Hi Christoph, (I've added Ccs, hoping for more expertise than we have in linux-mm.) On Fri, 30 Oct 2009, Christoph Hellwig wrote: > > since 6a6ba83175c029c7820765bae44692266b29e67a the swap code > unconditionally calls blkdev_issue_discard when swap clusters get freed. > So far this was harmless because only the mtd driver has discard support > wired up and it's pretty fast there (entirely done in-kernel). > > We're now adding support for real UNMAP/TRIM support for SCSI arrays and > SSDs, and so far all the real life ones we've dealt with have too many > performance issues to just issue the discard requests on the fly. > Because of that unconditionally enabling this code is a bad idea, it > really needs an option to disable it or even better just leave it > disabled by default for now with an option to enable it. Thanks for the info. Yes, in practice TRIM seems a huge disappointment: is there a device on which it is really implemented, and not more trouble than it's worth? I'd been waiting for OCZ to get a Vertex 1.4* firmware out of Beta before looking at swap discard again; but even then, the Linux ATA support is still up in the air, so far as I know. You don't mention swap's discard of the whole partition (or all extents of the swapfile) at swapon time: do you believe that usage is okay to retain? Is it likely on some devices to take so long, that I ought to make it asynchronous? Assuming that initial swap discard is good, I wonder whether just to revert the discard of swap clusters for now: until such time as we find devices (other than mtd) that can implement it efficiently. If we do retain the discard of swap clusters, under something more than an #if 0, any ideas for what I should make it conditional upon? Something near /sys/block/sda/queue/rotational (nicely rw these days) seems appropriate: any chance of a /sys/block/sda/queue/discard_is_useful? I think I'd prefer that to a new option to swapon. Or is there a sensible measurement I could make in swapfile.c: for example, does discard of a range complete faster than write of the same range? (But my guess is that those devices we'd want to avoid discard on, would give erratic answers to any such test; never mind the noise of what other I/Os are concurrent to the same device.) Something I should almost certainly revert: at one stage I made the non-rotational case spread its swapping evenly over the partition, in case the device's wear-levelling was inadequate (localized). But now I think it's better to ignore that possibility, and anchor swapping to the start of the partition just as in the rotational case: in the rotational case it's done to minimize seeking, in the non- rotational case it would be to minimize encroaching upon that initially discarded total extent. Hugh -- 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
