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 CENTOS
gnome-disks
GNOME-DISKS(1) gnome-disk-utility GNOME-DISKS(1)NAME
gnome-disks - the GNOME Disks application
SYNOPSIS
gnome-disks [OPTIONS]
DESCRIPTION
gnome-disks is the command to launch the GNOME Disks application. Disks provides a way to inspect, format, partition and configure disks
and block devices.
The Disks application is single-instance. What this means is that if the application is not already running when the gnome-disks command is
invoked, it will get launched and the command invocation will block until the application exits. Otherwise the existing application
instance will be used and the gnome-disks command will exit immediately.
OPTIONS
The following options are understood:
--block-device DEVICE
Switches to the Disks application and selects the block device given by DEVICE (for example, /dev/sda).
--block-device DEVICE --format-device [--xid WINDOW-ID]
Shows the "Format Volume" dialog for the block device given by DEVICE (for example, /dev/sdb1). If WINDOW-ID is given, makes the dialog
transient to the given XID.
--help
Prints a short help text and exits.
AUTHOR
Written by David Zeuthen <zeuthen@gmail.com> with a lot of help from many others.
BUGS
Please send bug reports to either the distribution bug tracker or the upstream bug tracker at
https://bugzilla.gnome.org/enter_bug.cgi?product=gnome-disk-utility.
SEE ALSO gnome-disk-image-mounter(1), udisks(8)GNOME March 2013 GNOME-DISKS(1)