Re: [libvirt] [Qemu-block] [RFC] finegrained disk driver options control
On Sat 18 Mar 2017 10:54:16 AM CET, Daniel P. Berrange wrote:
> If you don't care about that at all, the optimal
configuration in
> terms of performance is to give qemu a cache large enough that the
> metadata of the whole image fits in it. When setting
> cache-clean-interval, this could actually be reasonable even for
> large images because the memory wouldn't be used forever but just as
> long as the guest is submitting the problematic I/O patterns - but it
> still means that temporarily qemu could really use all of this
> memory.
Is there some easy algorithm that libvirt can use to determine the
size of the L2 tables, and thus report what the maximum useful cache
size would be ?
Yes, the disk size that can be covered with a certain L2 cache is:
disk_size = l2_cache_size * cluster_size / 8
(all sizes in bytes)
Note that increasing the L2 cache size also increases the refcount
cache, which you might want to keep small.
It's all explained in detail here:
https://github.com/qemu/qemu/blob/master/docs/qcow2-cache.txt
https://blogs.igalia.com/berto/2015/12/17/improving-disk-io-performance-i...
Berto