Understanding persistent storage

Per Buer perbu at varnish-software.com
Tue Apr 19 10:01:06 CEST 2011

On Tue, Apr 19, 2011 at 9:34 AM, Yang Zhang <yanghatespam at gmail.com> wrote:

> On Tue, Apr 19, 2011 at 12:16 AM, Traian Bratucu
> <traian.bratucu at eea.europa.eu> wrote:
> > Well, varnish documentation tends to kind of suck. The "-s file" does not
> mean persistent storage, but simply that the file will be mmapped, so even
> if you have 2Gb of RAM, you can use a mmapped file of 10Gb. There is a new
> "-s persistence" which is documented for now as "new, shiny, better", you
> can make whatever you want out of that (
> http://www.varnish-cache.org/docs/2.1/reference/varnishd.html - Storage
> Types).
> > I am not a varnish developer, just using varnish. Perhaps one of the
> developers may explain more.
> This was precisely my limited understanding as well.  I'm still
> curious whether -s file could be hacked to be persistent and what
> exactly -s persistence is.

-sfile cannot be hacked into persistence. Varnish has no idea whatsoever
what the file contains. It just writes to memory and the kernel takes care
of the rest. It's the best way to allocate truly virtual memory. You can
probably get a thorough explanation in a C book or something.

-spersistence is experimental and I have intensionally left the
documentation blank on purpose. -spersistence hasn't got any working LRU
code yet. I expect -spersistence ready in the autumn.


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