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