help understanding backend_health varnishlog line

Michael Alger varnish at
Wed Sep 1 04:54:16 CEST 2010

On Tue, Aug 31, 2010 at 03:42:37PM -0700, David Birdsong wrote:
> I'm trying to understand why varnish's health checks marks my
> backend down at random intervals.  Could somebody explain the
> columns in these lines?
> varnishlog  -i backend_health  | grep -i sick
>     0 Backend_health - mogfe Went sick 4--X--- 2 3 3 0.000000 0.000387

It's documented on this page of the wiki:

But it does look to me like varnish think it's sending the request okay,
and not receiving any response.

> My backend is nginx on the same machine.  Varnish connects over
>  The check stanza in nginx simply returns a 1x1 gif that
> nginx keeps around in memory so I'm really confused what could be
> causing the bad response.

Maybe double-check that your probes are closing the connection each
request (with the Connection: close header)? As a temporary workaround
you could perhaps bump the window size so a single failed request
won't knock the backend offline.

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