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Date: Sun, 29 Aug 2010 20:14:41 -0400 From: Josef Bacik <josef@...hat.com> To: Tomasz Chmielewski <mangoo@...g.org> Cc: linux-kernel@...r.kernel.org, linux-btrfs@...r.kernel.org, hch@...radead.org, gg.mariotti@...il.com, "Justin P. Mattock" <justinmattock@...il.com>, mjt@....msk.ru, josef@...hat.com, tytso@....edu Subject: Re: BTRFS: Unbelievably slow with kvm/qemu On Sun, Aug 29, 2010 at 09:34:29PM +0200, Tomasz Chmielewski wrote: > Christoph Hellwig wrote: > >> There are a lot of variables when using qemu. >> >> The most important one are: >> >> - the cache mode on the device. The default is cache=writethrough, >> which is not quite optimal. You generally do want to use cache=none >> which uses O_DIRECT in qemu. >> - if the backing image is sparse or not. >> - if you use barrier - both in the host and the guest. > > I noticed that when btrfs is mounted with default options, when writing > i.e. 10 GB on the KVM guest using qcow2 image, 20 GB are written on the > host (as measured with "iostat -m -p"). > > > With ext4 (or btrfs mounted with nodatacow), 10 GB write on a guest > produces 10 GB write on the host. > Whoa 20gb? That doesn't sound right, COW should just mean we get quite a bit of fragmentation, not write everything twice. What exactly is qemu doing? Thanks, Josef -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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