linux-kernel - Re: BTRFS: Unbelievably slow with kvm/qemu

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Date:	Sun, 18 Jul 2010 03:15:08 -0400
From:	Christoph Hellwig <hch@...radead.org>
To:	Ted Ts'o <tytso@....edu>, Christoph Hellwig <hch@...radead.org>,
	Giangiacomo Mariotti <gg.mariotti@...il.com>,
	linux-kernel@...r.kernel.org
Subject: Re: BTRFS: Unbelievably slow with kvm/qemu

On Sat, Jul 17, 2010 at 06:28:06AM -0400, Ted Ts'o wrote:
> Thanks so much for running these benchmarks.  It's been on my todo
> list ever since the original complaint came across on the linux-ext4
> list, but I just haven't had time to do the investigation.  I wonder
> exactly what qemu is doing which is impact btrfs in particularly so
> badly.  I assume that using the qcow2 format with cache=writethrough,
> it's doing lots of effectively file appends whih require allocation
> (or conversion of uninitialized preallocated blocks to initialized
> blocks in the fs metadata) with lots of fsync()'s afterwards.

This is using raw images.  So what we're doing there is hole filling.
No explicit fsyncs are done for cache=writethrough.  cache=writethrough
translates to using O_DSYNC, which makes every write synchronous, which
these days translates to an implicity ->fsync call on every write.

> P.S.  I assume since you listed "sparse" that you were using a raw
> disk and not a qcom2 block device image?

All of these are using raw images.  sparse means just doing a truncate
to the image size, preallocated means using fallocate to pre-allocate
the space.

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