dd bytesize Post: 302515149

Sponsored Content

Top Forums UNIX for Advanced & Expert Users dd bytesize Post 302515149 by programAngel on Tuesday 19th of April 2011 08:29:40 AM

04-19-2011

Registered User

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

programAngel

View Public Profile for programAngel

Find all posts by programAngel

LEARN ABOUT SUSE

blockdev

BLOCKDEV(8)						      System Manager's Manual						       BLOCKDEV(8)

NAME

       blockdev - call block device ioctls from the command line

SYNOPSIS

       blockdev [options] commands devices
       blockdev --report [devices]

DESCRIPTION

       The utility blockdev allows one to call block device ioctls from the command line.

OPTIONS

       -V     Print version and exit.

       -q     Be quiet.

       -v     Be verbose.

       --report
	      Print a report for devices.  Note that the partition StartSec is in 512-byte sectors.

COMMANDS

       --setro
	      Set read-only.

       --setrw
	      Set read-write.

       --getro
	      Get read-only. Print 1 if the device is read-only, 0 otherwise.

       --getss
	      Print sectorsize in bytes - usually 512.

       --getbsz
	      Print blocksize in bytes.

       --setbsz N
	      Set blocksize to N bytes.

       --getsize
	      Print device size in sectors (BLKGETSIZE). Deprecated in favor of the --getsz option.

       --getsize64
	      Print device size in bytes (BLKGETSIZE64)

       --getsz
	      Get size in 512-byte sectors (BLKGETSIZE64 / 512).

       --setra N
	      Set readahead to N 512-byte sectors.

       --getra
	      Print readahead (in 512-byte sectors).

       --setfra N
	      Set filesystem readahead (same like --setra on 2.6 kernels).

       --getfra
	      Get filesystem readahead.

       --flushbufs
	      Flush buffers.

       --rereadpt
	      Reread partition table.

AUTHOR

       blockdev was written by Andries E. Brouwer.

AVAILABILITY

       The blockdev command is part of the util-linux-ng package and is available from ftp://ftp.kernel.org/pub/linux/utils/util-linux-ng/.

								     Jun 2007							       BLOCKDEV(8)