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

[James Bottomley]

On Thu, 2009-08-13 at 08:13 -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.
> 
> > 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.
> 
> It turns out there are a lot of tradeoffs involved with discard, and
> they're different between TRIM and UNMAP.
> 
> Let's start with UNMAP.  This SCSI command is used by giant arrays.
> They want to do Thin Provisioning, so allocate physical storage to virtual
> LUNs on demand, and want to deallocate it when they get an UNMAP command.
> They allocate storage in large chunks (hundreds of kilobytes at a time).
> They only care about discards that enable them to free an entire chunk.
> The vast majority of users *do not care* about these arrays, because
> they don't have one, and will never be able to afford one.  We should
> ignore the desires of these vendors when designing our software.

Fundamentally, unmap, trim and write_same do similar things, so
realistically they all map to discard in linux.

Ignoring the desires of the enterprise isn't an option, since they are a
good base for us.  However, they really do need to step up with a useful
patch set for discussion that does what they want, so in the interim I'm
happy with any proposal that doesn't actively damage what the enterprise
wants to do with trim/write_same.

> 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.

That's a bit of a silly protocol oversight ... I assume there's no way
it can be corrected?

> 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.
> 
> Thoughts on that are welcome.

What you're basically planning is discard accumulation ... it's
certainly closer to what the enterprise is looking for, so no objections
from me.

James


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