Varnish 503ing on ~1/100 POSTs

Ronan Mullally ronan at
Wed Mar 9 16:38:28 CET 2011

Hi Stephan,

On Wed, 9 Mar 2011, Caunter, Stefan wrote:

> I don't think pass or pipe is the issue. 503 means the backend can't
> answer, and calling pipe won't change that.

> Set up a "patient" back end; you can collect your back ends into a
> patient director.

Ah, the penny drops.  I was thinking of "patient" in the context of
health checks (ie a sick backend).

I'll give it a go, but my gut feeling is that the backends aren't at
fault.  I'm seeing this error when they are both backends lightly loaded
(load average around 1 on an 8 core box), and the rate of incidence does
not appear to be related to the load - I actually saw a slightly lower
rate (under 1%) last night when utilisation was higher, and as I said
previously when I used pound instead of varnish this wasn't a problem.

I'll try the patient backend and keep a close eye on the error rate vs
utilisation over the next few days.  Thanks for your help.


> -----Original Message-----
> >From: varnish-misc-bounces at
> [mailto:varnish-misc-bounces at] On Behalf Of Ronan
> Mullally
> Sent: March-08-11 5:32 PM
> To: Stewart Robinson
> Cc: varnish-misc at
> Subject: Re: Varnish 503ing on ~1/100 POSTs
> On Tue, 8 Mar 2011, Stewart Robinson wrote:
> > Whilst this may not be a fix to a possible bug in varnish have you
> tried
> > switching posts to pipe instead of pass?
> This might well help, but I'd have no way of knowing for sure.  The
> backend servers indicate the requests via varnish are processed
> correctly.
> I'm not able to reproduce the problem at will so I'd be relying on user
> feedback to determine if the problem still occurs and that's unreliable
> at
> best.
> It is of course better than having the problem occur, but I'd rather
> take
> the opportunity to try and get to the bottom of it while I can.  I only
> deployed varnish a couple of days ago.  The site will be fairly quiet
> until the end of the week.  I'll resort to pipe if I've not got a fix by
> then.
> -Ronan
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