MD: "sync_action" issues: pausing resync/recovery automatically restarts.

[Benjamin ESTRABAUD <be@mpstor.com>]

Hi everybody,

I am getting a weird issue when I am writing values to 
"/sys/block/mdX/md/sync_action".
For instance, I would like to pause a resync or/and a recovery when they 
are happening.
I create a RAID 5 as follow:

mdadm --create -vvv --force --run --metadata=1.2 /dev/md/d0 --level=5 
--size=9429760 --chunk=64 --name=1056856 -n5 --bitmap=internal 
--bitmap-chunk=4096 --layout=ls /dev/sde2 /dev/sdb2 /dev/sdc2 /dev/sdf2 
/dev/sdd2

The RAID is resyncing:

# cat /proc/mdstat
Personalities : [linear] [raid0] [raid1] [raid10] [raid6] [raid5] [raid4]
md_d0 : active raid5 sdd2[4] sdf2[3] sdc2[2] sdb2[1] sde2[0]
      37719040 blocks super 1.2 level 5, 64k chunk, algorithm 2 [5/5] 
[UUUUU]
      [====>................]  resync = 22.2% (2101824/9429760) 
finish=2.6min speed=46186K/sec
      bitmap: 1/1 pages [64KB], 4096KB chunk

unused devices: <none>

I then decide to pause its resync:

# echo idle > /sys/block/md_d0/md/sync_action

The RAID resync should have paused by now, let's check the sys properties:

# cat /sys/block/md_d0/md/sync_action
resync

The resync seems to have not stopped/restarted, let's check dmesg:

[157287.049715] raid5: raid level 5 set md_d0 active with 5 out of 5 
devices, algorithm 2
[157287.057601] RAID5 conf printout:
[157287.060909]  --- rd:5 wd:5
[157287.063700]  disk 0, o:1, dev:sde2
[157287.067182]  disk 1, o:1, dev:sdb2
[157287.070664]  disk 2, o:1, dev:sdc2
[157287.074147]  disk 3, o:1, dev:sdf2
[157287.077628]  disk 4, o:1, dev:sdd2
[157287.086813] md_d0: bitmap initialized from disk: read 1/1 pages, set 
2303 bits
[157287.094134] created bitmap (1 pages) for device md_d0
[157287.113475] md: resync of RAID array md_d0
[157287.117650] md: minimum _guaranteed_  speed: 1000 KB/sec/disk.
[157287.123555] md: using maximum available idle IO bandwidth (but not 
more than 200000 KB/sec) for resync.
[157287.133011] md: using 2048k window, over a total of 9429760 blocks.
[157345.158535] md: md_do_sync() got signal ... exiting
[157345.166057] md: checkpointing resync of md_d0.
[157345.179819] md: resync of RAID array md_d0
[157345.183993] md: minimum _guaranteed_  speed: 1000 KB/sec/disk.
[157345.189899] md: using maximum available idle IO bandwidth (but not 
more than 200000 KB/sec) for resync.
[157345.199353] md: using 2048k window, over a total of 9429760 blocks.

The resync seem to stop at some stage since:

[157345.158535] md: md_do_sync() got signal ... exiting

But it seems to be restarting right after this:

[157345.179819] md: resync of RAID array md_d0

I read in the md.txt documentation that pausing a resync could sometimes 
not work if a n event or trigger was triggering it to automatically 
restart. However, I don't think I have any trigger that would cause it 
to restart.
it then builds perfectly fine.

I now want to check if the same issue occurs while recovering, after 
all, I especially want to be able to pause a recovery, while I don't 
really need to pause/restart resyncs.

Let's say I pull a disk from the bay, fail it and remove it as follow:

# mdadm --fail /dev/md/d0 /dev/sde2
mdadm: set /dev/sde2 faulty in /dev/md/d0

# mdadm --remove /dev/md/d0 /dev/sde2
mdadm: hot removed /dev/sde2

Now let's add a spare:

# /opt/soma/bin/mdadm/mdadm --add /dev/md/d0 /dev/sda2  
raid manager: added /dev/sda2

The RAID is now recovering:

# cat /proc/mdstat
Personalities : [linear] [raid0] [raid1] [raid10] [raid6] [raid5] [raid4]
md_d0 : active raid5 sda2[5] sdd2[4] sdf2[3] sdc2[2] sdb2[1]
      37719040 blocks super 1.2 level 5, 64k chunk, algorithm 2 [5/4] 
[_UUUU]
      [>....................]  recovery =  1.7% (169792/9429760) 
finish=0.9min speed=169792K/sec
      bitmap: 0/1 pages [0KB], 4096KB chunk

unused devices: <none>

# cat /sys/block/md_d0/md/sync_action
recover

Let's try and stop this recovery:

# echo idle > /sys/block/md_d0/md/sync_action

[157641.618291]  disk 3, o:1, dev:sdf2
[157641.621774]  disk 4, o:1, dev:sdd2
[157641.632057] md: recovery of RAID array md_d0
[157641.636413] md: minimum _guaranteed_  speed: 1000 KB/sec/disk.
[157641.642314] md: using maximum available idle IO bandwidth (but not 
more than 200000 KB/sec) for recovery.
[157641.651940] md: using 2048k window, over a total of 9429760 blocks.
[157657.120722] md: md_do_sync() got signal ... exiting
[157657.267055] RAID5 conf printout:
[157657.270381]  --- rd:5 wd:4
[157657.273171]  disk 0, o:1, dev:sda2
[157657.276650]  disk 1, o:1, dev:sdb2
[157657.280129]  disk 2, o:1, dev:sdc2
[157657.283605]  disk 3, o:1, dev:sdf2
[157657.287087]  disk 4, o:1, dev:sdd2
[157657.290568] RAID5 conf printout:
[157657.293876]  --- rd:5 wd:4
[157657.296660]  disk 0, o:1, dev:sda2
[157657.300139]  disk 1, o:1, dev:sdb2
[157657.303615]  disk 2, o:1, dev:sdc2
[157657.307096]  disk 3, o:1, dev:sdf2
[157657.310579]  disk 4, o:1, dev:sdd2
[157657.320835] md: recovery of RAID array md_d0
[157657.325194] md: minimum _guaranteed_  speed: 1000 KB/sec/disk.
[157657.331091] md: using maximum available idle IO bandwidth (but not 
more than 200000 KB/sec) for recovery.
[157657.340713] md: using 2048k window, over a total of 9429760 blocks.
[157657.347047] md: resuming recovery of md_d0 from checkpoint.

I am getting the same issue, the recovery stops, but restarts 200 
milliseconds later.

This clearly indicates that some sort of trigger is automatically 
restarting the resync and recovery, but I have no clue as of what could 
it be.

Would anyone here had a similar experience with trying to stop resyncs? 
Is there a "magic" variable that would enable or disable automatic 
restart of resync/recoveries?

Would anyone know of a standard event or trigger that would cause a 
resync or recovery to automatically restart?

Thank you very much in advance for your help.

My Kernel version is:

2.6.26.3

Ben.