Varnish seems to be doing gunzip a lot

Question

I would like your help to clarify this n_gunzip on my varnish setup.

These are my stats for one server that is running a couple of websites.

   34837         0.00         0.50 cache_hit - Cache hits
    1022         0.00         0.01 cache_hitpass - Cache hits for pass
    4672         0.00         0.07 cache_miss - Cache misses
    2175          .            .   n_expired - N expired objects
      85         0.00         0.00 n_gzip - Gzip operations
    3512         0.00         0.05 n_gunzip - Gunzip operations

The problem is I am seeing what I think is a lot of gunzips, about 7% of all hits. I really do not believe users would be accessing my websites with browsers that do not support gzip so I cannot understand why the gunzips are happenning.

All I have related to encoding on my VCL is the following:

    sub vcl_recv {  
     if (req.http.Accept-Encoding) {
            if (req.http.Accept-Encoding ~ "gzip") {
                # If the browser supports it, we'll use gzip.
                set req.http.Accept-Encoding = "gzip";
            }
            else if (req.http.Accept-Encoding ~ "deflate") {
                # Next, try deflate if it is supported.
                set req.http.Accept-Encoding = "deflate";
            }
            else {
                # Unknown algorithm. Remove it and send unencoded.
                unset req.http.Accept-Encoding;
            }
        }
...

Is my Varnish behaving correctly? Is this normal behavior?

Answer 1

Gunzip is pretty cheap, so I wouldn't worry about this for your traffic levels.

If this is a Varnish 3.0 server, you can safely remove the Accept-Encoding scrubbing in vcl_recv. This will be done behind the scenes by Varnish itself.

As for a root cause, my guess is that this is your service monitoring probe that forgets to set Accept-Encoding: gzip. Your probe URL (front page/favicon/probe.txt) is stored gzip-ed inside Varnish, and these repeated checks are skewing your gzip rate.