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 -- To unsubscribe from this list: send the line "unsubscribe linux-scsi" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
