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


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