncheck(8) delim off
icheck(8)