Messages in this thread | | | Date | Sun, 12 Sep 2010 19:55:47 +0800 | From | Tao Ma <> | Subject | Re: [PATCH 02/41] block/loop: queue ordered mode should be DRAIN_FLUSH |
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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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