Otdfbt

Recently, I had to resize a linux software RAID array. It has been a little bit complex as I was forced to do many steps to grow the device size while shrinking the number of members from 14 to 6. It nearly took a week. However, everything went fine and the LVM inside has not been harmed. Now the array seems to be fine, but shows only 4/6 devices as active:

[root@kvm24 ~]# cat /proc/mdstat 
Personalities : [raid10] [raid0] 
md3 : active raid10 sdh3[7] sdn3[6] sdl3[10] sda3[17] sdf3[19] sdc3[18]
 5559542784 blocks super 1.2 128K chunks 2 near-copies [6/4] [UUUUUU]

In the details, I can't see a problem:

[root@kvm24 ~]# mdadm --detail /dev/md3 
/dev/md3:
 Version : 1.2
 Creation Time : Wed Nov 1 23:53:09 2017
 Raid Level : raid10
 Array Size : 5559542784 (5301.99 GiB 5692.97 GB)
 Used Dev Size : 1853180928 (1767.33 GiB 1897.66 GB)
 Raid Devices : 6
 Total Devices : 6
 Persistence : Superblock is persistent

 Update Time : Tue May 7 13:28:06 2019
 State : active 
 Active Devices : 6
 Working Devices : 6
 Failed Devices : 0
 Spare Devices : 0

 Layout : near=2
 Chunk Size : 128K

Consistency Policy : unknown

 Name : kvm24:3 (local to host kvm24)
 UUID : 35833398:1c8291c5:8e817efc:6f99d541
 Events : 582653

 Number Major Minor RaidDevice State
 17 8 3 0 active sync set-A /dev/sda3
 10 8 179 1 active sync set-B /dev/sdl3
 18 8 35 2 active sync set-A /dev/sdc3
 7 8 115 3 active sync set-B /dev/sdh3
 19 8 83 4 active sync set-A /dev/sdf3
 6 8 211 5 active sync set-B /dev/sdn3

To be honest, I didn't even realize that, until Check_MK told me CRIT - disk state is [6/4] [UUUUUU] (expected 4 disks to be up).

What might be the problem with the array?

Everything is alright with your RAID, at least according to these logs. Check dmesg for more details on what happend with your RAID earlier. I don't know what is your Check_Ml, but RAID will re-sync from time to time and it's something normal. If you're refering to RAID state, this could be: clean, but also active or active sync. If you see [UUUUUU], then each U here means that each drive is synchronized. You should be worried if one of them will be replaced with _, meaning that one drive is inactive/failed. You can read about it at https://www.kernel.org/doc/html/v4.16/admin-guide/md.html