Re: [Qemu-devel] [PATCH 20/25] block/nbd: Comment on discard/flush silently failing

[Paolo Bonzini]

On 25/02/2015 19:08, Max Reitz wrote:
> If some operation cannot be performed by a block driver, it is normally
> supposed to return an error. In these cases, however, it is fine to
> pretend the operations were carried out successfully because if the NBD
> block driver would not implement discard or flush in the first place,
> this is exactly what the block layer would do.
> 
> Because this may not be obvious, add a comment for it.
> 
> Signed-off-by: Max Reitz <address@hidden>
> ---
>  block/nbd-client.c | 2 ++
>  1 file changed, 2 insertions(+)
> 
> diff --git a/block/nbd-client.c b/block/nbd-client.c
> index be6803d..ab13607 100644
> --- a/block/nbd-client.c
> +++ b/block/nbd-client.c
> @@ -315,6 +315,7 @@ int nbd_client_co_flush(BlockDriverState *bs)
>      ssize_t ret;
>  
>      if (!(client->nbdflags & NBD_FLAG_SEND_FLUSH)) {
> +        /* This mirrors the behavior of bdrv_co_flush() in block.c */
>          return 0;
>      }
>  
> @@ -350,6 +351,7 @@ int nbd_client_co_discard(BlockDriverState *bs, int64_t 
> sector_num,
>      ssize_t ret;
>  
>      if (!(client->nbdflags & NBD_FLAG_SEND_TRIM)) {
> +        /* This mirrors the behavior of bdrv_co_discard() in block.c */
>          return 0;

Should this return -EOPNOTSUPP instead?

Paolo

>      }
>      request.from = sector_num * 512;
>