[Nottingham] Diagnosing a slow network

Nick Leverton nick at leverton.org
Sun Nov 25 14:36:07 UTC 2012

On Sun, Nov 25, 2012 at 01:53:12PM +0000, Jason Irwin wrote:
> I'm running an rsync from my desktop (Ubuntu 10.10) to a server (Ubuntu
> 12.04) as a paranoid back-up before upgrading the desktop.
> The desktop and server are both SATA units (desktop is RAID0, server is
> RAID1), they are connected to the same gigabit switch by CAT6 cables.
> There's nothing else really running, so why is rsync showing speed of
> only 14MB/s or so?
> Surely I should be seeing 100MB/s or something?  (I'm assuming rsync is
> displaying mega-bytes and not mega-bits)
> How do I set about determining where the bottleneck is?
> Or has my total inability to understand anything network-y reared its
> head again?

I have seen similar slowdown on rsync, and I think it is because by
default it tunnels over ssh.  The encryption and decryption slows things
down quite a surprising amount, and more so if you have ssh compression on
(doubly so I would imagine if you also ask rsync to do compression with
the -z flag which IME is rarely worth it across a fast local network).

One time, I started up rsync server daemon on one of the hosts and let
rsync on the other talk to it directly.  Alternatively perhaps you could
install rsh and tell rsync to use it with "-e rsh" (see rsync man page
for alternative transport shells and options).

I am a bit hung over today for some reason so can't remember more but
perhaps this may give you an avenue to explore.



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