RE: Question about synchronous write on SSD

[Kyungmin Park]

> >
> > 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?

Kyungmin Park

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