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

Markus Trippelsdorf
2009-08-13T16:26:28Z

On Thu, Aug 13, 2009 at 08:13:12AM -0700, Matthew Wilcox wrote:
> On Wed, Aug 12, 2009 at 11:48:27PM +0100, Hugh Dickins wrote:
> > But fundamentally, though I can see how this cutdown communication
> > path is useful to compcache, I'd much rather deal with it by the more
> > general discard route if we can. (I'm one of those still puzzled by
> > the way swap is mixed up with block device in compcache: probably
> > because I never found time to pay attention when you explained.)
> >
> > You're right to question the utility of the current swap discard
> > placement. That code is almost a year old, written from a position
> > of great ignorance, yet only now do we appear to be on the threshold
> > of having an SSD which really supports TRIM (ah, the Linux ATA TRIM
> > support seems to have gone missing now, but perhaps it's been
> > waiting for a reality to check against too - Willy?).
>
> I am indeed waiting for hardware with TRIM support to appear on my
> desk before resubmitting the TRIM code. It'd also be nice to be able to
> get some performance numbers.
>

OCZ just released a new firmware with full TRIM support for their Vertex
SSDs.

> > I won't be surprised if we find that we need to move swap discard
> > support much closer to swap_free (though I know from trying before
> > that it's much messier there): in which case, even if we decided to
> > keep your hotline to compcache (to avoid allocating bios etc.), it
> > would be better placed alongside.
>
>
> Solid State Drives are introducing an ATA command called TRIM. SSDs
> generally have an intenal mapping layer, and due to their low, low seek
> penalty, will happily remap blocks anywhere on the flash. They want
> to know when a block isn't in use any more, so they don't have to copy
> it around when they want to erase the chunk of storage that it's on.
> The unfortunate thing about the TRIM command is that it's not NCQ, so
> all NCQ commands have to finish, then we can send the TRIM command and
> wait for it to finish, then we can send NCQ commands again.
>
> 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.
>
> I am planning a complete overhaul of the discard work. Users can send
> down discard requests as frequently as they like. The block layer will
> cache them, and invalidate them if writes come through. Periodically,
> the block layer will send down a TRIM or an UNMAP (depending on the
> underlying device) and get rid of the blocks that have remained unwanted
> in the interim.

That is a very good idea. I've tested your original TRIM implementation on
my Vertex yesterday and it was awful ;-). The SSD needs hundreds of
milliseconds to digest a single TRIM command. And since your implementation
sends a TRIM for each extent of each deleted file, the whole system is
unusable after a short while.
An optimal solution would be to consolidate the discard requests, bundle
them and send them to the drive as infrequent as possible.

--
Markus