Re: [Qemu-devel] [Qemu-block] [PATCH] qcow2: Optimize L2 table cache size based on image and cluster sizes

[Alberto Garcia]

On Tue 04 Oct 2016 05:51:26 PM CEST, Max Reitz wrote:

>> At least giving users a way to skip the math would be an improvement.
>> Would you be okay with an explicitly-set option like
>> l2_cache_size=auto or =max that optimizes for performance at the
>> expense of memory?
>
> That (cache-size=max) is actually something Stefan Hajnoczi has
> proposed at KVM Forum 2015.
>
> I agree that implementing the formula in qemu's qcow2 driver itself
> makes sense and will help users; however, I do think it is appropriate
> to expect the user to pass cache-size=max if they need it.

Frank Myhr's suggestion (in bugzilla) is that we allow specifying a % of
the disk size, so

l2-cache-size=100%  (that would be cache-size=max)
l2-cache-size=80%
...

https://bugzilla.redhat.com/show_bug.cgi?id=1377735#c3

Either one looks good to me. They would change however the data type of
'l2-cache-size' from int to string in BlockdevOptionsQcow2, can we
actually do that?

Berto