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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?
Alex
Re: [Qemu-devel] [PATCH 1/2] Add no-op aio emulation stub, Stefan Hajnoczi, 2010/05/11
[Qemu-devel] Re: [PATCH 1/2] Add no-op aio emulation stub, Kevin Wolf, 2010/05/11
Re: [Qemu-devel] [PATCH 0/2] Enable qemu block layer to not flush, Anthony Liguori, 2010/05/10
- Re: [Qemu-devel] [PATCH 0/2] Enable qemu block layer to not flush,
Alexander Graf <=
- Re: [Qemu-devel] [PATCH 0/2] Enable qemu block layer to not flush, Anthony Liguori, 2010/05/10
- Re: [Qemu-devel] [PATCH 0/2] Enable qemu block layer to not flush, Jamie Lokier, 2010/05/11
- Re: [Qemu-devel] [PATCH 0/2] Enable qemu block layer to not flush, Stefan Hajnoczi, 2010/05/12
- Re: [Qemu-devel] [PATCH 0/2] Enable qemu block layer to not flush, Jamie Lokier, 2010/05/12
- Re: [Qemu-devel] [PATCH 0/2] Enable qemu block layer to not flush, Stefan Hajnoczi, 2010/05/12
- Re: [Qemu-devel] [PATCH 0/2] Enable qemu block layer to not flush, Jamie Lokier, 2010/05/12