/dev/sdb failed
1. find out UUID and details of the array
Use --examine to look at an existing disk
mdadm --examine -v /dev/sde
output:
Magic : a92b4efc
Version : 1.2
Feature Map : 0x0
Array UUID : a94ed4cc:acd0f975:28f03379:fe085d38
There is the Array UUID
I didn't have a mdadm.conf file, so I had to create one (it's apparently optional and I didn't create one when I first build the raid).
mkdir /etc/mdadm/mdadm.conf (this is where centos keeps it I believe)
Add these lines and make sure all others are commented out
DEVICE /dev/sdb /dev/sdc /dev/sdd /dev/sde
ARRAY /dev/md0 UUID=a94ed4cc:acd0f975:28f03379:fe085d38
My array has 4 drives as you can see
Start the array ( you might have to stop it first)
mdadm --assemble /dev/md0 --scan --force
Re-start the array (force it) and add the drive that you've replaced
mdadm: Marking array /dev/md0 as 'clean'
mdadm: /dev/md0 has been started with 3 drives (out of 4).
[root@localhost ~]# mdadm --manage /dev/md0 --add /dev/sdb
mdadm: added /dev/sdb
Bingo!
mdadm --detail /dev/md0
/dev/md0:
Version : 1.2
Creation Time : Mon Feb 29 01:02:46 2016
Raid Level : raid5
Array Size : 2929893888 (2794.16 GiB 3000.21 GB)
Used Dev Size : 976631296 (931.39 GiB 1000.07 GB)
Raid Devices : 4
Total Devices : 4
Persistence : Superblock is persistent
Update Time : Tue Mar 13 07:25:23 2018
State : clean, degraded, recovering
Active Devices : 3
Working Devices : 4
Failed Devices : 0
Spare Devices : 1
Layout : left-symmetric
Chunk Size : 512K
Rebuild Status : 0% complete
Name : user-S5000PSL-2:my-raid
UUID : a94ed4cc:acd0f975:28f03379:fe085d38
Events : 3569
Number Major Minor RaidDevice State
5 8 16 0 spare rebuilding /dev/sdb
1 8 32 1 active sync /dev/sdc
2 8 48 2 active sync /dev/sdd
4 8 64 3 active sync /dev/sde
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