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

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On Thu, 13 Aug 2009, Matthew Wilcox wrote:

So TRIM isn't free, and there's a better way for the drive to find
out that the contents of a block no longer matter -- write some new
data to it.  So if we just swapped a page in, and we're going to swap
something else back out again soon, just write it to the same location
instead of to a fresh location.  You've saved a command, and you've
saved the drive some work, plus you've allowed other users to continue
accessing the drive in the meantime.

on the other hand, if you then end up swapping the page you read in out again and haven't dirtied it, you now have to actually write it as opposed to just throwing it away (knowing that you already have a copy of it stored on the swap device)

David Lang
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