On Thu, Aug 13, 2009 at 6:20 PM, Richard Sharpe<realrichardsharpe@xxxxxxxxx> wrote: > On Thu, Aug 13, 2009 at 2:28 PM, Greg Freemyer<greg.freemyer@xxxxxxxxx> wrote: >> On Thu, Aug 13, 2009 at 4:44 PM, <david@xxxxxxx> wrote: >>> On Thu, 13 Aug 2009, Greg Freemyer wrote: >>> >>>> On Thu, Aug 13, 2009 at 12:33 PM, <david@xxxxxxx> wrote: >>>>> >>>>> On Thu, 13 Aug 2009, Markus Trippelsdorf wrote: >>>>> >>>>>> On Thu, Aug 13, 2009 at 08:13:12AM -0700, Matthew Wilcox wrote: >>>>>>> >>>>>>> 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. >>>>> >>>>> or queue them up and send them when the drive is idle (you would need to >>>>> keep track to make sure the space isn't re-used) >>>>> >>>>> as an example, if you would consider spinning down a drive you don't hurt >>>>> performance by sending accumulated trim commands. >>>>> >>>>> David Lang >>>> >>>> An alternate approach is the block layer maintain its own bitmap of >>>> used unused sectors / blocks. Unmap commands from the filesystem just >>>> cause the bitmap to be updated. No other effect. >>> >>> how does the block layer know what blocks are unused by the filesystem? >>> >>> or would it be a case of the filesystem generating discard/trim requests to >>> the block layer so that it can maintain it's bitmap, and then the block >>> layer generating the requests to the drive below it? >>> >>> David Lang >> >> Yes, my thought.was that block layer would consume the discard/trim >> requests from the filesystem in realtime to maintain the bitmap, then >> at some later point in time when the system has extra resources it >> would generate the calls down to the lower layers and eventually the >> drive. > > Why should the block layer be forced to maintain something that is > probably of use for only a limited number of cases? For example, the > devices I work on already maintain their own mapping of HOST-visible > LBAs to underlying storage, and I suspect that most such devices do. > So, you are duplicating something that we already do, and there is no > way that I am aware of to synchronise the two. > > All we really need, I believe is for the UNMAP requests to come down > to us with writes barriered until we respond, and it is a relatively > cheap operation, although writes that are already in the cache and > uncommitted to disk present some issues if an UNMAP request comes down > for recently written blocks. > Richard, Quoting the original email I saw in this thread: > >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. > >Thoughts on that are welcome. >> My thought was that a bitmap was a better solution than a cache of discard commands. One of the biggest reasons is that a bitmap can coalesce the unused areas into much larger discard ranges than a queue that will only have a limited number of discards to coalesce. And both Enterprise scsi and mdraid are desirous of larger discard ranges. Greg -- Greg Freemyer Head of EDD Tape Extraction and Processing team Litigation Triage Solutions Specialist http://www.linkedin.com/in/gregfreemyer Preservation and Forensic processing of Exchange Repositories White Paper - <http://www.norcrossgroup.com/forms/whitepapers/tng_whitepaper_fpe.html> The Norcross Group The Intersection of Evidence & Technology http://www.norcrossgroup.com -- To unsubscribe from this list: send the line "unsubscribe linux-ide" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
