Re: Poor performance with qemu

[Gordan Bobic <gordan@bobich.net>]

Avi Kivity wrote:
> On 03/30/2010 03:56 PM, Chris Mason wrote:
>> On Sun, Mar 28, 2010 at 05:18:03PM +0200, Diego Calleja wrote:
>>   
>>> Hi, I'm using KVM, and the virtual disk (a 20 GB file using the "raw"
>>> qemu format according to virt-manager and, of course, placed on a btrfs
>>> filesystem, running the latest mainline git) is awfully slow, no matter
>>> what OS is running inside the VM. The PCBSD installer says it's copying
>>> data at a 40-50 KB/s rate. Is someone using KVM and having better 
>>> numbers
>>> than me? How can I help to debug this workload?
>>>      
>> The problem is that qemu uses O_SYNC by default, which makes btrfs do
>> log commits for every write.
>>    
> 
> Problem is, btrfs takes the 50 KB/s guest rate and inflates it to 
> something much larger (megabytes/sec).  Are there plans to reduce the 
> amount of O_SYNC overhead writes?
> 
> I saw this too, but with 2.6.31 or 2.6.32 IIRC.
> 
>> Once the O_DIRECT read patch is in, you can switch to that, or tell qemu
>> to use a writeback cache instead.
>>    
> 
> Even with writeback qemu will issue a lot of fsyncs.
> 

My understanding was that with cache=writeback qemu shouldn't issue any 
fsyncs at all, but I could be wrong.

Gordan