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

[Max Reitz <mreitz@redhat.com>]

On 2015-03-11 at 07:31, Paolo Bonzini wrote:
>
> 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 <mreitz@redhat.com>
>> ---
>>   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?

That's what this patch is for. I asked myself the same thing, and it 
turns out, the functions deliberately return 0 because bdrv_co_flush() 
and bdrv_co_discard() do the same if the block driver does not support 
these functions at all, so that's why I'm adding these comments.

Max

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