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Full Discussion: dd bytesize
Top Forums UNIX for Advanced & Expert Users dd bytesize Post 302515149 by programAngel on Tuesday 19th of April 2011 08:29:40 AM
Old 04-19-2011
Quote:
Originally Posted by pludi
"bs" is the block size, not the byte size (a byte is always 8 bits). Reasons you might want to use it:
  • To transfer a certain amount of fixed-size records (using bs and count options)
  • To match the block size of a device, thus reducing I/O blocking (eg. for hard disks and tapes)
  • To make use of a large buffer, thus reducing I/O requests

Simple example: you want to create a backup of the master boot record prior to changing the layout, it's as simple as dd if=/dev/sda of=part_table.backup bs=512 count=1, since the MBR is exactly the 512 bytes at the beginning of the disk (including boot loader, partition table, ...)
thanks you.

regarding the second use "To match the block size of a device", who can I know the block size of a device?
like for example my hard disk
 
WREN(3) 						     Library Functions Manual							   WREN(3)

NAME
wren, ata - hard disk interface SYNOPSIS
bind #H[drive] /dev bind #w[target[.lun]] /dev /dev/hd0disk /dev/hd0partition /dev/sd0disk /dev/sd0partition ... DESCRIPTION
The hard disk interfaces (wren, #w, is a SCSI disk; ata, #H, is an IDE or ATA disk) serve a one-level directory giving access to the hard disk partitions. The parameter to attach defines the numerical SCSI target and logical unit number or the IDE drive number to access. Both default to zero. Each partition name is prefixed by hd and the numeric drive identifier. The partition always exists and covers the entire disk. The size of each partition as reported by stat(2) is the number of bytes in the partition, so the size of is the size of the entire disk. The partition also always exists; it is the last block on the disk for SCSI, second to last for IDE. If it contains valid partition data, those partitions will be visible as well. Every time the device is bound, the partitions are updated to reflect any changes in the parti- tion file. The format of the partition file is the string plan9 partitions on a line, followed by partition specifications, one per line, consisting of a name and textual strings for the block start and limit for each partition on the disk. The program prep(8) writes the partition table for the disk; its use is preferred to writing it by hand. SEE ALSO
prep(8), scsi(3) SOURCE
/sys/src/9/port/devwren.c /sys/src/9/pc/devata.c WREN(3)
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