On Thu, Apr 08, 2010 at 05:58:17PM +0300, 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.
With O_DIRECT the writeback rates are very reasonable. I'll work up a way to pass the barrier down from the guest to btrfs to force logging of updated metadata when required. > > >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. Oh, I didn't see that when I was testing, when does it fsync? -chris -- To unsubscribe from this list: send the line "unsubscribe linux-btrfs" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
