** Description changed: BugLink: https://bugs.launchpad.net/bugs/1896578 [Impact] Block discard is very slow on Raid10, which causes common use cases which invoke block discard, such as mkfs and fstrim operations, to take a very long time. For example, on a i3.8xlarge instance on AWS, which has 4x 1.9TB NVMe devices which support block discard, a mkfs.xfs operation on Raid 10 takes between 8 to 11 minutes, where the same mkfs.xfs operation on Raid 0, takes 4 seconds. The bigger the devices, the longer it takes. The cause is that Raid10 currently uses a 512k chunk size, and uses this for the discard_max_bytes value. If we need to discard 1.9TB, the kernel splits the request into millions of 512k bio requests, even if the underlying device supports larger requests. For example, the NVMe devices on i3.8xlarge support 2.2TB of discard at once: $ cat /sys/block/nvme0n1/queue/discard_max_bytes 2199023255040 $ cat /sys/block/nvme0n1/queue/discard_max_hw_bytes 2199023255040 Where the Raid10 md device only supports 512k: $ cat /sys/block/md0/queue/discard_max_bytes 524288 $ cat /sys/block/md0/queue/discard_max_hw_bytes 524288 If we perform a mkfs.xfs operation on the /dev/md array, it takes over 11 minutes and if we examine the stack, it is stuck in blkdev_issue_discard() $ sudo cat /proc/1626/stack [<0>] wait_barrier+0x14c/0x230 [raid10] [<0>] regular_request_wait+0x39/0x150 [raid10] [<0>] raid10_write_request+0x11e/0x850 [raid10] [<0>] raid10_make_request+0xd7/0x150 [raid10] [<0>] md_handle_request+0x123/0x1a0 [<0>] md_submit_bio+0xda/0x120 [<0>] __submit_bio_noacct+0xde/0x320 [<0>] submit_bio_noacct+0x4d/0x90 [<0>] submit_bio+0x4f/0x1b0 [<0>] __blkdev_issue_discard+0x154/0x290 [<0>] blkdev_issue_discard+0x5d/0xc0 [<0>] blk_ioctl_discard+0xc4/0x110 [<0>] blkdev_common_ioctl+0x56c/0x840 [<0>] blkdev_ioctl+0xeb/0x270 [<0>] block_ioctl+0x3d/0x50 [<0>] __x64_sys_ioctl+0x91/0xc0 [<0>] do_syscall_64+0x38/0x90 [<0>] entry_SYSCALL_64_after_hwframe+0x44/0xa9 [Fix] Xiao Ni has developed a patchset which resolves the block discard performance problems. These commits have now landed in 5.10-rc1. commit cf78408f937a67f59f5e90ee8e6cadeed7c128a8 Author: Xiao Ni <x...@redhat.com> Date: Thu Feb 4 15:50:43 2021 +0800 Subject: md: add md_submit_discard_bio() for submitting discard bio Link: https://github.com/torvalds/linux/commit/cf78408f937a67f59f5e90ee8e6cadeed7c128a8 commit c2968285925adb97b9aa4ede94c1f1ab61ce0925 Author: Xiao Ni <x...@redhat.com> Date: Thu Feb 4 15:50:44 2021 +0800 Subject: md/raid10: extend r10bio devs to raid disks Link: https://github.com/torvalds/linux/commit/c2968285925adb97b9aa4ede94c1f1ab61ce0925 commit f2e7e269a7525317752d472bb48a549780e87d22 Author: Xiao Ni <x...@redhat.com> Date: Thu Feb 4 15:50:45 2021 +0800 Subject: md/raid10: pull the code that wait for blocked dev into one function Link: https://github.com/torvalds/linux/commit/f2e7e269a7525317752d472bb48a549780e87d22 commit d30588b2731fb01e1616cf16c3fe79a1443e29aa Author: Xiao Ni <x...@redhat.com> Date: Thu Feb 4 15:50:46 2021 +0800 Subject: md/raid10: improve raid10 discard request Link: https://github.com/torvalds/linux/commit/d30588b2731fb01e1616cf16c3fe79a1443e29aa commit 254c271da0712ea8914f187588e0f81f7678ee2f Author: Xiao Ni <x...@redhat.com> Date: Thu Feb 4 15:50:47 2021 +0800 Subject: md/raid10: improve discard request for far layout Link: https://github.com/torvalds/linux/commit/254c271da0712ea8914f187588e0f81f7678ee2f There is also an additional commit which is required, and was merged after "md/raid10: improve raid10 discard request" was merged. The following commits enable Radid10 to use large discards, instead of splitting into many bios, since the technical hurdles have now been removed. commit ca4a4e9a55beeb138bb06e3867f5e486da896d44 Author: Mike Snitzer <snit...@redhat.com> Date: Fri Apr 30 14:38:37 2021 -0400 Subject: dm raid: remove unnecessary discard limits for raid0 and raid10 Link: https://github.com/torvalds/linux/commit/ca4a4e9a55beeb138bb06e3867f5e486da896d44 The commits more or less cherry pick to the 5.11, 5.8, 5.4 and 4.15 kernels, with the following minor backports: 1) submit_bio_noacct() needed to be renamed to generic_make_request() since it was recently changed in: commit ed00aabd5eb9fb44d6aff1173234a2e911b9fead Author: Christoph Hellwig <h...@lst.de> Date: Wed Jul 1 10:59:44 2020 +0200 Subject: block: rename generic_make_request to submit_bio_noacct Link: https://github.com/torvalds/linux/commit/ed00aabd5eb9fb44d6aff1173234a2e911b9fead - 2) bio_split(), mempool_alloc(), bio_clone_fast() all needed their + 2) In the 4.15, 5.4 and 5.8 kernels, trace_block_bio_remap() needs to + have its request_queue argument put back in place. It was recently + removed in: + + commit 1c02fca620f7273b597591065d366e2cca948d8f + Author: Christoph Hellwig <h...@lst.de> + Date: Thu Dec 3 17:21:38 2020 +0100 + Subject: block: remove the request_queue argument to the block_bio_remap tracepoint + Link: https://github.com/torvalds/linux/commit/1c02fca620f7273b597591065d366e2cca948d8f + + 3) bio_split(), mempool_alloc(), bio_clone_fast() all needed their "address of" '&' removed for one of their arguments for the 4.15 kernel, due to changes made in: commit afeee514ce7f4cab605beedd03be71ebaf0c5fc8 Author: Kent Overstreet <kent.overstr...@gmail.com> Date: Sun May 20 18:25:52 2018 -0400 Subject: md: convert to bioset_init()/mempool_init() Link: https://github.com/torvalds/linux/commit/afeee514ce7f4cab605beedd03be71ebaf0c5fc8 - 3) The 4.15 kernel does not need "dm raid: fix discard limits for raid1 - and raid10" and "dm raid: remove unnecessary discard limits for raid10" - due to not having the following commit, which was merged in 5.1-rc1: + 4) The 4.15 kernel does not need "dm raid: remove unnecessary discard + limits for raid0 and raid10" due to not having the following commit, + which was merged in 5.1-rc1: commit 61697a6abd24acba941359c6268a94f4afe4a53d Author: Mike Snitzer <snit...@redhat.com> Date: Fri Jan 18 14:19:26 2019 -0500 Subject: dm: eliminate 'split_discard_bios' flag from DM target interface Link: https://github.com/torvalds/linux/commit/61697a6abd24acba941359c6268a94f4afe4a53d - 4) The 4.15 kernel needed bio_clone_blkg_association() to be renamed to + 5) The 4.15 kernel needed bio_clone_blkg_association() to be renamed to bio_clone_blkcg_association() due to it changing in: commit db6638d7d177a8bc74c9e539e2e0d7d061c767b1 Author: Dennis Zhou <den...@kernel.org> Date: Wed Dec 5 12:10:35 2018 -0500 Subject: blkcg: remove bio->bi_css and instead use bio->bi_blkg https://github.com/torvalds/linux/commit/db6638d7d177a8bc74c9e539e2e0d7d061c767b1 [Testcase] You will need a machine with at least 4x NVMe drives which support block discard. I use a i3.8xlarge instance on AWS, since it has all of these things. $ lsblk xvda 202:0 0 8G 0 disk └─xvda1 202:1 0 8G 0 part / nvme0n1 259:2 0 1.7T 0 disk nvme1n1 259:0 0 1.7T 0 disk nvme2n1 259:1 0 1.7T 0 disk nvme3n1 259:3 0 1.7T 0 disk Create a Raid10 array: $ sudo mdadm --create --verbose /dev/md0 --level=10 --raid-devices=4 /dev/nvme0n1 /dev/nvme1n1 /dev/nvme2n1 /dev/nvme3n1 Format the array with XFS: $ time sudo mkfs.xfs /dev/md0 real 11m14.734s $ sudo mkdir /mnt/disk $ sudo mount /dev/md0 /mnt/disk Optional, do a fstrim: $ time sudo fstrim /mnt/disk real 11m37.643s There are test kernels for 5.8, 5.4 and 4.15 available in the following PPA: https://launchpad.net/~mruffell/+archive/ubuntu/lp1896578-test If you install a test kernel, we can see that performance dramatically improves: $ sudo mdadm --create --verbose /dev/md0 --level=10 --raid-devices=4 /dev/nvme0n1 /dev/nvme1n1 /dev/nvme2n1 /dev/nvme3n1 $ time sudo mkfs.xfs /dev/md0 real 0m4.226s user 0m0.020s sys 0m0.148s $ sudo mkdir /mnt/disk $ sudo mount /dev/md0 /mnt/disk $ time sudo fstrim /mnt/disk real 0m1.991s user 0m0.020s sys 0m0.000s The patches bring mkfs.xfs from 11 minutes down to 4 seconds, and fstrim from 11 minutes to 2 seconds. Performance Matrix (AWS i3.8xlarge): Kernel | mkfs.xfs | fstrim --------------------------------- 4.15 | 7m23.449s | 7m20.678s 5.4 | 8m23.219s | 8m23.927s 5.8 | 2m54.990s | 8m22.010s 4.15-test | 0m4.286s | 0m1.657s 5.4-test | 0m6.075s | 0m3.150s 5.8-test | 0m2.753s | 0m2.999s The test kernel also changes the discard_max_bytes to the underlying hardware limit: $ cat /sys/block/md0/queue/discard_max_bytes 2199023255040 [Where problems can occur] A problem has occurred once before, with the previous revision of this patchset. This has been documented in bug 1907262, and caused a worst case scenario of data loss for some users, in this particular case, on the second and onward disks. This was due to two two faults: the first, incorrectly calculating the start offset for block discard for the second and extra disks. The second bug was an incorrect stripe size for far layouts. The kernel team was forced to revert the patches in an emergency and the faulty kernel was removed from the archive, and community users urged to avoid the faulty kernel. These bugs and a few other minor issues have now been corrected, and we have been testing the new patches since mid February. The patches have been tested against the testcase in bug 1907262 and do not cause the disks to become corrupted. The regression potential is still the same for this patchset though. If a regression were to occur, it could lead to data loss on Raid10 arrays backed by NVMe or SSD disks that support block discard. If a regression happens, users need to disable the fstrim systemd service as soon as possible, plan an emergency maintainance window, and downgrade the kernel to a previous release, or upgrade to a corrected kernel.
-- You received this bug notification because you are a member of Ubuntu Bugs, which is subscribed to Ubuntu. https://bugs.launchpad.net/bugs/1896578 Title: raid10: Block discard is very slow, causing severe delays for mkfs and fstrim operations To manage notifications about this bug go to: https://bugs.launchpad.net/ubuntu/+source/linux/+bug/1896578/+subscriptions -- ubuntu-bugs mailing list ubuntu-bugs@lists.ubuntu.com https://lists.ubuntu.com/mailman/listinfo/ubuntu-bugs
