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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 DEBIAN

readdvd

readdvd(1)							    readdvd 1.0 							readdvd(1)

NAME

       readdvd - is creating an image of your source dvd media or medias even if it has / they have corrupted blocks

SYNOPSIS

       readdvd [-l] [-h]
       readdvd -d DEVICE1 [-d DEVICE2] [-d ...] -o file.iso [-s #] [-v] [-vv]

DESCRIPTION

       readdvd reads even a corrupted dvd and writes the the result into a new image file on your harddisk.

DEVICE

       can be an IDE, SCSI, SATA, USB or FireWire connected optical drive. Not all SATA controller support all Plextor features.

       Linux:
       /dev/hdX: IDE device
       /dev/scdX: Linux 2.4: SATA, SCSI, USB device, or IDE device via ide-scsi emulation
       /dev/srX: Linux 2.6: IDE device via new ATA layer, SCSI or USB device

       OpenBSD/NetBSD:
       /dev/rcdX

       FreeBSD:
       /dev/cd: SCSI device
       /dev/acd: ATA device

       MacOS X:
       /dev/disk:

       win32:
       C:,D:,E:, ... X:,Y:,Z:

OPTIONS

       -l	 scan busses for all available CD and DVD devices

       -h	 help shows available options.

       -o file.iso
		 write data to imagefile named file.iso

       -s #	 read source media with selected speed

       -v	 use verbose mode

       -vv	 use extended verbose mode

INTERACTIVE MODUS

       q    stop reading media and exit

       w    save sector map which is currently read and continue with the next one

EXAMPLES

       readdvd -d /dev/sr0 -o filename.iso -s 8 -v
	    create an image filename.bin of inserted media in device /dev/sr0 with read speed 8 in verbose mode.

       please report man page improvements to T.Maguin@web.de

Gennady ShultZ Kozlov						  10. April, 2009							readdvd(1)
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