Checking and monitoring software RAID status (Linux)

• Log in to the server as an administrator (root).

• To check the status of the software RAID, enter the following command:

  [root@localhost ~]# cat /proc/mdstat

Status with an intact RAID

If all drives are correctly integrated, the output will look like this:

Dedicated Server with 2 SSDs and RAID 1

[root@localhost ~]# cat /proc/mdstat

Personalities : [raid1]
read_ahead 1024 sectors
md2 : active raid1 sda3[1] sdb3[0]
262016 blocks [2/2] [UU]

md1 : active raid1 sda2[1] sdb2[0]
119684160 blocks [2/2] [UU]

md0 : active raid1 sda1[1] sdb1[0]
102208 blocks [2/2] [UU]

unused devices: <none>

This example shows three RAID devices (md0, md1, md2). At the end of the line for each md device, the status of the RAID arrays is displayed in square brackets.

[2/2] means: 2 out of 2 devices are active. Each U stands for a device in the "Up" (functional) state.

Dedicated Server with 2 SSDs, 4 hard drives, and RAID 5

Personalities: [raid1] [raid5]
md127: active raid1 sda2[0] sdd2[1]
1428416 blocks super 1.0 [2/2] [UU]

md11 : active raid5 sdb1[1] sde1[0] sdc1[2] sdf1[3]
23442074112 blocks super 1.0 level 5, 512k chunk [4/4] [UUUU]

This example shows the RAID devices md127 (RAID 1) and md11 (RAID 5).

[2/2] or [4/4]: Indicates how many drives are designated for the array and how many of them are active.

[UU]: Each U stands for "Up" (functional). A RAID 5 with four hard drives consequently shows [UUUU].

Status in case of a defective or missing drive

If a drive has failed or is missing, the display changes:

Dedicated Server with 2 SSDs and RAID 1

[root@localhost ~]# cat /proc/mdstat
Personalities : [raid1]
read_ahead 1024 sectors
md0 : active raid1 sda1[1]
102208 blocks [2/1] [U_]

md2 : active raid1 sda3[1]
262016 blocks [2/1] [U_]

md1 : active raid1 sda2[1]
119684160 blocks [2/1] [U_]

unused devices: <none>

Dedicated Server with 2 SSDs, 3 hard drives, and RAID 5

md127 : active raid1 sda2[0]
1428416 blocks super 1.0 [2/1] [U_]

The _ symbol in the [U_] entry indicates that a device in the array is missing or faulty. [2/1] indicates that only one of the two devices is still active. In this case, urgent action is required. If necessary, contact IONOS Customer Support for a hardware replacement.

Displaying details of a RAID device

To view more details about a RAID device, use the mdadm command. This is necessary, for example, if you need to know exactly which physical hard drive is defective.

For a deeper analysis of a specific array, use the command mdadm -D /dev/md1.

• Adapt the following command and enter it:

  [root@localhost ~]# mdadm -D /dev/mdX

  Example:

  [root@localhost ~]# mdadm -D /dev/md127

• Pay close attention to the following lines in the output:

  • State: A healthy RAID is displayed as clean. Critical states are degraded or dirty.

  • Active Devices / Working Devices: Should match the number of RAID devices.
  • Failed Devices: Shows the number of defective drives.
  • RaidDevice State: Displays the state of individual partitions (e.g., /dev/sda1). The status active sync means synchronized, while faulty or removed indicates a defect.
  • Rebuild Status: If you have just replaced a drive, the rebuilding progress will be shown here in percent.

Examples of a functioning RAID:

Dedicated Server with 2 SSDs and RAID 1

[root@localhost ~]#mdadm -D /dev/md1

/dev/md1:
Version: 0.90
Creation Time: Tue Sep 11 21:33:24 2012
RAID Level: raid1
Array Size: 4194240 (4.00 GiB 4.29 GB)
Used Dev Size: 4194240 (4.00 GiB 4.29 GB)
Raid Devices: 2
Total Devices: 2
Preferred Minor: 1
Persistence: Superblock is persistent

Update Time: Wed 10 Jun 11:11:05 2015
State: clean
Active Devices: 2
Working Devices: 2
Failed Devices: 0
Spare Devices: 0

UUID: 2db35cef:bde058e8:1f51fb89:78ee93fe
Events: 0.251

Number Major Minor RaidDevice State
0 8 1 0 active sync /dev/sda1
1 8 17 1 active sync /dev/sdb1

Dedicated Server with 2 SSDs, 3 hard drives, and RAID 5

[root@localhost ~]# mdadm -D /dev/md127
/dev/md127:
Version : 1.0
Creation Time : Mon Feb 23 10:05:22 2026
RAID Level : raid1
Array Size : 1428416 (1394.94 MiB 1462.70 MB)
Used Dev Size : 1428416 (1394.94 MiB 1462.70 MB)
Raid Devices : 2
Total Devices : 2
Persistence : Superblock is persistent

Update Time : Tue Feb 24 14:24:22 2026
State : clean
Active Devices : 2
Working Devices : 2
Failed Devices : 0
Spare Devices : 0

Consistency Policy : resync

Name : punix:2
UUID : 92475c4d:a0880ac6:3192b41e:d1db328b
Events : 43

Number Major Minor RaidDevice State
0 8 2 0 active sync /dev/sda2
2 8 50 1 active sync /dev/sdd2

If a drive is missing, the output looks similar to this:

mdadm -D /dev/md0

/dev/md0:
Version: 00.90.00
Creation Time: Thu Aug 21 12:22:43 2003
Raid Level: raid1
Array Size: 102208 (99.81 MiB 104.66 MB)
Device Size: 102208 (99.81 MiB 104.66 MB)
Raid Devices: 2
Total Devices: 1
Preferred Minor: 0
Persistence: Superblock is persistent

Update Time: Fri 15 Oct 06:25:45 2004
State: dirty, no-errors
Active Devices: 1
Working Devices: 1
Failed Devices: 1
Spare Devices: 0

Number Major Minor RaidDevice State
0 0 0 0 faulty removed
1 3 1 1 active sync /dev/sda1
UUID : f9401842:995dc86c:b4102b57:f2996278

Set up continuous monitoring

If you have configured your server's operating system to send emails, you can receive an error notification via email if a drive fails.

Preparing email delivery

Ensure that your server can send emails.