Re: [Qemu-devel] [PATCH 26/27] block/parallels: optimize linear image expansion

[Stefan Hajnoczi]

On Wed, Mar 11, 2015 at 01:28:20PM +0300, Denis V. Lunev wrote:
> Plain image expansion spends a lot of time to update image file size.
> This seriously affects the performance. The following simple test
>   qemu_img create -f parallels -o cluster_size=64k ./1.hds 64G
>   qemu_io -n -c "write -P 0x11 0 1024M" ./1.hds
> could be improved if the format driver will pre-allocate some space
> in the image file with a reasonable chunk.
> 
> This patch preallocates 128 Mb using bdrv_write_zeroes, which should
> normally use fallocate() call inside. Fallback to older truncate()
> could be used as a fallback using image open options thanks to the
> previous patch.
> 
> The benefit is around 15%.

qcow2 doesn't use bdrv_truncate() at all.  It simply writes past the end
of bs->file to grow the file.  Can you use this approach instead?

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