Re: [PATCH] block: fix DISCARD_BARRIER requests

[Jens Axboe <axboe@kernel.dk>]

On 2010-06-17 13:49, Matthew Wilcox wrote:
> On Thu, Jun 17, 2010 at 10:10:18AM +0200, Jens Axboe wrote:
>> On 2010-06-17 09:54, Christoph Hellwig wrote:
>>> Filesystems assume that DISCARD_BARRIER are full barriers, so that they
>>> don't have to track in-progress discard operation when submitting new I/O.
>>> But currently we only treat them as elevator barriers, which don't
>>> actually do the nessecary queue drains.
>>>
>>> Also remove the unlikely around both the DISCARD and BARRIER requests -
>>> the happen far too often for a static mispredict.
>>
>> Thanks, applied. There was a recent problem report on btrfs using
>> discard, could possibly explain it if Chris assumed it was a full
>> barrier.
> 
> If it was on real hardware (ie a SATA disc), it can't be this problem,
> since TRIM isn't NCQ so there was already an implicit queue drain ahead
> and behind the TRIM.  I hear rumours of an NCQ TRIM coming 'RSN', but
> haven't seen any details on it yet.

Good point, it will drain the queue on SATA of course. The problem was
reported on MMC, I doubt the do queuing at all (though I have not
checked).

-- 
Jens Axboe