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Re: [Qemu-devel] [RFC PATCH 06/17] block: use bdrv_{co, aio}_discard for
From: |
Paolo Bonzini |
Subject: |
Re: [Qemu-devel] [RFC PATCH 06/17] block: use bdrv_{co, aio}_discard for write_zeroes operations |
Date: |
Fri, 16 Mar 2012 10:34:02 +0100 |
User-agent: |
Mozilla/5.0 (X11; Linux x86_64; rv:10.0.1) Gecko/20120216 Thunderbird/10.0.1 |
Il 16/03/2012 01:47, Richard Laager ha scritto:
> On Thu, 2012-03-15 at 10:36 +0100, Paolo Bonzini wrote:
>> Changing across guest boots is a minor problem, but changing across
>> migration must be avoided at all costs.
>>
>> BTW, after this discussion I think we can instead report
>> discard_granularity = 512 and discard_zeroes_data=0 and get most of the
>> benefit, at least on file-backed storage.
>
> Are you going to report that to guests all the time, or only when the
> host supports discard? If you don't report it all the time, you could
> still be "changing across migration". If you do report it all the time,
> then you're incurring a performance penalty on systems that don't
> support discard, as the guest will be sending discard requests that QEMU
> has to throw away (but by which time some work has been wasted).
I don't think that should be that bad. Discard requests should be
relatively rare.
> And either way, what you're proposing means that users with
> discard_zeros_data = 1 hosts can't get the (albeit small) benefits of
> that because some other QEMU user might want to do a migration across
> heterogeneous storage.
Yes, discard_zeroes_data can be made configurable on top, and either
rejected or emulated if the storage does not support it.
> Finally, I see your proposal of advertising fixed discard support
It does not really have to be fixed, it's just a default.
Paolo
- Re: [Qemu-devel] [RFC PATCH 06/17] block: use bdrv_{co, aio}_discard for write_zeroes operations, (continued)
- Re: [Qemu-devel] [RFC PATCH 06/17] block: use bdrv_{co, aio}_discard for write_zeroes operations, Kevin Wolf, 2012/03/14
- Re: [Qemu-devel] [RFC PATCH 06/17] block: use bdrv_{co, aio}_discard for write_zeroes operations, Christoph Hellwig, 2012/03/24
- Re: [Qemu-devel] [RFC PATCH 06/17] block: use bdrv_{co, aio}_discard for write_zeroes operations, Christoph Hellwig, 2012/03/24
- Re: [Qemu-devel] [RFC PATCH 06/17] block: use bdrv_{co, aio}_discard for write_zeroes operations, Richard Laager, 2012/03/26
- Re: [Qemu-devel] [RFC PATCH 06/17] block: use bdrv_{co, aio}_discard for write_zeroes operations, Christoph Hellwig, 2012/03/24
- Re: [Qemu-devel] [RFC PATCH 06/17] block: use bdrv_{co, aio}_discard for write_zeroes operations, Daniel P. Berrange, 2012/03/26
- Re: [Qemu-devel] [RFC PATCH 06/17] block: use bdrv_{co, aio}_discard for write_zeroes operations, Christoph Hellwig, 2012/03/26
- Re: [Qemu-devel] [RFC PATCH 06/17] block: use bdrv_{co, aio}_discard for write_zeroes operations, Richard Laager, 2012/03/14
- Re: [Qemu-devel] [RFC PATCH 06/17] block: use bdrv_{co, aio}_discard for write_zeroes operations, Paolo Bonzini, 2012/03/15
- Re: [Qemu-devel] [RFC PATCH 06/17] block: use bdrv_{co, aio}_discard for write_zeroes operations, Richard Laager, 2012/03/15
- Re: [Qemu-devel] [RFC PATCH 06/17] block: use bdrv_{co, aio}_discard for write_zeroes operations,
Paolo Bonzini <=
- Re: [Qemu-devel] [RFC PATCH 06/17] block: use bdrv_{co, aio}_discard for write_zeroes operations, Christoph Hellwig, 2012/03/24
- Re: [Qemu-devel] [RFC PATCH 06/17] block: use bdrv_{co, aio}_discard for write_zeroes operations, Richard Laager, 2012/03/26
- Re: [Qemu-devel] [RFC PATCH 06/17] block: use bdrv_{co, aio}_discard for write_zeroes operations, Christoph Hellwig, 2012/03/27
[Qemu-devel] [RFC PATCH 04/17] qed: implement bdrv_aio_discard, Paolo Bonzini, 2012/03/08
[Qemu-devel] [RFC PATCH 10/17] ide/scsi: add discard_zeroes_data property, Paolo Bonzini, 2012/03/08
[Qemu-devel] [RFC PATCH 09/17] ide: issue discard asynchronously but serialize the pieces, Paolo Bonzini, 2012/03/08