Re: [Qemu-devel] [PATCH 1/4] block: Fix dirty bitmap in bdrv_co_discard

[Paolo Bonzini <pbonzini@redhat.com>]

On 06/05/2015 11:50, Fam Zheng wrote:
>  #   src can_write_zeroes_with_unmap       target can_write_zeroes_with_unmap
> --------------------------------------------------------------------------------
>  1              true                                   true
>  2              true                                   false
>  3              false                                  true
>  4              false                                  false

1 should replicate WRITE SAME, in case the unmap granularity of the
target is different from that of the source.  In that case, a discard on
the target might leave some sectors untouched.  Writing zeroes would
ensure matching data between the source and the target.

2 should _not_ discard: it should write zeroes even at the cost of
making the target fully provisioned.  Perhaps you can optimize it by
looking at bdrv_get_block_status for the target, and checking the answer
for BDRV_ZERO.

3 and 4 can use discard on the target.

So it looks like only the source setting matters.

We need to check the cost of bdrv_co_get_block_status for "raw", too.
If it's too expensive, that can be a problem.

Paolo

> For case 2 & 3 it's probably better to mirror the actual reading of source.
> 
> I'm not sure about 4.

Even in case 1, discard could be "UNMAP" and write zeroes could be
"WRITE SAME".  If the unmap granularity of the target is For unaligned
sectors, UNMAP might leave some sectors aside while WRITE SAME will
write with zeroes.