Re: Re: [PATCH 1/2] rbd: RBD_DEV_FLAG_THICK rbd_dev_flags bit

[Ilya Dryomov]

On Thu, Mar 22, 2018 at 12:57 PM, äääå / KAMEIïHITOSHI
<hitoshi.kamei.xm@xxxxxxxxxxx> wrote:
> Hi Yang,
>
>> I am not sure is this the best way for this case, what about adding an option in "rbd map -o thick rbd/test"?
>
> I will add such option to the rbd map command to manipulate image settings. So, the end-user
> do not change the settings directly via sysfs file.
>
>>       @@ -4011,6 +4012,15 @@ static void rbd_queue_workfn(struct work_struct *work)
>>                       goto err;
>>               }
>>
>>       +       /* Ignore/skip discard requests for thick-provision image */
>>
>> Just ignore? or return -EOPNOTSUPP?
>
> Thanks, I think -EOPNOTSUPP is better because user programs cannot know
> the result of requested operation when the kernel rbd driver ignores
> discard request. The result of requested operation when the kernel rbd driver
> ignores discard requests, which probably misleads the user programs.
>
>> In addition, we should not ignore the REQ_OP_WRITE_ZEROES.
>
> Relating to the above, the return code of REQ_OP_WRITE_ZEROS request
> is also -EOPNOTSUPP instead of ignoring. I think the result of
> -EOPNOTSUPP is also better for this request because the kernel
> rbd driver can expect that user programs write zero data by itself.

REQ_OP_WRITE_ZEROES should continue to work, we just need to make
sure it never issues truncates or deletes and instead writes zeroes
explicitly.

I think we should be explicit about the fact that discard is not
supported instead of accepting the discard request and failing it in
rbd_queue_workfn().  Attached patch is what I have in mind.

Thanks,

                Ilya