Re: about TRIM/DISCARD support and barriers

[James Bottomley]

On Mon, 2008-11-24 at 18:52 +0000, 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.
> 
> > > 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.
> 
> They're already treated as a special command and you can special-case
> them wherever you like, so I'm not entirely sure what you're suggesting.

I mean that since it's not a bug, you don't have to do it for every
write, just between a write and a discard, thus special casing the
overlap checking code.

James


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