Replacing a disk in a RAID array

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Updated at February 17, 2025

Remove the defective disk from the RAID array
Request physical replacement of the disk
Add the new disk to your RAID array

If there is a RAID array disk failure on a BareMetal server, you must remove the defective disk from the array, request support to replace the physical drive on the server, and then add the new disk to the RAID array.

Note

This guide does not apply to disk failures in RAID 0 arrays. Such arrays are not fault-tolerant, so if one of the disks fails, all the array data will be lost and the array will have to be completely rebuilt.

Remove the defective disk from the RAID array

Connect to the server over SSH:
```
ssh root@<server_public_IP_address>
```
You can also connect to the server through the KVM console using your username and password.

Get information about the RAID array's current disk and partition layout:

cat /proc/mdstat

Result:

Personalities : [raid1] [linear] [multipath] [raid0] [raid6] [raid5] [raid4] [raid10]
md2 : active raid1 sdb3[1] sda3[0]
      6287360 blocks super 1.2 [2/2] [UU]

md3 : active raid1 sdb4[1] sda4[0]
      849215488 blocks super 1.2 [2/2] [UU]
      bitmap: 4/7 pages [16KB], 65536KB chunk

md1 : active raid1 sdb2[1] sda2[0]
      10477568 blocks super 1.2 [2/2] [UU]

The above example shows a RAID array consisting of three partitions: md1 (disk partitions sdb2 and sda2), md2 (disk partitions sdb3 and sda3), and md3 (disk partitions sdb4 and sda4).

Get information about the roles of the RAID array partitions:

lsblk

Result:

NAME    MAJ:MIN RM   SIZE RO TYPE  MOUNTPOINTS
sda       8:0    0 838.4G  0 disk
├─sda1    8:1    0   299M  0 part
├─sda2    8:2    0    10G  0 part
│ └─md1   9:1    0    10G  0 raid1 /boot
├─sda3    8:3    0     6G  0 part
│ └─md2   9:2    0     6G  0 raid1 [SWAP]
└─sda4    8:4    0   810G  0 part
  └─md3   9:3    0 809.9G  0 raid1 /
sdb       8:16   0 838.4G  0 disk
├─sdb1    8:17   0   299M  0 part
├─sdb2    8:18   0    10G  0 part
│ └─md1   9:1    0    10G  0 raid1 /boot
├─sdb3    8:19   0     6G  0 part
│ └─md2   9:2    0     6G  0 raid1 [SWAP]
└─sdb4    8:20   0   810G  0 part
  └─md3   9:3    0 809.9G  0 raid1 /

In the above example:

md1: /boot partition.
md2: SWAP partition.
md3: / partition with the root file system.

Let's assume the /dev/sdb disk is down. Remove the /dev/sdb disk's partitions from the RAID array's partitions:
```
mdadm /dev/md1 --remove /dev/sdb2
mdadm /dev/md2 --remove /dev/sdb3
mdadm /dev/md3 --remove /dev/sdb4
```
The mdadm utility will not allow you to remove a disk from a RAID array if it considers it operational or if the action can cause the array failure. In which case you will be notified that the device is busy:
```
mdadm: hot remove failed for /dev/sdb2: Device or resource busy
```
If this is the case, tentatively mark the disk as defective and retry the removal:
```
mdadm /dev/md1 --fail /dev/sdb2
mdadm /dev/md1 --remove /dev/sdb2
mdadm /dev/md2 --fail /dev/sdb3
mdadm /dev/md2 --remove /dev/sdb3
mdadm /dev/md3 --fail /dev/sdb4
mdadm /dev/md3 --remove /dev/sdb4
```

Get the defective disk's ID:

fdisk -l

Result:

...
Disk /dev/sdb: 838.36 GiB, 900185481216 bytes, 1758174768 sectors
Disk model: SAMSUNG MZ7GE900
Units: sectors of 1 * 512 = 512 bytes
Sector size (logical/physical): 512 bytes / 512 bytes
I/O size (minimum/optimal): 512 bytes / 512 bytes
Disklabel type: gpt
Disk identifier: CD2ACB4C-1618-4BAF-A6BB-D2B9********
...

Save the defective disk's ID (Disk identifier): you will need it to report the problem to tech support.

Request physical replacement of the disk

Create a disk replacement request to support stating the BareMetal server's and defective disk's IDs.

Wait for the data center engineers to replace the defective disk.

Add the new disk to your RAID array

Once the physical drive is replaced on the server, you must partition the drive and add it to the existing RAID array.

Use the gdisk utility to set the partition table type: GPT or MBR. Install gdisk for your server's OS if needed.

Run the command stating the ID of the RAID array's remaining operational disk:
```
gdisk -l /dev/sda
```
Depending on the partition table type, the result will be as follows:
GPT

MBR
```
Partition table scan:
  MBR: protective
  BSD: not present
  APM: not present
  GPT: present
...
```
```
Partition table scan:
  MBR: MBR only
  BSD: not present
  APM: not present
  GPT: not present
...
```
Copy the partition table layout from the RAID array's remaining operational disk to the new disk:
GPT

MBR
If the source disk uses a GPT partition table:
1. Create a copy of the source disk partition table:
  sgdisk --backup=table /dev/sda
  Result:
  The operation has completed successfully.
2. Recover the partition table from the copy to the new disk:
  sgdisk --load-backup=table /dev/sdb
  Result:
  The operation has completed successfully.
3. Assign a new random UUID to the new disk:
  sgdisk -G /dev/sdb
  Result:
  The operation has completed successfully.
If the source disk uses a MBR partition table:
1. Copy the partition table:
  sfdisk -d /dev/sda | sfdisk /dev/sdb
  Where:
  - /dev/sda: RAID array's remaining source disk to copy the partition table from.
  - /dev/sdb: Target (new) disk to copy the partition table to from the source disk.
2. If the partitions are not displayed after copying, re-read the partition table:
  sfdisk -R /dev/sdb
Add the disk to the RAID array by adding the corresponding disk partitions to the RAID partitions one by one. The mapping between these partitions was done earlier in Remove the defective disk from the RAID array.

Run the following commands:
```
mdadm /dev/md1 --add /dev/sdb2
mdadm /dev/md2 --add /dev/sdb3
mdadm /dev/md3 --add /dev/sdb4
```
Once a disk is added to the array, synchronization begins, its speed depending on disk size and type (ssd/hdd).

Result:
```
mdadm: added /dev/sdb2
mdadm: added /dev/sdb3
mdadm: added /dev/sdb4
```

Make sure the new disk is added to the RAID array:

cat /proc/mdstat

Result:

Personalities : [raid1] [linear] [multipath] [raid0] [raid6] [raid5] [raid4] [raid10]
md2 : active raid1 sdb3[2] sda3[0]
      6287360 blocks super 1.2 [2/2] [UU]

md3 : active raid1 sdb4[2] sda4[0]
      849215488 blocks super 1.2 [2/2] [UU]
      bitmap: 4/7 pages [16KB], 65536KB chunk

md1 : active raid1 sdb2[2] sda2[0]
      10477568 blocks super 1.2 [2/2] [UU]

unused devices: <none>

Install the Linux OS bootloader on the new disk:

grub-install /dev/sdb

Result:

Installing for i386-pc platform.
Installation finished. No error reported.

Replacing a disk in a RAID array

Remove the defective disk from the RAID array

Request physical replacement of the disk

Add the new disk to your RAID array

See also

Was the article helpful?

Replacing a disk in a RAID array

Remove the defective disk from the RAID arrayRemove the defective disk from the RAID array

Request physical replacement of the diskRequest physical replacement of the disk

Add the new disk to your RAID arrayAdd the new disk to your RAID array

See alsoSee also

Was the article helpful?

Remove the defective disk from the RAID array

Request physical replacement of the disk

Add the new disk to your RAID array

See also