Re: [Qemu-block] [Qemu-devel] [PATCH 2/3] qemu-img: Use zero writes after source backing EOF

[Max Reitz]

On 18.04.19 20:59, Eric Blake wrote:
> On 7/13/18 6:14 AM, Max Reitz wrote:
>> Past the end of the source backing file, we memset() buf_old to zero, so
>> it is clearly easy to use blk_pwrite_zeroes() instead of blk_pwrite()
>> then.
>>
>> Signed-off-by: Max Reitz <address@hidden>
>> ---
>>  qemu-img.c | 11 +++++++++--
>>  1 file changed, 9 insertions(+), 2 deletions(-)
>>
> 
>> @@ -3458,8 +3461,12 @@ static int img_rebase(int argc, char **argv)
>>                  if (compare_buffers(buf_old + written, buf_new + written,
>>                                      n - written, &pnum))
>>                  {
>> -                    ret = blk_pwrite(blk, offset + written,
>> -                                     buf_old + written, pnum, 0);
>> +                    if (buf_old_is_zero) {
>> +                        ret = blk_pwrite_zeroes(blk, offset + written, 
>> pnum, 0);
> 
> Should we allow BDRV_REQ_MAY_UNMAP here, either unconditionally, or
> based on a command line knob that told us whether the user is more
> interested in a sparse result (that still reads as zero) vs. a
> fully-allocated result?

I wouldn’t trust that.  We haven’t yet switched to the new backing file
at this point, so I think a driver would be correct in handling such
requests by punching a hole in the file -- which would lead to the new
backing file poking through once we’ve switched to it.

Max

> Reviewed-by: Eric Blake <address@hidden>

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