Re: [Qemu-devel] [PATCH v3 3/5] qcow2: Introduce an option for sufficient L2 cache for the entire image

[Daniel P . Berrangé]

On Wed, Jul 25, 2018 at 06:23:45PM +0300, Leonid Bloch wrote:
> On 07/25/2018 04:32 PM, Kevin Wolf wrote:
> > >          Another interesting question is whether 'full' shouldn't keep 
> > > meaning
> > >          full throughout the lifetime of the BlockDriverState, i.e. 
> > > should it
> > >          keep adapting to the new size when the image size changes?
> > > 
> > > 
> > >      Do we even resize the cache now for image size changes? If we use an 
> > > enum,
> > >      we could have two different values depending on whether the chosen 
> > > cache
> > >      size remains fixed or also tries to resize when the image grows.
> > 
> > We don't because we only support absolute cache sizes today. 'full'
> > would be the first one that is relative to the image size.
> > 
> > > Is it even possible to change the virtual disk image size online?
> > 
> > Yes, this is what qcow2_co_truncate() does (can be invoked, amongst
> > others, with the QMP command 'block_resize').
> 
> Cool! This does look like a good idea to resize the L2 cache accordingly,
> but maybe this is out of scope for now as well? The purpose of the current
> series is just to provide an option to automatically calculate the needed L2
> cache size for covering the entire image, instead of using an external
> script to do that and feed the output to l2-cache-size.

Personally if I saw the description of the option and then found it didn't
"do the right thing" when resizing I'd consider the option broken. So I
think we should make it deal with resizes right away rather than implementing
a known bug.


Regards,
Daniel
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