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

From: Alexander Graf
Subject: Re: [Qemu-devel] [PATCH 0/2] Enable qemu block layer to not flush
Date: Tue, 11 May 2010 00:03:55 +0200

On 10.05.2010, at 23:59, Anthony Liguori wrote:

> 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.

The gain is little on my netbook where I did the test on. This is part of 
performance regressions from 0.10 to 0.12 where we're talking build times of 2 
minutes going to 30. While writeback was most of the chunk, flushing still at 
least doubled the build times which is unacceptable for us.

I also fail to see where it's asking for trouble. If we don't flush volatile 
data, things are good, no?


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