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    SubjectRe: Discard support (was Re: [PATCH] swap: send callback when swap slot is freed)
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    On Fri, Aug 14, 2009 at 6:30 PM, Greg Freemyer<greg.freemyer@gmail.com> wrote:
    > On Fri, Aug 14, 2009 at 8:19 PM, Chris Worley<worleys@gmail.com> 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.
    >>
    >> Chris
    >
    > Why?
    >
    > AIUI, on every write a high performing device allocates a new erase
    > block from its free lists, writes to it, and puts the now unused erase
    > block on the free list.

    So erase blocks are 512 bytes (if I write 512 bytes, an erase block is
    now freed)? Not true.

    >  That erase block becomes available for reuse
    > some milliseconds later.
    >
    > As long as the SSD has enough free erase blocks to work with I see no
    > disadvantage in delaying a discard by minutes, hours or days in most
    > cases.  The exception is when the filesystem is almost full and the
    > SSD is short of erase blocks to work with.

    That "exception..." is another good reason why.

    >
    > In that case it will want to get as many free erase blocks as it can
    > as fast as it can get them.

    Exactly.

    Chris
    >
    > Greg
    >
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