[clug] Diagnosing SSH performance problems [SEC=UNCLASSIFIED]

Brian Morris brian at netspeed.com.au
Thu Dec 10 15:35:55 MST 2009

I have also seen this problem using rsync.

I suspected it was either [a] MTU size too large [b] Excessive CPU load (as
it must do the SSH encryption on the fly) or [c] a problem with the write
caching on the HDD controller.

In my case it was as simple as the RAID controller battery requiring
replacement.  A flat battery on the controller meant that it would not cache
HDD writes and therefore slowed the whole thing down.

I hope this helps.

Regards,  Brian Morris

On 10/12/09 5:18 PM, "Roppola, Antti - BRS" <Antti.Roppola at daff.gov.au>

> Hi all,
> So I have an SSH problem where pushing data over an SSH connation gets
> me less than 9600 baud and numerous halts and timeouts. Pulling from the
> same host just rips along as you'd expect.
> Nothing obvious seen using "ssh -v". Nothing relevant leaps out at me in
> sshd_config.
> To me this suggests the issue is deeper down in the transport, network,
> data link of physical layer. Viz, I can't think of anything in OpenSSH
> that could be mis-configured to give asymmetric symptoms.
> Does this seem like a reasonable hypothesis?
> Cheers,
> Antti
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