linux-ext4 - Re: [PATCH 1/2] fs: Do not dispatch FITRIM through separate super

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Date:	Thu, 18 Nov 2010 12:55:40 -0500
From:	Chris Mason <chris.mason@...cle.com>
To:	James Bottomley <james.bottomley@...e.de>
Cc:	Christoph Hellwig <hch@...radead.org>,
	Matthew Wilcox <matthew@....cx>,
	Josef Bacik <josef@...hat.com>,
	Lukas Czerner <lczerner@...hat.com>, tytso <tytso@....edu>,
	linux-ext4 <linux-ext4@...r.kernel.org>,
	linux-kernel <linux-kernel@...r.kernel.org>,
	linux-fsdevel <linux-fsdevel@...r.kernel.org>,
	sandeen <sandeen@...hat.com>
Subject: Re: [PATCH 1/2] fs: Do not dispatch FITRIM through separate super_operation

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
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