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Date: Sun, 10 May 2015 18:26:00 +0200
From: Stefan Weil <sw@weilnetz.de>
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Subject: Re: [Qemu-devel] [PATCH v4] block/vdi: Use bdrv_flush after
	metadata updates
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To: Paolo Bonzini <pbonzini@redhat.com>, phoeagon <phoeagon@gmail.com>, Kevin Wolf <kwolf@redhat.com>, Max Reitz <mreitz@redhat.com>
Cc: qemu-devel@nongnu.org, qemu-block@nongnu.org

Am 10.05.2015 um 18:10 schrieb Paolo Bonzini:
> On 10/05/2015 18:02, Stefan Weil wrote:
>>> Since the default qemu-img convert case isn't slowed down, I
>>> would think that correctness trumps performance.  Nevertheless,
>>> it's a huge difference.
>> I doubt that the convert case isn't slowed down.
> The *default* convert case isn't slowed down because "qemu-img convert"
> defaults to the "unsafe" cache mode.
>
> The *non-default* convert case with flushes was slowed down indeed: 2x
> in total (if you include the final flush done by bdrv_close), and 10x if
> you only consider the sequential write part of convert.
>
> Paolo


For those who might be interested:

The relevant GPL source code from VirtualBox is available here:

https://www.virtualbox.org/browser/vbox/trunk/src/VBox/Storage

If I interpret that code correctly, blocks are normally written 
asynchronously,
but changes of metadata (new block allocation) are written synchronously.

See file VDI.cpp (function vdiBlockAllocUpdate) and VD.cpp 
(vdIOIntWriteMeta).

Stefan