Re: [Qemu-devel] [PATCH v5 13/14] nbd: Implement NBD_CMD_WRITE_ZEROES on server

[Eric Blake <eblake@redhat.com>]

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On 07/19/2016 09:34 PM, Fam Zheng wrote:
> On Tue, 07/19 17:45, Paolo Bonzini wrote:
>>
>>
>> On 19/07/2016 17:28, Eric Blake wrote:
>>>> If I'm reading the NBD proto.md correctly, this is not enough if
>>>> NBD_CMD_FLAG_NO_HOLE is specified. We probably need to use a zeroed buffer with
>>>> blk_pwrite, or pass a new flag (BDRV_RED_NO_HOLE) to blk_pwrite_zeroes to
>>>> enforce the bdrv_driver_pwritev() branch in bdrv_co_do_pwrite_zeroes().
>>
>> I agree with Eric's interpretation.  It's a bit weird to have the
>> direction inverted, but I'm not sure I see the ambiguity.  Can you explain?
> 
> Write zeroes _means_ "punch hole" on a raw file.
> 
> In block/raw-posix.c:handle_aiocb_write_zeroes():
>> #ifdef CONFIG_FALLOCATE_PUNCH_HOLE
>>     if (s->has_discard && s->has_fallocate) {
>>         int ret = do_fallocate(s->fd,
>>                                FALLOC_FL_PUNCH_HOLE | FALLOC_FL_KEEP_SIZE,
>>                                aiocb->aio_offset, aiocb->aio_nbytes);
>>         if (ret == 0) {
>>             ret = do_fallocate(s->fd, 0, aiocb->aio_offset, aiocb->aio_nbytes);

That is just implementation: punch a hole, BUT THEN reallocate it back,
so that in the end, the file is still not sparse in that region.  Or am
I reading it wrong?

But the implementation under the hood is not visible to the guest - as
long as the end result is that a guest requesting NO_HOLE ends up with a
non-sparse file, and the data reads back as all 0, the client doesn't
care whether the zeros were written byte-by-byte or sped up by punching
a hole then reallocating.

> 
> And unmap is translated to "punch hole", too.
> 
> In block/raw-posix.c:handle_aiocb_discard():
>> #ifdef CONFIG_FALLOCATE_PUNCH_HOLE
>>         ret = do_fallocate(s->fd, FALLOC_FL_PUNCH_HOLE | FALLOC_FL_KEEP_SIZE,
>>                            aiocb->aio_offset, aiocb->aio_nbytes);
>> #endif

No, this call is different - it punches a hole, then stops.  There is no
followup do_fallocate(,0,,) to reallocate, so the file remains sparse.

> 
> So I agree that NBD_CMD_FLAG_NO_HOLE is a poorly named flag, because there is
> always going to be a hole event if it's set.

If we are punching holes even when BDRV_REQ_MAY_UNMAP is not set, that
seems like we have a bug in qemu (unless we are immediately then
reallocating so that there is no resulting hole).

-- 
Eric Blake   eblake redhat com    +1-919-301-3266
Libvirt virtualization library http://libvirt.org


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