Problems with varnish backend health

[Winkelmann, Thomas (Radio/Tele FFH - Online)]

Hello,

we have a setup with two varnish servers (version 3.0.2) and two backend servers. This setup works fine, if we split the requests with an loadbalancer to the two varnish servers. Around 500req/s 20MBit/s bandwidth on each server.

In Addition we have defined a backend probe which will tests the backend each 10s. This also works fine.

Now, if we switch all requests to one varnish server, after some time (around 20 seconds) the backends will be marked as sick, but they are both up and answering all requests. Also without any backend probe defined in VCL, varnish gets in trouble serving content. Some months ago, we had no problems with this and served more than 400Mbit/s from one single server.

What could be the bottleneck?

Kind regards

Thomas Winkelmann
Web-Entwickler
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FFH Digital
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