Re: [Qemu-block] [RFC] finegrained disk driver options control

[Denis V. Lunev]

On 03/16/2017 06:29 PM, Alberto Garcia wrote:
> On Thu 16 Mar 2017 04:08:50 PM CET, Daniel P. Berrange wrote:
>>> OK. How could I change L2 cache size for QCOW2 image?
>>>
>>> For 1 Tb disk, fragmented in guest, the performance loss is
>>> around 10 times. 10 TIMES. 1000%. The customer could not
>>> wait until proper fix in the next QEMU release especially
>>> if we are able to provide the kludge specifically for him.
>> We can explicitly allow L2 cache size set in the XML but that
>> is a pretty poor solution to the problem IMHO, as the mgmt
>> application has no apriori knowledge of whether a particular
>> cache size is going to be right for a particular QCow2 image.
>>
>> For a sustainable solution, IMHO this really needs to be fixed
>> in QEMU so it has either a more appropriate default, or if a
>> single default is not possible, have QEMU auto-tune its cache
>> size dynamically to suit the characteristics of the qcow2 image.
> Related bug report (and discussion):
>
> https://bugzilla.redhat.com/show_bug.cgi?id=1377735
>
> Berto
Very useful! Thank you.

I have different approach for the case. Though I do not have numbers, but
with a big data block the price for cache miss is too big. I'll try to come
back with this later on.

Den