> On 23 Jan 2022, at 21:34, Paul Menzel <pmenzel@xxxxxxxxxxxxx> wrote: > > Dear Roger, > > > Am 23.01.22 um 19:00 schrieb Lukas Straub: >> CC'ing Song Liu (md-raid maintainer) and linux-raid mailing list. >> On Fri, 21 Jan 2022 16:38:03 +0000 Roger Willcocks wrote: > >>> we noticed a thirty percent drop in performance on one of our raid >>> arrays when switching from CentOS 6.5 to 8.4; it uses raid0-like > > For those outside the CentOS universe, what Linux kernel versions are those? > 2.6.32 (and backported changes) and 4.18.0 (sim.) >>> striping to balance (by time) access to a pair of hardware raid-6 >>> arrays. The underlying issue is also present in the native raid0 >>> driver so herewith the gory details; I'd appreciate your thoughts. >>> >>> -- >>> >>> blkdev_direct_IO() calls submit_bio() which calls an outermost >>> generic_make_request() (aka submit_bio_noacct()). >>> >>> md_make_request() calls blk_queue_split() which cuts an incoming >>> request into two parts with the first no larger than get_max_io_size() >>> bytes (which in the case of raid0, is the chunk size): >>> >>> R -> AB >>> blk_queue_split() gives the second part 'B' to generic_make_request() >>> to worry about later and returns the first part 'A'. >>> >>> md_make_request() then passes 'A' to a more specific request handler, >>> In this case raid0_make_request(). >>> >>> raid0_make_request() cuts its incoming request into two parts at the >>> next chunk boundary: >>> >>> A -> ab >>> >>> it then fixes up the device (chooses a physical device) for 'a', and >>> gives both parts, separately, to generic make request() >>> >>> This is where things go awry, because 'b' is still targetted to the >>> original device (same as 'B'), but 'B' was queued before 'b'. So we >>> end up with: >>> >>> R -> Bab >>> >>> The outermost generic_make_request() then cuts 'B' at >>> get_max_io_size(), and the process repeats. Ascii art follows: >>> >>> >>> /---------------------------------------------------/ incoming rq >>> >>> /--------/--------/--------/--------/--------/------/ max_io_size >>> |--------|--------|--------|--------|--------|--------|--------| chunks >>> >>> |...=====|---=====|---=====|---=====|---=====|---=====|--......| rq out >>> a b c d e f g h i j k l >>> >>> Actual submission order for two-disk raid0: 'aeilhd' and 'cgkjfb' >>> >>> -- >>> >>> There are several potential fixes - >>> >>> simplest is to set raid0 blk_queue_max_hw_sectors() to UINT_MAX >>> instead of chunk_size, so that raid0_make_request() receives the >>> entire transfer length and cuts it up at chunk boundaries; >>> >>> neatest is for raid0_make_request() to recognise that 'b' doesn't >>> cross a chunk boundary so it can be sent directly to the physical >>> device; >>> >>> and correct is for blk_queue_split to requeue 'A' before 'B'. >>> >>> -- >>> >>> There's also a second issue - with large raid0 chunk size (256K), the >>> segments submitted to the physical device are at least 128K and >>> trigger the early unplug code in blk_mq_make_request(), so the >>> requests are never merged. There are legitimate reasons for a large >>> chunk size so this seems unhelpful. >>> >>> -- >>> >>> As I said, I'd appreciate your thoughts. > > Thank you for the report and the analysis. > > Is the second issue also a regression? If not, I suggest to split it into a separate thread. > Yes this is also a regression, both issues above have to be addressed to recover the original performance. Specifically, an md raid0 array with 256K chunk size interleaving two x 12-disk raid6 devices (Adaptec 3154 controller, 50MB files stored contiguously on disk, four threads) can achieve a sequential read rate of 3800 MB/sec with the (very) old 6.5 kernel; this falls to 2500 MB/sec with the relatively newer kernel. This change to raid0.c: - blk_queue_max_hw_sectors(mddev->queue, mddev->chunk_sectors); + blk_queue_max_hw_sectors(mddev->queue, UINT_MAX); improves things somewhat, the sub-chunk requests are now submitted in order but we still only get 2800 MB/sec because no merging takes place; the controller struggles to keep up with the large number of sub-chunk transfers. This additional change to blk-mq.c: - if (request_count >= BLK_MAX_REQUEST_COUNT || (last && + if (request_count >= BLK_MAX_REQUEST_COUNT || (false && last && blk_rq_bytes(last) >= BLK_PLUG_FLUSH_SIZE)) { blk_flush_plug_list(plug, false); Brings performance back to 6.5 levels. > > Kind regards, > > Paul > -- dm-devel mailing list dm-devel@xxxxxxxxxx https://listman.redhat.com/mailman/listinfo/dm-devel