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    SubjectRe: Discard support (was Re: [PATCH] swap: send callback when swap slot is freed)
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    On Sun, 2009-08-16 at 12:59 -0400, Christoph Hellwig wrote:
    > On Sun, Aug 16, 2009 at 10:52:07AM -0500, James Bottomley wrote:
    > > However, the enterprise has been doing UNMAP for a while, so we can draw
    > > inferences from them since the SSD FTL will operate similarly. For
    > > them, UNMAP is the same cost in terms of time regardless of the number
    > > of extents. The reason is that it's moving the blocks from the global
    > > in use list to the global free list. Part of the problem is that this
    > > involves locking and quiescing, so UNMAP ends up being quite expensive
    > > to the array but constant in terms of cost (hence they want as few
    > > unmaps for as many sectors as possible).
    >
    > How are they doing the unmaps? Using something similar to Mark's wiper
    > script and using SG_IO? Because right now we do not actually implement
    > UNMAP support in the kernel. I'd really love to test the XFS batched
    > discard support with a real UNMAP implementation.

    You mean how is the array vendor testing their implementation? Using
    SG_IO ... without any filesystem, I believe.

    The testing was initially done to see if the initial maximal discard
    proposal from LSF09 was a viable approach (which it wasn't given the
    time taken to UNMAP).

    James




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