Re: [Qemu-devel] [RFC PATCH 18/18] nbd: Implement NBD_CMD_WRITE_ZEROES on client

[Alex Bligh]

On 8 Apr 2016, at 23:05, Eric Blake <address@hidden> wrote:

> RFC because there is still discussion on the NBD list about
> adding an NBD_OPT_ to let the client suggest server defaults
> related to scanning for zeroes during NBD_CMD_WRITE, which may
> tweak this patch.
> 
> 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 lots of zeroes if we return -ENOTSUP because the server
> does not have WRITE_ZEROES.
> 
> Signed-off-by: Eric Blake <address@hidden>
> ---
> block/nbd-client.h |  2 ++
> block/nbd-client.c | 34 ++++++++++++++++++++++++++++++++++
> block/nbd.c        | 23 +++++++++++++++++++++++
> 3 files changed, 59 insertions(+)
> 
> diff --git a/block/nbd-client.h b/block/nbd-client.h
> index bc7aec0..2fe6654 100644
> --- a/block/nbd-client.h
> +++ b/block/nbd-client.h
> @@ -47,6 +47,8 @@ void nbd_client_close(BlockDriverState *bs);
> int nbd_client_co_discard(BlockDriverState *bs, int64_t sector_num,
>                           int nb_sectors);
> int nbd_client_co_flush(BlockDriverState *bs);
> +int nbd_client_co_write_zeroes(BlockDriverState *bs, int64_t sector_num,
> +                               int nb_sectors, int *flags);
> int nbd_client_co_writev(BlockDriverState *bs, int64_t sector_num,
>                          int nb_sectors, QEMUIOVector *qiov, int *flags);
> int nbd_client_co_readv(BlockDriverState *bs, int64_t sector_num,
> diff --git a/block/nbd-client.c b/block/nbd-client.c
> index f013084..4be83a8 100644
> --- a/block/nbd-client.c
> +++ b/block/nbd-client.c
> @@ -291,6 +291,40 @@ int nbd_client_co_readv(BlockDriverState *bs, int64_t 
> sector_num,
>     return nbd_co_readv_1(bs, sector_num, nb_sectors, qiov, offset);
> }
> 
> +int nbd_client_co_write_zeroes(BlockDriverState *bs, int64_t sector_num,
> +                               int nb_sectors, int *flags)
> +{
> +    ssize_t ret;
> +    NbdClientSession *client = nbd_get_client_session(bs);
> +    struct nbd_request request = { .type = NBD_CMD_WRITE_ZEROES };
> +    struct nbd_reply reply;
> +
> +    if (!(client->nbdflags & NBD_FLAG_SEND_WRITE_ZEROES)) {
> +        return -ENOTSUP;
> +    }
> +
> +    if ((*flags & BDRV_REQ_FUA) && (client->nbdflags & NBD_FLAG_SEND_FUA)) {
> +        *flags &= ~BDRV_REQ_FUA;
> +        request.flags |= NBD_CMD_FLAG_FUA;
> +    }
> +    if (!(*flags & BDRV_REQ_MAY_UNMAP)) {
> +        request.flags |= NBD_CMD_FLAG_NO_HOLE;
> +    }
> +
> +    request.from = sector_num * 512;
> +    request.len = nb_sectors * 512;
> +
> +    nbd_coroutine_start(client, &request);
> +    ret = nbd_co_send_request(bs, &request, NULL, 0);
> +    if (ret < 0) {
> +        reply.error = -ret;
> +    } else {
> +        nbd_co_receive_reply(client, &request, &reply, NULL, 0);
> +    }
> +    nbd_coroutine_end(client, &request);
> +    return -reply.error;
> +}
> +
> int nbd_client_co_writev(BlockDriverState *bs, int64_t sector_num,
>                          int nb_sectors, QEMUIOVector *qiov, int *flags)
> {
> diff --git a/block/nbd.c b/block/nbd.c
> index f7ea3b3..f5119c0 100644
> --- a/block/nbd.c
> +++ b/block/nbd.c
> @@ -355,6 +355,26 @@ static int nbd_co_readv(BlockDriverState *bs, int64_t 
> sector_num,
>     return nbd_client_co_readv(bs, sector_num, nb_sectors, qiov);
> }
> 
> +static int nbd_co_write_zeroes(BlockDriverState *bs, int64_t sector_num,
> +                               int nb_sectors, BdrvRequestFlags orig_flags)
> +{
> +    int flags = orig_flags;
> +    int ret;
> +
> +    ret = nbd_client_co_write_zeroes(bs, sector_num, nb_sectors, &flags);
> +    if (ret < 0) {
> +        return ret;
> +    }
> +
> +    /* The flag wasn't sent to the server, so we need to emulate it with an
> +     * explicit flush */

Surely you only need to do this is the flag wasn't sent to the server,
i.e. if !(client->nbdflags & NBD_FLAG_SEND_FUA)

If you've sent a FUA request, no need to flush the whole thing.

nbd_co_writev_flags seems to have the same issue, which is where I guess
you got that from.

> +    if (flags & BDRV_REQ_FUA) {
> +        ret = nbd_client_co_flush(bs);
> +    }
> +
> +    return ret;
> +}
> +
> static int nbd_co_writev_flags(BlockDriverState *bs, int64_t sector_num,
>                                int nb_sectors, QEMUIOVector *qiov, int flags)
> {
> @@ -476,6 +496,7 @@ static BlockDriver bdrv_nbd = {
>     .bdrv_parse_filename        = nbd_parse_filename,
>     .bdrv_file_open             = nbd_open,
>     .bdrv_co_readv              = nbd_co_readv,
> +    .bdrv_co_write_zeroes       = nbd_co_write_zeroes,
>     .bdrv_co_writev             = nbd_co_writev,
>     .bdrv_co_writev_flags       = nbd_co_writev_flags,
>     .supported_write_flags      = BDRV_REQ_FUA,
> @@ -496,6 +517,7 @@ static BlockDriver bdrv_nbd_tcp = {
>     .bdrv_parse_filename        = nbd_parse_filename,
>     .bdrv_file_open             = nbd_open,
>     .bdrv_co_readv              = nbd_co_readv,
> +    .bdrv_co_write_zeroes       = nbd_co_write_zeroes,
>     .bdrv_co_writev             = nbd_co_writev,
>     .bdrv_co_writev_flags       = nbd_co_writev_flags,
>     .supported_write_flags      = BDRV_REQ_FUA,
> @@ -516,6 +538,7 @@ static BlockDriver bdrv_nbd_unix = {
>     .bdrv_parse_filename        = nbd_parse_filename,
>     .bdrv_file_open             = nbd_open,
>     .bdrv_co_readv              = nbd_co_readv,
> +    .bdrv_co_write_zeroes       = nbd_co_write_zeroes,
>     .bdrv_co_writev             = nbd_co_writev,
>     .bdrv_co_writev_flags       = nbd_co_writev_flags,
>     .supported_write_flags      = BDRV_REQ_FUA,
> -- 
> 2.5.5
> 
> 

-- 
Alex Bligh