Re: about TRIM/DISCARD support and barriers

[Jens Axboe]

On Mon, Nov 24 2008, James Bottomley wrote:
> On Mon, 2008-11-24 at 19:57 +0100, Jens Axboe wrote:
> > On Mon, Nov 24 2008, David Woodhouse wrote:
> > > On Mon, 2008-11-24 at 13:42 -0500, James Bottomley wrote:
> > > > 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.
> > >  ...
> > > > 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.
> > > 
> > > We could argue about how much sense it makes to let two writes to the
> > > same sector actually happen in reverse order.
> > > 
> > > Especially given the fact that we actually _do_ preserve ordering in
> > > some cases; just not in others. (We preserve ordering only if the start
> > > and end of the duplicate writes are _precisely_ matching; if it's just
> > > overlapping (which may well happen in the presence of merges), then this
> > > check doesn't trigger.
> > > 
> > > But that's just semantics. Yes, changing it should be feasible. I talked
> > > to Jens about that at the kernel summit, and we agreed that it should
> > > probably be done.
> > 
> > The way this currently works is that we sort based on the first sector
> > in the request. So if you have have an overlap condition between rq1 and
> > rq2 and then a write gets merged into rq1, then you may have passing
> > writes. Linux has never guarenteed any write ordering for non-barriers,
> > so we've never attempted to handle it. Direct aliases (matching first
> > sectors) are handled as you mention, but that's more of an algorithmic
> > thing than by design.
> > 
> > My main worry is that this will add considerable overhead to request
> > sorting. For the rbtree based sorting, we'd have to do a rb_next/rb_prev
> > to look at adjacent requests. For CFQ it's even worse, since there's no
> > per-queue big rbtree for sorting.
> 
> Which is why I suggest special casing:  Only invoke the expensive
> overlap checking if one of the requests is a discard.  Otherwise use the
> standard path for writes.

Good point, we can easily track if we have discard requests pending.
That doesn't really make it a lot better for CFQ though, currently it'll
still have to dump all queues if a discard comes in.

-- 
Jens Axboe

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