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

[Pavel Borzenkov <pborzenkov@virtuozzo.com>]

On Sat, Apr 09, 2016 at 11:57:57AM +0100, Alex Bligh wrote:
> 
> On 8 Apr 2016, at 23:05, Eric Blake <eblake@redhat.com> 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 <eblake@redhat.com>
> > ---
> > 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.

In this case BDRV_REQ_FUA is cleared from 'flags' by
nbd_client_co_write_zeroes() and this condition becomes false.

> 
> 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
> 
> 
> 
> 
>