How to fix the problem with mdadm disks

I received three email messages from one of the servers on Hetzner with information about raids md0, md1, md2:

DegradedArray event on /dev/md/0:example.com
This is an automatically generated mail message from mdadm
running on example.com
A DegradedArray event had been detected on md device /dev/md/0.
Faithfully yours, etc.
P.S. The /proc/mdstat file currently contains the following:
Personalities : [raid6] [raid5] [raid4] [raid1]
md2 : active raid6 sdb3[1] sdd3[3]
208218112 blocks super 1.0 level 6, 512k chunk, algorithm 2 [4/2] [_U_U]
md1 : active raid1 sdb2[1] sdd2[3]
524224 blocks super 1.0 [4/2] [_U_U]
md0 : active raid1 sdb1[1] sdd1[3]
12582784 blocks super 1.0 [4/2] [_U_U]
unused devices:

I looked at the information about RAID and disks:

cat /proc/mdstat
cat /proc/partitions
mdadm --detail /dev/md0
mdadm --detail /dev/md1
mdadm --detail /dev/md2
fdisk -l | grep '/dev/sd'
fdisk -l | less

I was going to send a ticket to the tech support and plan to replace the dropped SSD disks.
SMART recorded information about the dropped discs in the files, there was also their serial number:

smartctl -x /dev/sda > sda.log
smartctl -x /dev/sdc > sdc.log

Remove disks from the raid if you can:

mdadm /dev/md0 -r /dev/sda1
mdadm /dev/md1 -r /dev/sda2
mdadm /dev/md2 -r /dev/sda3

mdadm /dev/md0 -r /dev/sdc1
mdadm /dev/md1 -r /dev/sdc2
mdadm /dev/md2 -r /dev/sdc3

If any partition of the disk is displayed as working, and the disk needs to be extracted, then first mark the partition not working and then delete, for example, if /dev/sda1, /dev/sda2 are dropped, and /dev/sda3 works:

mdadm /dev/md0 -f /dev/sda3
mdadm /dev/md0 -r /dev/sda3

In my case, having looked at the information about the dropped discs, I found that they are whole and working, even better than active ones.

I looked at the disk partitions:

fdisk /dev/sda
p
q
fdisk /dev/sdc
p
q

They were marked the same way as before:

Disk /dev/sda: 120.0 GB, 120034123776 bytes
255 heads, 63 sectors/track, 14593 cylinders
Units = cylinders of 16065 * 512 = 8225280 bytes
Sector size (logical/physical): 512 bytes / 512 bytes
I/O size (minimum/optimal): 512 bytes / 512 bytes
Disk identifier: 0x00015e3f
Device Boot Start End Blocks Id System
/dev/sda1 1 1567 12582912+ fd Linux raid autodetect
/dev/sda2 1567 1633 524288+ fd Linux raid autodetect
/dev/sda3 1633 14594 104109528+ fd Linux raid autodetect

Therefore, after waiting for the synchronization of each returned these discs back to the raid:

mdadm /dev/md0 -a /dev/sda1
mdadm /dev/md1 -a /dev/sda2
mdadm /dev/md2 -a /dev/sda3

mdadm /dev/md0 -a /dev/sdc1
mdadm /dev/md1 -a /dev/sdc2
mdadm /dev/md2 -a /dev/sdc3

At the end, the command cat /proc/mdstat was already displayed with [UUUU].

If the disks are replaced with new ones, then they need to be broken in the same way as the ones installed.
An example of partitioning the disk /dev/sdb is similar to /dev/sda with MBR:

sfdisk -d /dev/sda | sfdisk --force /dev/sdb

Example of partitioning /dev/sdb with GPT and assigning a random UUID disk:

sgdisk -R /dev/sdb /dev/sda
sgdisk -G /dev/sdb

Also on the newly installed disk you need to install the bootloader:

grub-install --version
grub-install /dev/sdb
update-grub

Either through the menu grub:

cat /boot/grub/device.map
grub
device (hd0) /dev/sdb
root (hd0,1)
setup (hd0)
quit

If the grub installation is performed from the rescue disk, you need to look at the partition list and mount it, for example if RAID is not used:

ls /dev/[hsv]d[a-z]*[0-9]*
mount /dev/sda3 /mnt

If you are using software RAID:

ls /dev/md*
mount /dev/md2 /mnt

Either LVM:

ls /dev/mapper/*
mount /dev/mapper/vg0-root /mnt

And execute chroot:

chroot-prepare /mnt
chroot /mnt

After mounting, you can restore GRUB as I wrote above.

See also my other articles:
The solution to the error “md: kicking non-fresh sda1 from array”
The solution to the warning “mismatch_cnt is not 0 on /dev/md*”
mdadm – utility for managing software RAID arrays
Description of RAID types
Diagnostics HDD using smartmontools
Recovering GRUB Linux

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