Re: [Qemu-devel] [PATCH] qcow2: Order concurrent AIO requests on the same unallocated cluster

[Kevin Wolf <kwolf@redhat.com>]

Avi Kivity schrieb:
> On 09/01/2009 01:50 PM, Kevin Wolf wrote:
>>   You
>> are worried about image fragmentation? I think we could do something
>> about it with a cleverer cluster allocation.
>>    
> 
> Not only image fragmentation - the odd requests will require RMW.

How that?

The case you're thinking of is that the first and third request are
already completed and then the second one starts, right? Assuming that
request 2 involves some sectors in the last cluster of 1 and the first
one of 3.

Then request 2 is written in three phases: The first one overwrites the
last sectors of requests 1 (cluster already allocated => no RMW). The
second one writes to unallocated, cluster-aligned space (writing
complete clusters => no RMW). The third one overwrites the first sectors
of request 3 (cluster already allocated => no RMW).

>>> Do you have performance numbers?
>>>      
>> No really detailed numbers. Installation time for RHEL on qcow2/virtio
>> went down from 34 min to 19 min, though.
>>    
> 
> That's very impressive.  cache=none or cache=wt?

This was with cache=none as are most of my ad-hoc tests (with cache=none
being the default for RHEL).

Kevin