Mailing List Archive

James Quacinella <james at nyi.net> writes:
> From the man page: "vcl_pass: Called upon entering pass mode. In this
> mode, the request is passed on to the backend, and the backend's
> response is passed on to the client, but is not entered into the
> cache. Subsequent requests submitted over the same client connection
> are handled normally."
>
> So when running varnishstat, what does 'Cache hits for pass' mean?
> Sicne the above says that pass mode does not enter anything in the
> cache, but it can still use the cache?

It means we found a record in the cache that indicates that a particular
object should be handled in pass mode. This can happen if vcl_fetch()
ended with "pass", e.g. if the backend sent a cookie.

DES
--
Dag-Erling Sm?rgrav
Senior Software Developer
Linpro AS - www.linpro.no

In message <46A79303.2090001 at nyi.net>, James Quacinella writes:

>So when running varnishstat, what does 'Cache hits for pass' mean? Sicne
>the above says that pass mode does not enter anything in the cache, but
>it can still use the cache?

If you select "pass" in vcl_fetch(), an place-holder object will be
inserted in the cache which is maked "pass this one".

The next request to hit this object, will get a cache-hit, and then
use pass-processing.

The reason for this is that when we have a cache_miss, any
subsequent requests are "stalled" until the backend replies, so
finding out that an object shouldn't be cached all the way down
in vcl_fetch() can potentially be a bottleneck.

Inserting the "pass" object, means that the subsequent requests
of this object don't risk this pile-up.


--
Poul-Henning Kamp | UNIX since Zilog Zeus 3.20
phk at FreeBSD.ORG | TCP/IP since RFC 956
FreeBSD committer | BSD since 4.3-tahoe
Never attribute to malice what can adequately be explained by incompetence.