Re: Poor performance with qemu

[Avi Kivity <avi@redhat.com>]

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.

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