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From: Peter Lieven <pl@kamp.de>
To: Paolo Bonzini <pbonzini@redhat.com>, Kevin Wolf <kwolf@redhat.com>
Cc: qemu-devel@nongnu.org, stefanha@redhat.com, mreitz@redhat.com
Subject: Re: [Qemu-devel] [RFC PATCH] block: optimize zero writes with bdrv_write_zeroes
Date: Mon, 24 Feb 2014 14:01:38 +0100	[thread overview]
Message-ID: <530B42B2.1040909@kamp.de> (raw)
In-Reply-To: <530B2143.1030808@redhat.com>

On 24.02.2014 11:38, Paolo Bonzini wrote:
> Il 24/02/2014 11:26, Peter Lieven ha scritto:
>>
>> I personally do not need this for QCOW2 but for iSCSI. Here the optimization
>> is basically saved bandwidth since a zero write becomes a WRITESAME.
>
> It saves bandwidth, but at the potential cost of extra host CPU utilization.  I would be fine with having this automatically, but drv->bdrv_co_write_zeroes is not the right check because it is true for qcow2 and raw formats.  Something using 
> bdrv_get_info is probably better, because it would have fewer or no false positives.
>
>> In all cases if unmap=on it would additionally save disk space.
>
> It would also cause worse performance though.  I think the automatic addition BDRV_REQ_MAY_UNMAP is what should be a separate option. Perhaps you can have a three-state option, detect-zeros=no/yes/unmap.

What would be the desired way to store this flag in the BlockDriverState?

Peter

  parent reply	other threads:[~2014-02-24 13:02 UTC|newest]

Thread overview: 18+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2014-02-22 13:00 [Qemu-devel] [RFC PATCH] block: optimize zero writes with bdrv_write_zeroes Peter Lieven
2014-02-22 16:45 ` Fam Zheng
2014-02-23 19:10   ` Peter Lieven
2014-02-24  1:01     ` Fam Zheng
2014-02-24 10:39       ` Paolo Bonzini
2014-02-24 11:33         ` Fam Zheng
2014-02-24 11:51           ` Paolo Bonzini
2014-02-24 12:04             ` Fam Zheng
2014-02-24 12:07             ` Kevin Wolf
2014-02-24 12:10               ` Paolo Bonzini
2014-02-24 12:22                 ` Kevin Wolf
2014-02-24 10:11 ` Kevin Wolf
2014-02-24 10:26   ` Peter Lieven
2014-02-24 10:38     ` Paolo Bonzini
2014-02-24 11:50       ` Peter Lieven
2014-02-24 13:01       ` Peter Lieven [this message]
2014-02-25 13:41         ` Kevin Wolf
2014-02-25 17:03           ` Peter Lieven

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