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
--
To unsubscribe from this list: send the line "unsubscribe linux-btrfs" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at http://vger.kernel.org/majordomo-info.html
