Subject: Re: [PATCH 1/2] fs: Do not dispatch FITRIM through separate
 super_operation
From: James Bottomley <James.Bottomley@suse.de>
To: Christoph Hellwig <hch@infradead.org>
Cc: Matthew Wilcox <matthew@wil.cx>, Josef Bacik <josef@redhat.com>,
        Lukas Czerner <lczerner@redhat.com>, tytso@mit.edu,
        linux-ext4@vger.kernel.org, linux-kernel@vger.kernel.org,
        linux-fsdevel@vger.kernel.org, sandeen@redhat.com
In-Reply-To: <20101118142918.GA18510@infradead.org>
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	 <20101118142918.GA18510@infradead.org>
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Date: Thu, 18 Nov 2010 11:19:10 -0600
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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.

James


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