Software RAID Status & Monitoring (Linux)

For Linux Dedicated Servers

You can read the status of the software RAID in the shell with the command cat /proc/mdstat.

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>

The example shows three RAID arrays (md0, md1, md2). In the second line in each case, the state of the individual partitions is shown at the end in square brackets. A U means that the respective device is in order (up).

If a hard disk (here sdb) is missing, the output looks similar to the following:

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>

You can tell from [U_] that something is wrong here.

Showing RAID Device Details

Detailed information about a RAID device is provided by the command mdadm -D /dev/md1.

Here is an example of a working RAID:

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 Jun 10 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

If the hard disk is missing, the output looks similar to the following:

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 Oct 15 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

Monitoring the RAID

Provided that you have configured your server's operating system to send emails, you can also have an error message sent to you by email in the event of a hard disk failure. For this purpose, start Mdadm as Deamon. Then, enter the following line in the file /etc/mdadm/mdadm.conf (Ubuntu) or /etc/mdadm.conf (CentOS 7):

MAILADDR admin@example.com

All events are also logged in the /var/log/syslog directory.

Example scenario:

The array should be checked every 300 seconds. If an event occurs, an email is to be sent and the event is to be noted in the system log file. The --daemonise option makes the monitoring program run continuously in the background. Thus, the command would look like this:

 ./mdadm --monitor --mail=root@localhost --syslog --delay=300 /dev/md0 --daemonise