Re: [Qemu-devel] [PATCH v1 01/13] qcow2: alloc space for COW in one chunk

[Kevin Wolf <kwolf@redhat.com>]

Am 19.05.2017 um 11:34 hat Anton Nefedov geschrieben:
> From: "Denis V. Lunev" <den@openvz.org>
> 
> Currently each single write operation can result in 3 write operations
> if guest offsets are not cluster aligned. One write is performed for the
> real payload and two for COW-ed areas. Thus the data possibly lays
> non-contiguously on the host filesystem. This will reduce further
> sequential read performance significantly.
> 
> The patch allocates the space in the file with cluster granularity,
> ensuring
>   1. better host offset locality
>   2. less space allocation operations
>      (which can be expensive on distributed storages)
> 
> Signed-off-by: Denis V. Lunev <den@openvz.org>
> Signed-off-by: Anton Nefedov <anton.nefedov@virtuozzo.com>

I don't think this is the right approach. You end up with two
write operations: One write_zeroes and then one data write. If the
backend actually supports efficient zero writes, write_zeroes won't
necessarily allocate space, but writing data will definitely split
the already existing allocation. If anything, we need a new
bdrv_allocate() or something that would call fallocate() instead of
abusing write_zeroes.

It seems much clearer to me that simply unifying the three write
requests into a single one is an improvement. And it's easy to do, I
even had a patch once to do this. The reason that I didn't send it was
that it seemed to conflict with the data cache approach

>  block/qcow2.c | 32 +++++++++++++++++++++++++++++++-
>  1 file changed, 31 insertions(+), 1 deletion(-)
> 
> diff --git a/block/qcow2.c b/block/qcow2.c
> index a8d61f0..2e6a0ec 100644
> --- a/block/qcow2.c
> +++ b/block/qcow2.c
> @@ -1575,6 +1575,32 @@ fail:
>      return ret;
>  }
>  
> +static void handle_alloc_space(BlockDriverState *bs, QCowL2Meta *l2meta)
> +{
> +    BDRVQcow2State *s = bs->opaque;
> +    BlockDriverState *file = bs->file->bs;
> +    QCowL2Meta *m;
> +    int ret;
> +
> +    for (m = l2meta; m != NULL; m = m->next) {
> +        uint64_t bytes = m->nb_clusters << s->cluster_bits;
> +
> +        if (m->cow_start.nb_bytes == 0 && m->cow_end.nb_bytes == 0) {
> +            continue;
> +        }
> +
> +        /* try to alloc host space in one chunk for better locality */
> +        ret = file->drv->bdrv_co_pwrite_zeroes(file, m->alloc_offset, bytes, 0);

No. This is what you bypass:

* All sanity checks that the block layer does

* bdrv_inc/dec_in_flight(), which is required for drain to work
  correctly. Not doing this will cause crashes.

* tracked_request_begin/end(), mark_request_serialising() and
  wait_serialising_requests(), which are required for serialising
  requests to work correctly

* Ensuring correct request alignment for file. This means that e.g.
  qcow2 with cluster size 512 on a host with a 4k native disk will
  break.

* blkdebug events

* before_write_notifiers. Not calling these will cause corrupted backups
  if someone backups file.

* Dirty bitmap updates

* Updating write_gen, wr_highest_offset and total_sectors

* Ensuring that bl.max_pwrite_zeroes and bl.pwrite_zeroes_alignment are
  respected

And these are just the obvious things. I'm sure I missed some.

> +        if (ret != 0) {
> +            continue;
> +        }
> +
> +        file->total_sectors = MAX(file->total_sectors,
> +                                  (m->alloc_offset + bytes) / BDRV_SECTOR_SIZE);

You only compensate for part of a single item in the list above, by
duplicating code with block/io.c. This is not how to do things.

As I said above, I think you don't really want write_zeroes anyway, but
if you wanted a write_zeroes "but only if it's efficient" (which I'm not
sure is a good thing to want), then a better way might be introducing a
new request flag.

> +    }
> +}
> +
>  static coroutine_fn int qcow2_co_pwritev(BlockDriverState *bs, uint64_t offset,
>                                           uint64_t bytes, QEMUIOVector *qiov,
>                                           int flags)

Kevin