On Tue, Feb 19 2008, Kyungmin Park wrote: > > > > > > Agree, however see the following sequence. > > > > > > __generic_make_request call q->make_request_fn(q, bio); > > > It was set by blk_init_queue_node with __make_request. > > > There are two ways in __make_request. > > > Case 1, get_rq > > > Case 2, out or merged (otherwise you mean unplug case) > > > > > > In case 1, if the BIO_RW_SYNC is set, the request gets the REQ_RW_SYNC > > > And REQ_RW_SYNC says > > > "include/linux/blkdev.h":112: __REQ_RW_SYNC, /* request is sync (O_DIRECT) */ > > > It means it acts as O_DIRECT flag. Is it right? > > > And it also is same as case 2. Unplug the device. > > > So next time it hasn't chance to merge??? > > > > But that still doesn't make it sync. I think you are working the wrong > > way. For ssd we still want merging and plugging also makes sense to some > > degree, though it probably should be minimized. It'll only cause an > > initial latency, for busy IO workloads you wont be plugging/unplugging > > much anyway. > > > > In fact your patch makes things WORSE, since the io schedulers will now > > treat the IO as sync and introduce idling for the disk head. And you > > definitely don't want that. > > Yes, you're right. It's for testing. > I just want to know the worst or corner case, if all writes are synchronous. > Of course I can measure the using tiotest "Do write synchronous" option. > Then you think it's the worse case? If you want to test when all writes are sync, then either mount with -o sync, open with O_SYNC or use O_DIRECT writes. You can't force that behaviour by changing the block layer code. Perhaps you could force O_SYNC when a file is opened, if you want to experiment with worst case generally. Not sure that makes a lot of sense, though. -- Jens Axboe - 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
