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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DD(1) General Commands Manual DD(1)
NAME
dd - disk dumper
SYNOPSIS
dd [option = value] ...
EXAMPLES
dd if=/dev/fd0 of=/dev/fd1
# Copy disk 0 to disk 1
dd if=x of=y bs=1w skip=4
# Copy x to y, skipping 4 words
dd if=x of=y count=3
# Copy three 512-byte blocks
DESCRIPTION
This command is intended for copying partial files. The block size, skip count, and number of blocks to copy can be specified. The
options are:
if = file - Input file (default is stdin)
of = file - Output file (default is standard output)
ibs = n - Input block size (default 512 bytes)
obs = n - Output block size (default is 512 bytes)
bs = n - Block size; sets ibs and obs (default is 512 bytes)
skip = n - Skip n input blocks before reading
seek = n - Skip n output blocks before writing
count = n - Copy only n input blocks
conv = lcase - Convert upper case letters to lower case
conv = ucase - Convert lower case letters to upper case
conv = swab - Swap every pair of bytes
conv = noerror- Ignore errors and just keep going
conv = silent- Suppress statistics (Minix specific flag)
Where sizes are expected, they are in bytes. However, the letters w, b, or k may be appended to the number to indicate words (2 bytes),
blocks (512 bytes), or K (1024 bytes), respectively. When dd is finished, it reports the number of full and partial blocks read and writ-
ten.
SEE ALSO
vol(1).
DD(1)