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    SubjectRe: Discard support (was Re: [PATCH] swap: send callback when swap slot is freed)
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    On Fri, 2009-08-14 at 18:19 -0600, Chris Worley wrote:
    > On Fri, Aug 14, 2009 at 5:45 PM, Matthew Wilcox<matthew@wil.cx> wrote:
    > > On Fri, Aug 14, 2009 at 05:21:32PM -0600, Chris Worley wrote:
    > >> Sooner is better than waiting to coalesce. The longer an LBA is
    > >> inactive, the better for any management scheme. If you wait until
    > >> it's reused, you might as well forgo the advantages of TRIM/UNMAP. If
    > >> a the controller wants to coalesce, let it coalesce.
    > >
    > > I'm sorry, you're wrong. There is a tradeoff point, and it's different
    > > for each drive model. Sending down a steady stream of tiny TRIMs is
    > > going to give terrible performance.
    >
    > Sounds like you might be using junk for a device?
    >
    > For junk, a little coalescing may be warranted... like in the I/O
    > schedular, but no more than 100usecs wait before posting, or then you
    > effect high performing devices too.

    Um, I think you missed the original point in all of this at the
    beginning of the thread: On ATA TRIM commands cannot be tagged. This
    means you have to drain the outstanding NCQ commands (stalling the
    device) before you can send a TRIM. If we do this for every discard,
    the performance impact will be pretty devastating, hence the need to
    coalesce. It's nothing really to do with device characteristics, it's
    an ATA protocol problem.

    James




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