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Hi9,<br>
<br>
I guess I would caution anyone trying this method of determining
network performance that the disk might be the slowest part of the
system. I think the only way to saturate a gigabit ethernet connection
while writing to disk, for example, is to have RAIDed hard drives on
separate controllers such that the disk throughput could be as fast as
the network.<br>
<br>
I think the whole idea of iSCSI is that the network might be way faster
than the physical disk, so you aren't really losing much performance
compared to a real disk, because the real disk locally connected is
about the same speed as a real disk connected via iSCSI. If you have
wizzy super-fast iSCSI target servers with fancy RAID arrays, your
iSCSI "disk" performance might actually be faster than a locally
connected disk, because the filer can give you I/O throughput of many
disks.<br>
<br>
You should use caution when trying to characterize network performance
in any way that involves a physical disk. As Dave notes, the
performance is as slow as the slowest part of the whole system, and I'm
postulating that the storage device might be way slower than the
network, especially if you have gigabit ethernet. If you can write to
/dev/null on the remote system that might be better, because there is
no disk I/O. For example this will copy 500MB over the network using
ssh but not use any disk I/O on either system:<br>
<br>
dd if=/dev/zero bs=1024k count=500 | ssh user@remote "dd of=/dev/null"<br>
<br>
I'm not sure if ssh is the best tool so if you really want to know your
link speed, you should probably just bite the bullet and use use a
purpose-written package like netperf. By the way I've used variable
size ping packets in the past to get an idea of link speed for slow
links like malfunctioning T1s.<br>
<br>
Jim<br>
-- <br>
Jim Ockers, P.E., P.Eng. (<a class="moz-txt-link-abbreviated" href="mailto:ockers@ockers.net">ockers@ockers.net</a>)<br>
Contact info: <a class="moz-txt-link-freetext" href="http://www.ockers.net/">http://www.ockers.net/</a><br>
<br>
David L. Willson wrote:
<blockquote
cite="mid:27bc578e-a049-4742-9835-eb2151f4fd8f@zimbra.thegeek.nu"
type="cite">
<pre wrap="">Here are the commands I use to test sustained read and write throughput.
# write speed, presumes device to test is exclusively mounted at /mnt (or that other sources of I/O are inconseqential)
dd if=/dev/zero of=/mnt/test.dd bs=4M count=256
# read speed, same presumptions
dd if=/mnt/test.dd of=/dev/null
Both commands output a summary of I/O speeds when they complete.
These commands to work equally well to test network throughput, but network throughput is the slowest of the network and any involved storage devices, minus a bit for management. I'll be trying netcat and netperf as time allows.
--
David L. Willson
Trainer, Engineer, Enthusiast
RHCE Network+ A+ Linux+ LPIC-1 Ubuntu
Mobile 720-333-LANS(5267)
This is a good time for a r3VOLution.
----- Original Message -----
</pre>
<blockquote type="cite">
<pre wrap="">So, to test hard drive transfer speeds, I use dd. I'll send the
commands when I'm on a real computer.
What do you use to test network speed?
--
David L. Willson
Trainer, Engineer, Enthusiast
RHCE Network+ A+ Linux+ LPIC-1 Ubuntu
Mobile 720-333-LANS(5267)
This is a good time for a r3VOLution.
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<br>
<br>
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