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

[Fam Zheng]

On Mon, 07/18 22:08, Eric Blake wrote:
> Upstream NBD protocol recently added the ability to efficiently
> write zeroes without having to send the zeroes over the wire,
> along with a flag to control whether the client wants a hole.
> 
> The generic block code takes care of falling back to the obvious
> write of lots of zeroes if we return -ENOTSUP because the server
> does not have WRITE_ZEROES.
> 
> Ideally, since NBD_CMD_WRITE_ZEROES does not involve any data
> over the wire, we want to support transactions that are much
> larger than the normal 32M limit imposed on NBD_CMD_WRITE.  But
> the server may still have a limit smaller than UINT_MAX, so
> until experimental NBD protocol additions for advertising various
> command sizes is finalized (see [1], [2]), for now we just stick to
> the same limits as normal writes.
> 
> [1] https://github.com/yoe/nbd/blob/extension-info/doc/proto.md
> [2] https://sourceforge.net/p/nbd/mailman/message/35081223/
> 
> Signed-off-by: Eric Blake <address@hidden>
> 
> ---
> v5: enhance commit message
> v4: rebase to byte-based limits
> v3: rebase, tell block layer about our support
> ---
>  block/nbd-client.h |  2 ++
>  block/nbd-client.c | 35 +++++++++++++++++++++++++++++++++++
>  block/nbd.c        |  4 ++++
>  3 files changed, 41 insertions(+)
> 
> diff --git a/block/nbd-client.h b/block/nbd-client.h
> index 044aca4..2cfe377 100644
> --- a/block/nbd-client.h
> +++ b/block/nbd-client.h
> @@ -48,6 +48,8 @@ int nbd_client_co_pdiscard(BlockDriverState *bs, int64_t 
> offset, int count);
>  int nbd_client_co_flush(BlockDriverState *bs);
>  int nbd_client_co_pwritev(BlockDriverState *bs, uint64_t offset,
>                            uint64_t bytes, QEMUIOVector *qiov, int flags);
> +int nbd_client_co_pwrite_zeroes(BlockDriverState *bs, int64_t offset,
> +                                int count, BdrvRequestFlags flags);
>  int nbd_client_co_preadv(BlockDriverState *bs, uint64_t offset,
>                           uint64_t bytes, QEMUIOVector *qiov, int flags);
> 
> diff --git a/block/nbd-client.c b/block/nbd-client.c
> index 7e9c3ec..104ba2f 100644
> --- a/block/nbd-client.c
> +++ b/block/nbd-client.c
> @@ -275,6 +275,41 @@ int nbd_client_co_pwritev(BlockDriverState *bs, uint64_t 
> offset,
>      return -reply.error;
>  }
> 
> +int nbd_client_co_pwrite_zeroes(BlockDriverState *bs, int64_t offset,
> +                                int count, BdrvRequestFlags flags)
> +{
> +    ssize_t ret;
> +    NbdClientSession *client = nbd_get_client_session(bs);
> +    struct nbd_request request = {
> +        .type = NBD_CMD_WRITE_ZEROES,
> +        .from = offset,
> +        .len = count,
> +    };
> +    struct nbd_reply reply;
> +
> +    if (!(client->nbdflags & NBD_FLAG_SEND_WRITE_ZEROES)) {
> +        return -ENOTSUP;
> +    }
> +
> +    if (flags & BDRV_REQ_FUA) {
> +        assert(client->nbdflags & NBD_FLAG_SEND_FUA);
> +        request.flags |= NBD_CMD_FLAG_FUA;
> +    }
> +    if (!(flags & BDRV_REQ_MAY_UNMAP)) {

Correct me if I'm wrong, I don't think we care about BDRV_REQ_MAY_UNMAP here,
the NBD protocol can never issue an unmap request. In other words I think
NO_HOLE and MAY_UNMAP are two different things.

Fam

> +        request.flags |= NBD_CMD_FLAG_NO_HOLE;
> +    }
> +
> +    nbd_coroutine_start(client, &request);
> +    ret = nbd_co_send_request(bs, &request, NULL);
> +    if (ret < 0) {
> +        reply.error = -ret;
> +    } else {
> +        nbd_co_receive_reply(client, &request, &reply, NULL);
> +    }
> +    nbd_coroutine_end(client, &request);
> +    return -reply.error;
> +}
> +
>  int nbd_client_co_flush(BlockDriverState *bs)
>  {
>      NbdClientSession *client = nbd_get_client_session(bs);
> diff --git a/block/nbd.c b/block/nbd.c
> index 8d57220..049d1bd 100644
> --- a/block/nbd.c
> +++ b/block/nbd.c
> @@ -357,6 +357,7 @@ static int nbd_co_flush(BlockDriverState *bs)
>  static void nbd_refresh_limits(BlockDriverState *bs, Error **errp)
>  {
>      bs->bl.max_pdiscard = NBD_MAX_BUFFER_SIZE;
> +    bs->bl.max_pwrite_zeroes = NBD_MAX_BUFFER_SIZE;
>      bs->bl.max_transfer = NBD_MAX_BUFFER_SIZE;
>  }
> 
> @@ -440,6 +441,7 @@ static BlockDriver bdrv_nbd = {
>      .bdrv_file_open             = nbd_open,
>      .bdrv_co_preadv             = nbd_client_co_preadv,
>      .bdrv_co_pwritev            = nbd_client_co_pwritev,
> +    .bdrv_co_pwrite_zeroes      = nbd_client_co_pwrite_zeroes,
>      .bdrv_close                 = nbd_close,
>      .bdrv_co_flush_to_os        = nbd_co_flush,
>      .bdrv_co_pdiscard           = nbd_client_co_pdiscard,
> @@ -458,6 +460,7 @@ static BlockDriver bdrv_nbd_tcp = {
>      .bdrv_file_open             = nbd_open,
>      .bdrv_co_preadv             = nbd_client_co_preadv,
>      .bdrv_co_pwritev            = nbd_client_co_pwritev,
> +    .bdrv_co_pwrite_zeroes      = nbd_client_co_pwrite_zeroes,
>      .bdrv_close                 = nbd_close,
>      .bdrv_co_flush_to_os        = nbd_co_flush,
>      .bdrv_co_pdiscard           = nbd_client_co_pdiscard,
> @@ -476,6 +479,7 @@ static BlockDriver bdrv_nbd_unix = {
>      .bdrv_file_open             = nbd_open,
>      .bdrv_co_preadv             = nbd_client_co_preadv,
>      .bdrv_co_pwritev            = nbd_client_co_pwritev,
> +    .bdrv_co_pwrite_zeroes      = nbd_client_co_pwrite_zeroes,
>      .bdrv_close                 = nbd_close,
>      .bdrv_co_flush_to_os        = nbd_co_flush,
>      .bdrv_co_pdiscard           = nbd_client_co_pdiscard,
> -- 
> 2.5.5
> 
>