dd bytesize Post: 302515149

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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

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LEARN ABOUT LINUX

blockdev

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

NAME

       blockdev - call block device ioctls from the command line

SYNOPSIS

       blockdev [-q] [-v] command [command...]	device [device...]
       blockdev --report [device...]

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  the  specified device. It is possible to give multiple devices. If none is given, all devices which appear in
	      /proc/partitions are shown. Note that the partition StartSec is in 512-byte sectors.

COMMANDS

       It is possible to give multiple devices and multiple commands.

       --flushbufs
	      Flush buffers.

       --getalignoff
	      Get alignment offset.

       --getbsz
	      Print blocksize in bytes.

       --getdiscardzeroes
	      Get discard zeroes support status.

       --getfra
	      Get filesystem readahead in 512-byte sectors.

       --getiomin
	      Get minimum I/O size.

       --getioopt
	      Get optimal I/O size.

       --getmaxsect
	      Get max sectors per request

       --getpbsz
	      Get physical block (sector) size.

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

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

       --getsize64
	      Print device size in bytes.

       --getsize
	      Print device size (32-bit!) in sectors. Deprecated in favor of the --getsz option.

       --getss
	      Print sectorsize in bytes - usually 512.

       --getsz
	      Get size in 512-byte sectors.

       --rereadpt
	      Reread partition table

       --setbsz bytes
	      Set blocksize.

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

       --setra sectors
	      Set readahead (in 512-byte sectors).

       --setro
	      Set read-only.

       --setrw
	      Set read-write.

AUTHOR

       blockdev was written by Andries E. Brouwer and rewritten by Karel Zak.

AVAILABILITY

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

								     Aug 2010							       BLOCKDEV(8)