Re: Swap on flash SSDs

[Ingo Molnar <mingo@elte.hu>]

* Dave Hansen <dave@linux.vnet.ibm.com> wrote:

> On Fri, 2009-12-18 at 11:17 -0800, Mike Travis wrote:
>
> > Interesting discussion about SSD's.  I was under the impression that with 
> > the finite number of write cycles to an SSD, that unnecessary writes were 
> > to be avoided?
> 
> I'm no expert, but my impression was that this was a problem with other 
> devices and with "bare" flash, and mostly when writing to the same place 
> over and over.
> 
> Modern, well-made flash SSDs and other flash devices have wear-leveling 
> built in so that they wear all of the flash cells evenly.  There's still a 
> discrete number of writes that they can handle over their life, but it 
> should be high enough that you don't notice.
> 
> http://en.wikipedia.org/wiki/Solid-state_drive

A quality SDD is supposed to wear off in continuous non-stop write traffic 
after its Mean Time Between Failures. (Obviously it will take a few years for 
drives to gather that kind of true physical track record - right now what we 
have is the claims of manufacturers and 1-2 years of a track record.)

And even when a cell does go bad and all the spares are gone, the failure mode 
is not catastrophic like with a hard disk, but that particular cell goes 
read-only and you can still recover the info and use the remaining cells.

Sidenote: i think we should make the Linux swap code resilient against write 
IO errors of that fashion and reallocate the swap entry to a free slot. Right 
now in mm/page_io.c's end_swap_bio_write() we do this:

                /*
                 * We failed to write the page out to swap-space.
                 * Re-dirty the page in order to avoid it being reclaimed.
                 * Also print a dire warning that things will go BAD (tm)
                 * very quickly.
                 *
                 * Also clear PG_reclaim to avoid rotate_reclaimable_page()
                 */
                set_page_dirty(page);
                printk(KERN_ALERT "Write-error on swap-device (%u:%u:%Lu)\n",
                                imajor(bio->bi_bdev->bd_inode),
                                iminor(bio->bi_bdev->bd_inode),
                                (unsigned long long)bio->bi_sector);
                ClearPageReclaim(page);

We could be more intelligent than printing a scary error: we could clear that 
page from the swap map [permanently] and retry. It will still have a long-term 
failure mode when all swap pages are depleted - but that's still quite a slow 
failure mode and it is actionable via servicing.

	Ingo

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