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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 8:38 PM, Chris Worley<worleys@gmail.com> wrote:
    > 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.

    Seriously, how do you know? Are you under NDA?

    The write paper I read about typical SSD design described a partial
    erase block write as:

    Internal logic/micro-controller performs:

    Read erase block, modify erase block, allocate new erase block, write
    new erase block, free now unused old erase block, old erase block
    added to a hardware erase queue the performs the actual erase in the
    background at the relatively slow speed of multiple milliseconds..

    The purpose of the trim/discard command being to allow the ssd to have
    enough free erase blocks ready to go that the writes don't have to
    stall while they wait for a erase block to pop out of the erase queue.

    Greg
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