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    SubjectRe: Discard support (was Re: [PATCH] swap: send callback when swap slot is freed)
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    On Sat, Aug 15, 2009 at 7:20 AM, Mark Lord<liml@rtr.ca> wrote:
    > Chris Worley wrote:
    > ..
    >>
    >> So erase blocks are 512 bytes (if I write 512 bytes, an erase block is
    >> now freed)?  Not true.
    >
    > ..
    >
    > No, erase blocks are typically 512 KILO-bytes, or 1024 sectors.
    > Logical write blocks are only 512 bytes,

    That was my point.

    The OS should not make assumptions of the size.

    > but most drives out there
    > now actually use 4096 bytes as the native internal write size.
    >
    > Lots of issues there.
    >
    > The only existing "in the wild" TRIM-capable SSDs today all incur
    > large overheads from TRIM


    SSD's yes, SSS no.

    > --> they seem to run a garbage-collection
    > and erase cycle for each TRIM command, typically taking 100s of milliseconds
    > regardless of the amount being trimmed.

    The OS should not assume a dumb algorithm on the part of the drive.

    >
    > So it makes send to gather small TRIMs into single larger TRIMs.

    If Linux is only to support slow legacy SAS/SATA.

    >
    > But I think, even better, is to just not bother with the bookkeeping,
    > and instead have the filesystem periodically just issue a TRIM for all
    > free blocks within a block group, cycling through the block groups
    > one by one over time.
    >
    > That's how I'd like it to work on my own machine here.
    > Server/enterprise users very likely want something different.

    Yes. I didn't realize this was a laptop-only fix.

    Thanks,

    Chris
    >
    > Pluggable architecture, anyone?  :)
    >
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