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 -- To unsubscribe from this list: send the line "unsubscribe linux-fsdevel" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
