Re: [PATCH 1/2] fs: Do not dispatch FITRIM through separate super_operation

[Ric Wheeler]

On 11/18/2010 12:55 PM, Chris Mason wrote:
> Excerpts from James Bottomley's message of 2010-11-18 12:19:10 -0500:
> > On Thu, 2010-11-18 at 09:29 -0500, Christoph Hellwig wrote:
> > > On Thu, Nov 18, 2010 at 07:19:58AM -0700, Matthew Wilcox wrote:
> > > > I guess I was assuming that, on receiving a FALLOC_FL_PUNCH_HOLE, a
> > > > filesystem that was TRIM-aware would pass that information down to the
> > > > block device that it's mounted on.  I strongly feel that we shouldn't
> > > > have two interfaces to do essentially the same thing.
> > > >
> > > > I guess I'm saying that you're going to have to learn about TRIM :-)
> > > Did you actually look Lukas FITRIM code (not the slight reordering here,
> > > but the original one).  It's the ext4 version of the batched discard
> > > model, that is a userspace ioctl to discard free space in the
> > > filesystem.
> > >
> > > hole punching will free the blocks into the free space pool.  If you do
> > > online discard it will also get discarded, but a filesystem that has
> > > online discard enabled doesn't need FITRIM.
> > Not stepping into the debate: I'm happy to see punch go to the mapping
> > data and FITRIM pick it up later.
> >
> > However, I think it's time to question whether we actually still want to
> > allow online discard at all.  Most of the benchmarks show it to be a net
> > lose to almost everything (either SSD or Thinly Provisioned arrays), so
> > it's become an "enable this to degrade performance" option with no
> > upside.
> I think we want to keep it.  In general we've (except for hch) spent
> almost zero time actually tuning online discard, and the benchmarking
> needs to be redone with the shiny new barrier code.
>
> -chris

Very belated response - I agree that we should keep the online discard support 
in (but off by default).

Some of the devices we have tested perform well with it and I expect that 
hardware vendors will get better now that we have the support for them to test with.

Ric


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