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Re: [Qemu-devel] [PATCH 0/2] Enable qemu block layer to not flush

From: Anthony Liguori
Subject: Re: [Qemu-devel] [PATCH 0/2] Enable qemu block layer to not flush
Date: Mon, 10 May 2010 16:59:39 -0500
User-agent: Mozilla/5.0 (X11; U; Linux x86_64; en-US; rv: Gecko/20091209 Fedora/3.0-4.fc12 Lightning/1.0pre Thunderbird/3.0

On 05/10/2010 04:51 PM, Alexander Graf wrote:
Thanks to recent improvements, qemu flushes guest data to disk when the guest
tells us to do so.

This is great if we care about data consistency on host disk failures. In cases
where we don't it just creates additional overhead for no net win. One such use
case is the building of appliances in SUSE Studio. We write the resulting images
out of the build VM, but compress it directly afterwards. So if possible we'd
love to keep it in RAM.

This patchset introduces a new block parameter to -drive called "flush" which
allows a user to disable flushing in odd scenarios like the above. To show the
difference in performance this makes, I have put together a small test case.
Inside the initrd, I call the following piece of code on a 500MB preallocated
vmdk image:

This seems like it's asking for trouble to me. I'm not sure it's worth the minor performance gain.


Anthony Liguori

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