On 05/05/2017 05:06 PM, Max Reitz wrote:
> On 04.05.2017 05:07, Eric Blake wrote:
>> We've already improved discards to operate efficiently on the tail
>> of an unaligned qcow2 image; it's time to make a similar improvement
>> to write zeroes.  The special case is only valid at the tail
>> cluster of a file, where we must recognize that any sectors beyond
>> the image end would implicitly read as zero, and therefore should
>> not penalize our logic for widening a partial cluster into writing
>> the whole cluster as zero.
>>

>>  $QEMU_IMG map --output=json "$TEST_IMG" | _filter_qemu_img_map
>>
>> +echo
>> +echo == unaligned image tail cluster, no allocation needed ==
> 
> [...]
> 
>> +# A preallocated cluster maintains its allocation, whether it stays as
>> +# data due to a partial write:
>> +# Convert 128m... | XX XX => ... | XX 00
>> +_make_test_img $((size + 1024))
>> +$QEMU_IO -c "write -P 1 $((size)) 1024" "$TEST_IMG" | _filter_qemu_io
>> +$QEMU_IO -c "write -z $((size + 512)) 512" "$TEST_IMG.base" | _filter_qemu_io
> 
> s/\.base//, I suppose?

D'oh. Serves me right for renaming which file I was working on.

> 
> (You should read your reference output. "Pattern verification failed" is
> never good. ;-))

Looks like I get to spin a v13 then.

> 
> With that (and the reference output) fixed:
> 
> Reviewed-by: Max Reitz <mreitz@redhat.com>
> 

-- 
Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3266
Virtualization:  qemu.org | libvirt.org