Re: Discard support (was Re: [PATCH] swap: send callback when swap slot is freed)

Arjan van de Ven
2009-08-16T15:32:21Z

On Sun, 16 Aug 2009 15:05:30 +0100
Alan Cox <alan@lxorguk.ukuu.org.uk> wrote:

> On Sat, 15 Aug 2009 08:55:17 -0500
> James Bottomley <James.Bottomley@suse.de> wrote:
>
> > On Sat, 2009-08-15 at 09:22 -0400, Mark Lord wrote:
> > > James Bottomley wrote:
> > > >
> > > > This means you have to drain the outstanding NCQ commands
> > > > (stalling the device) before you can send a TRIM. If we do
> > > > this for every discard, the performance impact will be pretty
> > > > devastating, hence the need to coalesce. It's nothing really
> > > > to do with device characteristics, it's an ATA protocol problem.
> > > ..
> > >
> > > I don't think that's really much of an issue -- we already have
> > > to do that for cache-flushes whenever barriers are enabled. Yes
> > > it costs, but not too much.
> >
> > That's not really what the enterprise is saying about flush
> > barriers. True, not all the performance problems are NCQ queue
> > drain, but for a steady workload they are significant.
>
> Flush barriers are nightmare for more than enterprise. You drive
> basically goes for a hike for a bit which trashes interactivity as
> well. If the device can't do trim and the like without a drain I
> don't see much point doing it at all, except maybe to wait for idle
> devices and run a filesystem managed background 'strimmer' thread to
> just weed out now idle blocks that have stayed idle - eg by adding an
> inode of all the deleted untrimmed blocks and giving it an irregular
> empty ?
>

trim is mostly for ssd's though, and those tend to not have the "goes
for a hike" behavior as much......

I wonder if it's worse to batch stuff up, because then the trim itself
gets bigger and might take longer.....

--
Arjan van de Ven Intel Open Source Technology Centre
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