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I'd like to create a copy-on-write qcow2 image on top of a read-only base image. However, the write performance of the copy-on-write top image is too slow for the intended workload. Is there a way to preallocate a portion of the copy-on-write image at creation time? I don't want to preallocate the whole thing, but a few tens of GBs should ensure we remain scalable without taking a performance hit the majority of the time.

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qcow2 doesn't work like that. – womble Aug 23 '11 at 23:27
@womble: As far as I can tell, I can mount a qcow2 image inside of a VM, create a 20gb trash file, delete it, and the image will remain 20gb. I guess I don't understand the difference in that and what I'm trying to do here - won't the filesystem try to place new files on those wiped sectors before allocating new ones, first filling that spare 20gb before growing the qcow image? – BigChief Aug 24 '11 at 2:37
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