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

[Alan Cox]

On Sat, 15 Aug 2009 08:55:17 -0500
James Bottomley <James.Bottomley@xxxxxxx> 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 ?

Alan
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