linux-kernel - Re: [PATCH 02/41] block/loop: queue ordered mode should be DRAIN

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Date:	Sun, 12 Sep 2010 19:55:47 +0800
From:	Tao Ma <tao.ma@...cle.com>
To:	Tejun Heo <tj@...nel.org>
CC:	linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org
Subject: Re: [PATCH 02/41] block/loop: queue ordered mode should be DRAIN_FLUSH



On 09/12/2010 07:41 PM, Tejun Heo wrote:
> Hello,
>
> On 09/12/2010 10:38 AM, Tao Ma wrote:
>> According to Documentation/block/barrier.txt,
>> QUEUE_ORDERED_DRAIN
>>          Requests are ordered by draining the request queue and cache
>>          flushing isn't needed.
>>
>>          Sequence: drain =>  barrier
>>
>> QUEUE_ORDERED_DRAIN_FLUSH
>>          Requests are ordered by draining the request queue and both
>>          pre-barrier and post-barrier cache flushings are needed.
>>
>>          Sequence: drain =>  preflush =>  barrier =>  postflush
>>
>> And for loop device, it call fsync in barrier request. See do_bio_filebacked in drivers/block/loop.c
>> bool barrier = !!(bio->bi_rw&  REQ_HARDBARRIER);
>>                  struct file *file = lo->lo_backing_file;
>>
>>                  if (barrier) {
>>                          if (unlikely(!file->f_op->fsync)) {
>>                                  ret = -EOPNOTSUPP;
>>                                  goto out;
>>                          }
>>
>>                          ret = vfs_fsync(file, 0);
>>                          if (unlikely(ret)) {
>>                                  ret = -EIO;
>>                                  goto out;
>>                          }
>>                  }
>>
>> So actually the sync is done in barrier and no extra flush is needed?
>> Or Do I misread the rules in barrier.txt?
>
> Hmmm... maybe the doc was a bit confusing.  Any device which has
> writeback cache should have FLUSH in the queue ordered configuration.
> The loop device used vfs_fsync() for cache flushing and didn't support
> ordered sequence.  As such, it should use draining for request
> ordering and suports FLUSH, so the mode to use is DRAIN_FLUSH.
oh, thanks for the explanation.

Regards,
Tao
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