Re: [Qemu-block] [PATCH 3/4] docs: document how to configure the qcow2 L2/refcount caches

[Max Reitz]

On 05.06.2015 17:56, Alberto Garcia wrote:
> On Fri 05 Jun 2015 05:52:29 PM CEST, Max Reitz wrote:
> > > +Reducing the memory usage
> > > +-------------------------
> > > +It is possible to clean unused cache entries in order to reduce the
> > > +memory usage during periods of low I/O activity.
> > > +
> > > +The parameter "cache-clean-interval" defines an interval (in seconds)
> > > +after which all unused cache entries are removed from memory.
> > > +
> > > +This example removes all unused cache entries every 15 minutes:
> > > +
> > > +   -drive file=hd.qcow2,cache-clean-interval=900
> > > +
> > > +If unset, the default value for this parameter is 0 and it disables
> > > +this feature.
> > > +
> > > +Note that this functionality currently relies on the MADV_DONTNEED
> > > +argument for madvise() to actually free the memory, so it is not
> > > +useful in systems that don't follow that behavior.
> > I'd like a definition of what "unused" means (I know because of this
> > series, but I wouldn't know just from this document).
> It means that it hasn't been accessed in that interval (because it was
> not needed). Are you thinking of any other possible interpretation?

Well... To me, intuitively, "unused" entries in a cache are just empty 
entries. Just replacing one occurrence of "unused" by "not accessed 
during the past interval" or something like that would resolve the 
riddle for me. :-)

Max