From mboxrd@z Thu Jan  1 00:00:00 1970
From: Gordan Bobic <gordan@bobich.net>
Subject: Re: Poor performance with qemu
Date: Thu, 08 Apr 2010 16:21:33 +0100
Message-ID: <4BBDF47D.60700@bobich.net>
References: <201003281718.03699.diegocg@gmail.com>	<20100330125623.GB13190@think> <4BBDEF09.70306@redhat.com>
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To: linux-btrfs@vger.kernel.org
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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