Re: [Qemu-devel] [PATCH 12/12] nbd: split requests

[Nicholas Thomas <nick@bytemark.co.uk>]

On 08/09/11 16:25, Paolo Bonzini wrote:
> qemu-nbd has a limit of slightly less than 1M per request.  Work
> around this in the nbd block driver.
> 
> Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
> ---
>  block/nbd.c |   52 ++++++++++++++++++++++++++++++++++++++++++++++------
>  1 files changed, 46 insertions(+), 6 deletions(-)
> 
> diff --git a/block/nbd.c b/block/nbd.c
> index 5a75263..468a517 100644
> --- a/block/nbd.c
> +++ b/block/nbd.c
> @@ -213,8 +213,9 @@ static int nbd_open(BlockDriverState *bs, const char* filename, int flags)
>      return result;
>  }
>  
> -static int nbd_co_readv(BlockDriverState *bs, int64_t sector_num,
> -                        int nb_sectors, QEMUIOVector *qiov)
> +static int nbd_co_readv_1(BlockDriverState *bs, int64_t sector_num,
> +                          int nb_sectors, QEMUIOVector *qiov,
> +                          int offset)
>  {
>      BDRVNBDState *s = bs->opaque;
>      struct nbd_request request;
> @@ -241,7 +242,7 @@ static int nbd_co_readv(BlockDriverState *bs, int64_t sector_num,
>          reply.error = EIO;
>          goto done;
>      }
> -    if (qemu_co_recvv(s->sock, qiov->iov, request.len, 0) != request.len) {
> +    if (qemu_co_recvv(s->sock, qiov->iov, request.len, offset) != request.len) {
>          reply.error = EIO;
>      }
>  
> @@ -251,8 +252,9 @@ done:
>  
>  }
>  
> -static int nbd_co_writev(BlockDriverState *bs, int64_t sector_num,
> -                         int nb_sectors, QEMUIOVector *qiov)
> +static int nbd_co_writev_1(BlockDriverState *bs, int64_t sector_num,
> +                           int nb_sectors, QEMUIOVector *qiov,
> +                           int offset)
>  {
>      BDRVNBDState *s = bs->opaque;
>      struct nbd_request request;
> @@ -273,7 +275,7 @@ static int nbd_co_writev(BlockDriverState *bs, int64_t sector_num,
>          reply.error = errno;
>          goto done;
>      }
> -    ret = qemu_co_sendv(s->sock, qiov->iov, request.len, 0);
> +    ret = qemu_co_sendv(s->sock, qiov->iov, request.len, offset);
>      if (ret != request.len) {
>          reply.error = EIO;
>          goto done;
> @@ -291,6 +293,44 @@ done:
>      return -reply.error;
>  }
>  
> +/* qemu-nbd has a limit of slightly less than 1M per request.  For safety,
> + * transfer at most 512K per request. */
> +#define NBD_MAX_SECTORS 1024

As far as I'm aware, the limit of 1MiB - header size is common to all
NBD servers. I'm not aware of anything at all that'll fail on a 768K
request but succeed in the exact same circumstances on a 512K request.
Again, this is a performance consideration - each request is relatively
slow, so you don't want them to be unnecessarily small.


> +
> +static int nbd_co_readv(BlockDriverState *bs, int64_t sector_num,
> +                        int nb_sectors, QEMUIOVector *qiov)
> +{
> +    int offset = 0;
> +    int ret;
> +    while (nb_sectors > NBD_MAX_SECTORS) {
> +        ret = nbd_co_readv_1(bs, sector_num, NBD_MAX_SECTORS, qiov, offset);
> +        if (ret < 0) {
> +            return ret;
> +        }
> +        offset += NBD_MAX_SECTORS * 512;
> +        sector_num += NBD_MAX_SECTORS;
> +        nb_sectors -= NBD_MAX_SECTORS;
> +    }
> +    return nbd_co_readv_1(bs, sector_num, nb_sectors, qiov, offset);
> +}
> +
> +static int nbd_co_writev(BlockDriverState *bs, int64_t sector_num,
> +                         int nb_sectors, QEMUIOVector *qiov)
> +{
> +    int offset = 0;
> +    int ret;
> +    while (nb_sectors > NBD_MAX_SECTORS) {
> +        ret = nbd_co_writev_1(bs, sector_num, NBD_MAX_SECTORS, qiov, offset);
> +        if (ret < 0) {
> +            return ret;
> +        }
> +        offset += NBD_MAX_SECTORS * 512;
> +        sector_num += NBD_MAX_SECTORS;
> +        nb_sectors -= NBD_MAX_SECTORS;
> +    }
> +    return nbd_co_writev_1(bs, sector_num, nb_sectors, qiov, offset);
> +}
> +
>  static int nbd_co_flush(BlockDriverState *bs)
>  {
>      BDRVNBDState *s = bs->opaque;