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From: "Daniel P. Berrange" <berrange@redhat.com>
To: Christoph Hellwig <hch@lst.de>
Cc: Kevin Wolf <kwolf@redhat.com>,
	Paolo Bonzini <pbonzini@redhat.com>,
	Richard Laager <rlaager@wiktel.com>,
	qemu-devel@nongnu.org
Subject: Re: [Qemu-devel] [RFC PATCH 06/17] block: use bdrv_{co, aio}_discard for write_zeroes operations
Date: Mon, 26 Mar 2012 10:44:07 +0100	[thread overview]
Message-ID: <20120326094407.GB2501@redhat.com> (raw)
In-Reply-To: <20120324152926.GB13014@lst.de>

On Sat, Mar 24, 2012 at 04:29:26PM +0100, Christoph Hellwig wrote:
> On Wed, Mar 14, 2012 at 01:01:35PM +0100, Kevin Wolf wrote:
> > Paolo mentioned a use case as a fast way for guests to write zeros, but
> > is it really faster than a normal write when we have to emulate it by a
> > bdrv_write with a temporary buffer of zeros? On the other hand we have
> > the cases where discard really means "I don't care about the data any
> > more" and emulating it by writing zeros is just a waste of resources there.
> 
> On raw a real discard also will destroy the preallocation, which might
> absolutely kill your performance on filesystems.

This suggests that there be a new command line param to '-drive' to turn
discard support on/off, since QEMU can't reliably know if the raw file
it is given is intended to be fully pre-allocated by the mgmt app.

Regards,
Daniel
-- 
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  reply	other threads:[~2012-03-26  9:44 UTC|newest]

Thread overview: 68+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2012-03-08 17:15 [Qemu-devel] [RFC PATCH 00/17] Improvements around discard and write zeroes Paolo Bonzini
2012-03-08 17:15 ` [Qemu-devel] [RFC PATCH 01/17] qemu-iotests: add a simple test for write_zeroes Paolo Bonzini
2012-03-08 17:15 ` [Qemu-devel] [RFC PATCH 02/17] qed: make write-zeroes bounce buffer smaller than a single cluster Paolo Bonzini
2012-03-08 17:15 ` [Qemu-devel] [RFC PATCH 03/17] block: add discard properties to BlockDriverInfo Paolo Bonzini
2012-03-09 16:47   ` Kevin Wolf
2012-03-08 17:15 ` [Qemu-devel] [RFC PATCH 04/17] qed: implement bdrv_aio_discard Paolo Bonzini
2012-03-09 16:31   ` Kevin Wolf
2012-03-09 17:53     ` Paolo Bonzini
2012-03-08 17:15 ` [Qemu-devel] [RFC PATCH 05/17] block: pass around qiov for write_zeroes operation Paolo Bonzini
2012-03-08 17:15 ` [Qemu-devel] [RFC PATCH 06/17] block: use bdrv_{co, aio}_discard for write_zeroes operations Paolo Bonzini
2012-03-09 16:37   ` Kevin Wolf
2012-03-09 18:06     ` Paolo Bonzini
2012-03-10 18:02       ` Richard Laager
2012-03-12 12:27         ` Paolo Bonzini
2012-03-12 13:04           ` Kevin Wolf
2012-03-13 19:13           ` Richard Laager
2012-03-14  7:41             ` Paolo Bonzini
2012-03-14 12:01               ` Kevin Wolf
2012-03-14 12:14                 ` Paolo Bonzini
2012-03-14 12:37                   ` Kevin Wolf
2012-03-14 12:49                     ` Paolo Bonzini
2012-03-14 13:04                       ` Kevin Wolf
2012-03-24 15:33                       ` Christoph Hellwig
2012-03-24 15:30                   ` Christoph Hellwig
2012-03-26 19:40                     ` Richard Laager
2012-03-27 10:20                       ` Kevin Wolf
2012-03-24 15:29                 ` Christoph Hellwig
2012-03-26  9:44                   ` Daniel P. Berrange [this message]
2012-03-26  9:56                     ` Christoph Hellwig
2012-03-15  0:42               ` Richard Laager
2012-03-15  9:36                 ` Paolo Bonzini
2012-03-16  0:47                   ` Richard Laager
2012-03-16  9:34                     ` Paolo Bonzini
2012-03-24 15:27         ` Christoph Hellwig
2012-03-26 19:40           ` Richard Laager
2012-03-27  9:08             ` Christoph Hellwig
2012-03-08 17:15 ` [Qemu-devel] [RFC PATCH 07/17] block: make high level discard operation always zero Paolo Bonzini
2012-03-08 17:55   ` Avi Kivity
2012-03-09 16:42     ` Kevin Wolf
2012-03-12 10:42       ` Avi Kivity
2012-03-12 11:04         ` Kevin Wolf
2012-03-12 12:03           ` Avi Kivity
2012-03-08 17:15 ` [Qemu-devel] [RFC PATCH 08/17] block: kill the write zeroes operation Paolo Bonzini
2012-03-08 17:15 ` [Qemu-devel] [RFC PATCH 09/17] ide: issue discard asynchronously but serialize the pieces Paolo Bonzini
2012-03-08 17:15 ` [Qemu-devel] [RFC PATCH 10/17] ide/scsi: add discard_zeroes_data property Paolo Bonzini
2012-03-08 18:13   ` Avi Kivity
2012-03-08 18:14     ` Avi Kivity
2012-03-08 17:15 ` [Qemu-devel] [RFC PATCH 11/17] ide/scsi: prepare for flipping the discard defaults Paolo Bonzini
2012-03-08 17:15 ` [Qemu-devel] [RFC PATCH 12/17] ide/scsi: turn on discard Paolo Bonzini
2012-03-08 18:17   ` Avi Kivity
2012-03-08 17:15 ` [Qemu-devel] [RFC PATCH 13/17] block: fallback from discard to writes Paolo Bonzini
2012-03-24 15:35   ` Christoph Hellwig
2012-03-08 17:15 ` [Qemu-devel] [RFC PATCH 14/17] block: support FALLOC_FL_PUNCH_HOLE trimming Paolo Bonzini
2012-03-09  8:20   ` Chris Wedgwood
2012-03-09  8:31     ` Paolo Bonzini
2012-03-09  8:35       ` Chris Wedgwood
2012-03-09  8:40         ` Paolo Bonzini
2012-03-09 10:31   ` Stefan Hajnoczi
2012-03-09 10:43     ` Paolo Bonzini
2012-03-09 10:53       ` Stefan Hajnoczi
2012-03-09 10:57         ` Paolo Bonzini
2012-03-09 20:36   ` Richard Laager
2012-03-12  9:34     ` Paolo Bonzini
2012-03-24 15:40     ` Christoph Hellwig
2012-03-08 17:15 ` [Qemu-devel] [RFC PATCH 15/17] raw: add get_info Paolo Bonzini
2012-03-08 17:15 ` [Qemu-devel] [RFC PATCH 16/17] qemu-io: fix the alloc command Paolo Bonzini
2012-03-08 17:15 ` [Qemu-devel] [RFC PATCH 17/17] raw: implement is_allocated Paolo Bonzini
2012-03-24 15:42   ` Christoph Hellwig

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