Re: Discard support (was Re: [PATCH] swap: send callback when swapslot is freed)

[Christoph Hellwig]

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.

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