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