Re: [Qemu-block] [PATCH 2/3] qcow2: add option to clean unused cache entries after some time

[Max Reitz]

On 28.05.2015 17:19, Alberto Garcia wrote:
> On Thu 28 May 2015 05:14:12 PM CEST, Max Reitz wrote:
> > > > >           'compat': 'str',
> > > > >           '*lazy-refcounts': 'bool',
> > > > >           '*corrupt': 'bool',
> > > > > -      'refcount-bits': 'int'
> > > > > +      'refcount-bits': 'int',
> > > > > +      'cache-clean-interval': 'int'
> > > > >       } }
> > > > I'm not too happy about making this part of ImageInfoSpecificQCow2.
> > > > Two reasons for this: First, it's eventually part of ImageInfo,
> > > > which is defined as "Information about a QEMU image file", but this
> > > > option cannot be set in the image file itself but is only a run-time
> > > > option.
> > > Can we mark the parameter optional, and only provide it when it is
> > > non-zero?  That way, qemu-img (which cannot set an interval) will not
> > > report it, and the only time it will appear is if it was set as part
> > > of opening the block device under qemu.
> > That sounds good.
> But what do we do with the other parameters (refcount-cache-size,
> l2-cache-size)? We cannot have the same solution there because they
> don't belong to the image file either, and they're never going go be
> zero.

Pssht, don't mention it, or Eric will notice. :-)

Well, one solution would be to remember whether they have been set 
explicitly or not. But that gets ugly really quickly. Maybe Kevin's 
series helps there, but I don't know.

Of course, the simplest solution is to worry about cache-clean-interval 
for now and worry about the cache sizes later… But that basically means 
"We'll never worry about them unless someone complains", so…

Max