How to scan and repair RAID 5 using mdadm

Raid5 on a server is having problems the last two times I rebooted. CentOS does boot up after 10-15 minutes. Is there a way I can scan the raid and repair it? I was reading the man page and mdadm --scan caught my attention but my command was unsuccessful. The raid has no data on it or anything at the moment.

Could this be one of the HDs failed? 2/3 drives are VERY old but aren't used much.

Then I saw the assemble option but the raid is already assembled no?

cat /proc/mdstat
Personalities : [raid6] [raid5] [raid4]
md0 : active raid5 sdd[1] sde[3] sdc[0] 3906762752 blocks super 1.2 level 5, 512k chunk, algorithm 2 [3/3] [UUU] bitmap: 0/15 pages [0KB], 65536KB chunk

And

mdadm --detail /dev/md?*
/dev/md0: Version : 1.2 Creation Time : Sat Jul 4 00:09:25 2020 Raid Level : raid5 Array Size : 3906762752 (3725.78 GiB 4000.53 GB) Used Dev Size : 1953381376 (1862.89 GiB 2000.26 GB) Raid Devices : 3 Total Devices : 3 Persistence : Superblock is persistent Intent Bitmap : Internal Update Time : Wed Sep 30 22:49:32 2020 State : clean Active Devices : 3 Working Devices : 3 Failed Devices : 0 Spare Devices : 0 Layout : left-symmetric Chunk Size : 512K
Consistency Policy : bitmap Name : orcacomputers.orcainbox:0 (local to host orcacomputers.orcainbox) UUID : 4ca9118c:3a557d0f:db723ff2:e8b9a521 Events : 5327 Number Major Minor RaidDevice State 0 8 32 0 active sync /dev/sdc 1 8 48 1 active sync /dev/sdd 3 8 64 2 active sync /dev/sde 

A little later,

lsblk /dev/sdf
NAME MAJ:MIN RM SIZE RO TYPE MOUNTPOINT
sdf 8:80 0 7.3T 0 disk

And

mdadm --examine /dev/sdf*
mdadm: No md superblock detected on /dev/sdf
NAME MAJ:MIN RM SIZE RO TYPE MOUNTPOINT
sda 8:0 0 447.1G 0 disk
├─sda1 8:1 0 200M 0 part /boot/efi
├─sda2 8:2 0 1G 0 part /boot
└─sda3 8:3 0 446G 0 part ├─centos-root 253:0 0 50G 0 lvm / ├─centos-swap 253:1 0 31.4G 0 lvm [SWAP] └─centos-home 253:2 0 364.5G 0 lvm /home
sdb 8:16 0 447.1G 0 disk /run/media/orca/ssd2
sdc 8:32 0 1.8T 0 disk
└─md0 9:0 0 3.7T 0 raid5 /mnt/raid5
sdd 8:48 0 1.8T 0 disk
└─md0 9:0 0 3.7T 0 raid5 /mnt/raid5
sde 8:64 0 1.8T 0 disk
└─md0 9:0 0 3.7T 0 raid5 /mnt/raid5
sdf 8:80 0 7.3T 0 disk

The hard drives look good, they have not failed as per

/dev/sda
smartctl 7.0 2018-12-30 r4883 [x86_64-linux-3.10.0-1127.19.1.el7.x86_64] (local build)
Copyright (C) 2002-18, Bruce Allen, Christian Franke,
=== START OF READ SMART DATA SECTION ===
SMART overall-health self-assessment test result: PASSED
----------------------------------------------------------------------------------------
/dev/sdb
smartctl 7.0 2018-12-30 r4883 [x86_64-linux-3.10.0-1127.19.1.el7.x86_64] (local build)
Copyright (C) 2002-18, Bruce Allen, Christian Franke,
=== START OF READ SMART DATA SECTION ===
SMART overall-health self-assessment test result: PASSED
----------------------------------------------------------------------------------------
/dev/sdc
smartctl 7.0 2018-12-30 r4883 [x86_64-linux-3.10.0-1127.19.1.el7.x86_64] (local build)
Copyright (C) 2002-18, Bruce Allen, Christian Franke,
=== START OF READ SMART DATA SECTION ===
SMART overall-health self-assessment test result: PASSED
----------------------------------------------------------------------------------------
/dev/sdd
smartctl 7.0 2018-12-30 r4883 [x86_64-linux-3.10.0-1127.19.1.el7.x86_64] (local build)
Copyright (C) 2002-18, Bruce Allen, Christian Franke,
=== START OF READ SMART DATA SECTION ===
SMART overall-health self-assessment test result: PASSED
----------------------------------------------------------------------------------------
/dev/sde
smartctl 7.0 2018-12-30 r4883 [x86_64-linux-3.10.0-1127.19.1.el7.x86_64] (local build)
Copyright (C) 2002-18, Bruce Allen, Christian Franke,
=== START OF READ SMART DATA SECTION ===
SMART overall-health self-assessment test result: PASSED
----------------------------------------------------------------------------------------
/dev/sdf
smartctl 7.0 2018-12-30 r4883 [x86_64-linux-3.10.0-1127.19.1.el7.x86_64] (local build)
Copyright (C) 2002-18, Bruce Allen, Christian Franke,
Read Device Identity failed: scsi error unsupported field in scsi command /dev/sdf is the combination of sdc sdd sde 

Here is a list of drives:

 /dev/sda 447.1G /dev/sdb 447.1G /dev/sdc 1.8T /dev/sdd 1.8T /dev/sde 1.8T /dev/sdf 7.3T

Thanks for any help. I did not set up this raid5.

2 Answers

/dev/sdf has failed and needs to be replaced. The buffer I/O errors are coming from the disk, not mdadm.

AFAIK there is no repair for mdadm as there is nothing to repair, but there is likely a verify option (which I don't think helps you) and, ifckurse, you can remove the failed device and add a new one.

The photograph of your console shows myraid errors from /dev/sdf. The output of /proc/mdstat shows that the RAID5 in question is built from /dev/sdd, /dev/sde, and /dev/sdc, with no missing component.

Therefore the RAID5 is not at risk. You have something else using /dev/sdf, and it is this that will be losing data. (Maybe an unassembled RAID array?)