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SubjectRe: [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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