Re: [PATCH] qemu-img convert: Don't pre-zero images

[Eric Blake <eblake@redhat.com>]

On 6/22/20 10:12 AM, Kevin Wolf wrote:
> Since commit 5a37b60a61c, qemu-img create will pre-zero the target image
> if it isn't already zero-initialised (most importantly, for host block
> devices, but also iscsi etc.), so that writing explicit zeros wouldn't
> be necessary later.
> 
> This could speed up the operation significantly, in particular when the
> source image file was only sparsely populated. However, it also means
> that some block are written twice: Once when pre-zeroing them, and then
> when they are overwritten with actual data. On a full image, the
> pre-zeroing is wasted work because everything will be overwritten.
> 
> In practice, write_zeroes typically turns out faster than writing
> explicit zero buffers, but slow enough that first zeroing everything and
> then overwriting parts can be a significant net loss.
> 
> Meanwhile, qemu-img convert was rewritten in 690c7301600 and zero blocks
> are now written to the target using bdrv_co_pwrite_zeroes() if the
> target could be pre-zeroed. This way we already make use of the faster
> write_zeroes operation, but avoid writing any blocks twice.
> 
> Remove the pre-zeroing because these days this former optimisation has
> actually turned into a pessimisation in the common case.

Indeed, it is both data-dependent (how sparse is the source) and 
target-dependent (does the reduced I/O from a single pre-zeroing pass 
outweigh the time spent on non-zero portions).  I'm fine with dropping 
this optimization, as we do not have proper heuristics to reliably 
detect when it is actually going to be a win.  (It would be nice if we 
_could_ have reliable heuristics, but that's a story for another day)

Reviewed-by: Eric Blake <eblake@redhat.com>

-- 
Eric Blake, Principal Software Engineer
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