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Full Discussion: DD command using block device as input
Special Forums Hardware DD command using block device as input Post 302666001 by nytty on Tuesday 3rd of July 2012 01:50:42 PM
Old 07-03-2012
DD command using block device as input
Hi,

I am trying to measure the speed of reading a given block size using the dd command. However depending on which input I use: a regular file (on the same device) or /dev/sdb1, I get some really different results.

Code:
$sudo sh -c 'echo 3 > /proc/sys/vm/drop_caches'; dd if=pirate of=/dev/null bs=512 count=1
1+0 records in
1+0 records out
512 bytes (512 B) copied, 0.0190755 s, 26.8 kB/s

$sudo sh -c 'echo 3 > /proc/sys/vm/drop_caches'; sudo dd if=/dev/sdb1 of=/dev/null bs=512 count=1
1+0 records in
1+0 records out
512 bytes (512 B) copied, 1.9649e-05 s, 26.1 MB/s

Note:
- Freshly formated ext4
ATA device, with non-removable media
Transport: Serial, SATA Rev 3.0

- I did the same experiment with varying blocksizes, and up to 128K the phenomena is the same.

- The file pirate is 100G and filefrag reports: 57 extents found

Questions:
- Is this normal?
- I am wondering if the speed of repositioning the disk head to the beginning of a device is fast comparing to any file offset movement. because reading 512B should be elementary ...

Thanks a lot
Last edited by methyl; 07-03-2012 at 07:36 PM.. Reason: please use code tags
 

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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)
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