Tracking down sporadic 503 errors

[Michal Taborsky - Netretail Holding]

Hi Justin,

not that I know anything about your problem, but we've seen this kind of 
behavior, when our Apaches were dying while processing the request. The 
apache log would not show any 503 (because there is no-one to log it), 
but Varnish was returning 503. In this case, you should see something 
like Segmentation fault in your error log on the backend (but I guess 
that depends on your platform).

Hope this helps.

Michal

Dne 30.3.2010 17:02, Justin Pasher napsal(a):
> Hello,
>
> I'm currently running Varnish r4633 (I can upgrade if absolutely 
> necessary), and we've been receiving very sporadic 503 errors listed 
> in the log files generated by varnishncsa. It's a very small 
> percentage, but nonetheless, when it happens on an important page, 
> it's noticeable.
>
> Stats from yesterday's log file show 116 "503" errors out of about 4.1 
> million hits. About 80% of the failed requests are POST requests, 
> which it setup in my VCL as a "pass through". If I look in the apache 
> logs (the backend server), I only see one 503 error returned by apache 
> itself, so maybe there's a timeout issue somewhere. I'm trying to 
> figure out the best way to troubleshoot this, since it's too 
> inconsistent to just sit watching the output of varnishlog.
>
> Perhaps it's hitting the default value for between_bytes_timeout (60 
> seconds)? If processing the data on the backend takes too long, then 
> varnish would time out after 60 seconds of no data, even if the 
> backend is still churning, right? I guess the question is what 
> situations cause Varnish to return a 503 aside from when the backend 
> itself returns a 503.
>
> I can post details of my VCL if needed, but it's pretty simple (mostly 
> taken from examples on the site).
>

-- 
Michal Táborský
chief systems architect
Netretail Holding, B.V.
http://www.nrholding.com

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