Log slow backend requests. VSL-query?

Geoff Simmons geoff at uplex.de
Tue Mar 21 19:28:10 CET 2017

On 03/21/2017 07:11 PM, Danila Vershinin wrote:
> Trying to log only the slow backend requests:
> varnishncsa -b -q 'Timestamp:Process[2] > 0.1’

Timestamp:Process only appears in the client-side logs, but with -b
you're filtering for the backend logs.

The "Backend fetch timestamps" section at the bottom of vsl(7) tells
you the names of the timestamps for backends:


The one you're probably looking for is Timestamp:Beresp[3]. Beresp is
the timestamp set after receiving backend response headers (the first
timestamp recorded after the backend starts sending a response). Field
3 is the time elapsed since the most recent timestamp, which would be
Bereq (backend request sent), so this is the best measurement of how
fast or slow a backend responds after receiving a request.

One thing to watch out for: if there is a fetch error such as a
timeout that results in no backend response being received at all,
then Timestamp:Beresp isn't recorded in the log. Instead of that you
get Timestamp:Error, so you might want to query for Timestamp:Error[3]
as well.

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