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Date: Thu, 08 Dec 2011 13:40:57 +0100
From: Kevin Wolf <kwolf@redhat.com>
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Subject: Re: [Qemu-devel] RFC: raw device support for block device targets
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To: Alex Bligh <alex@alex.org.uk>
Cc: qemu-devel@nongnu.org

Am 06.12.2011 17:20, schrieb Alex Bligh:
> qemu-img convert appears to support block devices as input, but not
> as output. That is irritating, as when using qemu-img convert to
> convert qcow to raw on a block partition, an intermediate file has
> to be used, which slows things down and pointlessly uses disk space.
> 
> The problem is that ftruncate() is being called on the output file
> in order ensure it is sufficiently large, and this fails on
> block devices.
> 
> I appreciate there may be other calls that fail depending on the
> input file format, but these will presumably be error checked
> at the time.
> 
> Is it therefore worth skipping the ftruncate() if the block device
> is large enough, and at least attempting to proceed further? Something
> like the following (not-even compile tested) patch?

Creating an image on a block device shouldn't even call raw_create(),
but only hdev_create(), which doesn't try to truncate the device, but
just uses lseek to make sure that it's large enough.

Which qemu version are you using and what's your command line?

Kevin