Varnish backend slow-start

Question

The situation:

I'm using Varnish in front of some heavy loaded mod-php apache servers. When a cold (empty xcache, cold template cache) apache gets started, the server may overload because of too many requests.

The question:

Is there any way to have something like slow-start from nginx (http://nginx.org/en/docs/http/ngx_http_upstream_module.html#upstream , 'slow_start=time') for varnish ?

Answer 1

Yes & no, when you define a server you have one parameter which is .max_connections you can use it to limit the maximum number of concurrent connections made to the backend, I think it's unlimited if unset.

If you have more than one server in the director you can do a several things:

• define the new server with less weight (only for the random & hash directors)
• define the new server with a lower .max_connections, and later switch to a production vcl with a higher .max_connections
• keep the new server outside the production pool and "prime" its cache with fake/clone requests
• Probably others that I haven't though...