** 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. It is currently in the md-next tree [1], and I am expecting the commits to be merged during the 5.10 merge window. [1] https://git.kernel.org/pub/scm/linux/kernel/git/song/md.git/log/?h =md-next commit 5b2374a6c221f28c74913d208bb5376a7ee3bf70 Author: Xiao Ni <x...@redhat.com> Date: Wed Sep 2 20:00:23 2020 +0800 Subject: md/raid10: improve discard request for far layout Link: https://git.kernel.org/pub/scm/linux/kernel/git/song/md.git/commit/?h=md-next&id=5b2374a6c221f28c74913d208bb5376a7ee3bf70 commit 8f694215ae4c7abf1e6c985803a1aad0db748d07 Author: Xiao Ni <x...@redhat.com> Date: Wed Sep 2 20:00:22 2020 +0800 Subject: md/raid10: improve raid10 discard request Link: https://git.kernel.org/pub/scm/linux/kernel/git/song/md.git/commit/?h=md-next&id=8f694215ae4c7abf1e6c985803a1aad0db748d07 commit 6fcfa8732a8cfea7828a9444c855691c481ee557 Author: Xiao Ni <x...@redhat.com> Date: Tue Aug 25 13:43:01 2020 +0800 Subject: md/raid10: pull codes that wait for blocked dev into one function Link: https://git.kernel.org/pub/scm/linux/kernel/git/song/md.git/commit/?h=md-next&id=6fcfa8732a8cfea7828a9444c855691c481ee557 commit 6f4fed152a5e483af2227156ce7b6263aeeb5c84 Author: Xiao Ni <x...@redhat.com> Date: Tue Aug 25 13:43:00 2020 +0800 Subject: md/raid10: extend r10bio devs to raid disks Link: https://git.kernel.org/pub/scm/linux/kernel/git/song/md.git/commit/?h=md-next&id=6f4fed152a5e483af2227156ce7b6263aeeb5c84 commit 7197f1a616caf85508d81c7f5c9f065ffaebf027 Author: Xiao Ni <x...@redhat.com> Date: Tue Aug 25 13:42:59 2020 +0800 Subject: md: add md_submit_discard_bio() for submitting discard bio Link: https://git.kernel.org/pub/scm/linux/kernel/git/song/md.git/commit/?h=md-next&id=7197f1a616caf85508d81c7f5c9f065ffaebf027 It follows a similar strategy which was implemented in Raid0 in the below commit, which was merged in 4.12-rc2: commit 29efc390b9462582ae95eb9a0b8cd17ab956afc0 Author: Shaohua Li <s...@fb.com> Date: Sun May 7 17:36:24 2017 -0700 Subject: md/md0: optimize raid0 discard handling Link: https://github.com/torvalds/linux/commit/29efc390b9462582ae95eb9a0b8cd17ab956afc0 [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 I built a test kernel based on 5.9-rc6 with the above patches, and 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. + The test kernel also changes the discard_max_bytes to the underlying + hardware limit: + + $ cat /sys/block/md0/queue/discard_max_bytes + 2199023255040 + [Regression Potential] If a regression were to occur, then it would affect operations which would trigger block discard operations, such as mkfs and fstrim, on Raid10 only. Other Raid levels would not be affected, although, I should note there will be a small risk of regression to Raid0, due to one of its functions being re-factored and split out, for use in both Raid0 and Raid10. The changes only affect block discard, so only Raid10 arrays backed by SSD or NVMe devices which support block discard will be affected. Traditional hard disks, or SSD devices which do not support block discard would not be affected. If a regression were to occur, users could work around the issue by running "mkfs.xfs -K <device>" which would skip block discard entirely.
-- 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
