Re: about TRIM/DISCARD support and barriers

[James Bottomley <James.Bottomley@HansenPartnership.com>]

On Mon, 2008-11-24 at 09:03 +0000, David Woodhouse wrote:
> On Mon, 2008-11-24 at 07:52 +0900, James Bottomley wrote:
> > On Sun, 2008-11-23 at 13:39 +0000, David Woodhouse wrote:
> > > > We don't attempt to put non-contiguous ranges into a single TRIM yet.
> > > 
> > > We don't even merge contiguous ranges -- I still need to fix the
> > > elevators to stop writes crossing writes,
> > 
> > I don't think we want to do that ... it's legal if the write isn't a
> > barrier and it will inhibit merging.  That may be just fine for a SSD,
> > but it's not for spinning media since they get better performance out of
> > merged writes.
> 
> No, I just mean writes _to the same sector_. At the moment, we happily
> let those cross each other in the queue.

That's legal ... if you want the ordering to matter, you either wait or
insert a barrier.

> We do notice this situation and preserve the ordering if the two
> requests cover _precisely_ the same range, but _overlapping_ writes may
> happen in any order.
> 
> We should fix that, and it's only for _that_ purpose that I'm saying we
> treat writes and discards as identical. And then we can drop the barrier
> flag on discards.

It's not a bug ... but changing it might be feasible ... as long as it
doesn't affect write performance too much (which I don't think it will),
since it is in the critical path.

> And _then_ we can think about special cases which let us merge
> non-contiguous discards.

I still think that treating discards as a special command from the
outset is the better way forwards.

James