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

[James Bottomley <James.Bottomley@suse.de>]

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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