Re: [PATCH 2/4] dm: implement REQ_FLUSH/FUA support

[Tejun Heo <tj@kernel.org>]

Hello,

On 08/27/2010 10:24 PM, Mike Snitzer wrote:
>> * As __blk_rq_prep_clone() copies REQ_FUA, just advertising FUA
>>   support is enough to pass through REQ_FUA to targets.
> 
> You're doing blk_queue_flush(md->queue, REQ_FLUSH | REQ_FUA); in 2
> places:
> 1) generic dm_init_md_queue -- used for bio-based and request-based
> 2) request-based specific dm_init_request_based_queue.

Well, there are two places creating queues.

> Interestingly, we never used the old blk_queue_ordered() method for
> bio-based DM yet it is now using blk_queue_flush().

Yeap, because now __make_request() filters out REQ_FLUSH bio's.

> But how can we blindly assume/advertise REQ_FUA?
>
> Should we be taking more care to check each block device that DM
> consumes to see if FUA is supported and only then advertise REQ_FUA?
> DM already does this for discard support (see:
> dm_table_supports_discards).

Nope, REQ_FUA will be interpreted by queues lower in the stack.
Drivers in the middle just need to pass them through.

>> Lightly tested linear, stripe, raid1, snap and crypt targets.
> 
> I tested the bio-based code with the LVM2 test suite and all tests
> passed.
> 
>> Please proceed with caution as I'm not familiar with the code base.
> 
> As I shared in an earlier (private) mail, I'm unfortunately having
> problems with request-based DM (when all patches in this series are
> applied).  I'll be working on that more.

Heh... I probably should set up a simple dm-mpath and test it.  I'll
do it this weekend.

> BTW, we can eliminate the dm_rq_is_flush_request() wrapper right?  I
> think hch mentioned this at some point in one of the various threads.

Sure, that's a rather silly wrapper at this point.

Thanks.

-- 
tejun