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. > I highlight the lower layers because mdraid is also going to have to > be in the mix if raid5/6 is in use. ie. At a minimum it will have to > adjust the block range to align with the stripe boundaries. > > Greg > -- > 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 > -- Regards, Richard Sharpe -- 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
