Re: [Qemu-devel] RFC: raw device support for block device targets

[Christoph Hellwig <hch@lst.de>]

On Tue, Dec 06, 2011 at 04:20:56PM +0000, Alex Bligh wrote:
> 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?

It probably should share the code with raw_truncate.  That is factor
a new raw_truncate_common that is equivalent to raw_truncate but
takes a file descriptor instead of a block driver state and then use
it for both raw_truncate and raw_create.