Dear Linux mdraid people,
we are using raid6 on several servers. Occasionally we had failures, where a mdX_raid6 process seems to go into a busy loop and all I/O to the md device blocks. We've seen this on various kernel versions.
The last time this happened (in this case with Linux 5.10.0-rc4), I took some data.
The triggering event seems to be the md_check cron job trying to pause the ongoing check operation in the morning with
echo idle > /sys/devices/virtual/block/md1/md/sync_action
This doesn't complete. Here's /proc/stack of this process:
root@done:~/linux_problems/mdX_raid6_looping/2020-11-27# ps -fp 23333
UID PID PPID C STIME TTY TIME CMD
root 23333 23331 0 02:00 ? 00:00:00 /bin/bash /usr/bin/mdcheck --continue --duration 06:00
root@done:~/linux_problems/mdX_raid6_looping/2020-11-27# cat /proc/23333/stack
[<0>] kthread_stop+0x6e/0x150
[<0>] md_unregister_thread+0x3e/0x70
[<0>] md_reap_sync_thread+0x1f/0x1e0
[<0>] action_store+0x141/0x2b0
[<0>] md_attr_store+0x71/0xb0
[<0>] kernfs_fop_write+0x113/0x1a0
[<0>] vfs_write+0xbc/0x250
[<0>] ksys_write+0xa1/0xe0
[<0>] do_syscall_64+0x33/0x40
[<0>] entry_SYSCALL_64_after_hwframe+0x44/0xa9
Note, that md0 has been paused successfully just before.
2020-11-27T02:00:01+01:00 done CROND[23333]: (root) CMD (/usr/bin/mdcheck --continue --duration "06:00")
2020-11-27T02:00:01+01:00 done root: mdcheck continue checking /dev/md0 from 10623180920
2020-11-27T02:00:01.382994+01:00 done kernel: [378596.606381] md: data-check of RAID array md0
2020-11-27T02:00:01+01:00 done root: mdcheck continue checking /dev/md1 from 11582849320
2020-11-27T02:00:01.437999+01:00 done kernel: [378596.661559] md: data-check of RAID array md1
2020-11-27T06:00:01.842003+01:00 done kernel: [392996.625147] md: md0: data-check interrupted.
2020-11-27T06:00:02+01:00 done root: pause checking /dev/md0 at 13351127680
2020-11-27T06:00:02.338989+01:00 done kernel: [392997.122520] md: md1: data-check interrupted.
[ nothing related following.... ]
After that, we see md1_raid6 in a busy loop:
PID USER PR NI VIRT RES SHR S %CPU %MEM TIME+ COMMAND
2376 root 20 0 0 0 0 R 100.0 0.0 1387:38 md1_raid6
Also, all processes doing I/O do the md device block.
This is /proc/mdstat:
Personalities : [linear] [raid0] [raid1] [raid6] [raid5] [raid4] [multipath]
md1 : active raid6 sdk[0] sdj[15] sdi[14] sdh[13] sdg[12] sdf[11] sde[10] sdd[9] sdc[8] sdr[7] sdq[6] sdp[5] sdo[4] sdn[3] sdm[2] sdl[1]
109394518016 blocks super 1.2 level 6, 512k chunk, algorithm 2 [16/16] [UUUUUUUUUUUUUUUU]
[==================>..] check = 94.0% (7350290348/7813894144) finish=57189.3min speed=135K/sec
bitmap: 0/59 pages [0KB], 65536KB chunk
md0 : active raid6 sds[0] sdah[15] sdag[16] sdaf[13] sdae[12] sdad[11] sdac[10] sdab[9] sdaa[8] sdz[7] sdy[6] sdx[17] sdw[4] sdv[3] sdu[2] sdt[1]
109394518016 blocks super 1.2 level 6, 512k chunk, algorithm 2 [16/16] [UUUUUUUUUUUUUUUU]
bitmap: 0/59 pages [0KB], 65536KB chunk
There doesn't seem to be any further progress.
I've taken a function_graph trace of the looping md1_raid6 process: https://owww.molgen.mpg.de/~buczek/2020-11-27_trace.txt (30 MB)
Maybe this helps to get an idea what might be going on?
Best
Donald
--
Donald Buczek
buczek@xxxxxxxxxxxxx
Tel: +49 30 8413 1433
