Varnish 503ing on ~1/100 POSTs

Ronan Mullally ronan at
Tue Apr 5 17:49:13 CEST 2011

Hi Sam,

On Tue, 5 Apr 2011, Sam Crawford wrote:

> I suspect you are right - Apache may be discarding the connection as you
> are sending the POST request, resulting in the 503. Incidentally, what
> are your Apache keep-alive settings? If I remember rightly, there is a
> KeepAliveTimeout (which it sounds like you've been using) and also a
> KeepAlive parameter (which controls the maximum number of requests on
> one TCP connection, irrespective of timeouts).

I've got the number of requests per session set high (thousands).  The
number of actual back-end requests is modest (10-20 per back-end per
second at most) so it won't hit that limit often.

> 1) If you disable keepalives completely at Apache, does the problem
> disappear?

Not entirely.  I did this eariler in the process but still saw occurances
(not as many as I saw with the Apache Keepalive timeout = 1, but not
significantly fewer than when it was = 5 either).

I've disabled keepalives again and will run with it that way overnight, so
far it looks like more of the same - I've seen one occurance in about 2000


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