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Configure software RAID on Linux using MDADM – How we do it

by | Mar 29, 2021

Wondering how to configure software raid on Linux using MDADM? We can help you.

RAID stands for Redundant Array of Inexpensive Disks. It allows us to turn multiple physical hard drives into a single logical hard drive.

To set up RAID, we can either use a hard drive controller or use any software to create it.

Here at Bobcares, we configure software RAID on Linux using MDADM for our customers as a part of our Server Management Services.

Today let’s see the steps that our Support Engineers follow to configure software RAID for our customers.

How to Configure Software RAID on Linux using MDADM

MDADM is a tool that allows to creation and management of software RAIDs on Linux.

Before going into the steps for configuring we will check the prerequisites for the setup.

Following are the prerequisites:

1. Firstly, we need to have a Linux distribution installed on a hard drive.
2. Then two hard drives  ‘/dev/vdb’ and ‘/dev/vdc’.
3. Next, we need a special file system on /dev/vdb and /dev/vdc.
4. Finally the RAID 1 array using the MDADM utility.

Steps to Configure Software RAID on Linux using MDADM

Following are the steps that our Support Engineers follow to configure software RAID:

Installing MDADM

We can run the following installation command depending upon the operating system:

For CentOS/Red Hat we can use the following command:

# yum install mdadm

For Ubuntu/Debian we can use the following command:

# apt-get install mdadm

This will install mdadm and the dependent libraries.

How to create RAID 1 (Mirror) using two disks on Linux

We can create RAID1 using the following steps:

1. Firstly, we have to zero all superblocks on the disks to be added to the RAID using the following command:

# mdadm --zero-superblock --force /dev/vd{b,c}

The two clean disks are vdb and vdc.

mdadm: Unrecognised md component device - /dev/vdb
mdadm: Unrecognised md component device - /dev/vdc

The above results mean that neither of the disks has ever been added to an array.

2. Then create a software RAID1 from two disks to the device /dev/md0, using the following command:

# mdadm --create --verbose /dev/md0 -l 1 -n 2 /dev/vd{b,c}

3. After that, to create RAID0 (Stripe) to improve read/write speed due to parallelizing commands between several physical disks, using the following command:

# mdadm --create --verbose /dev/md0 --level=0 --raid-devices=2 /dev/vdb /dev/vdc

For RAID 5 of three or more drives we can use the following command:

# mdadm --create --verbose /dev/md0 --level=5 --raid-devices=3 /dev/vdb /dev/ vdc /dev/vdd

4. When we list the information about our disks, we will see our RAID md0 drive:

We can use the following command for listing:

# lsblk

NAME MAJ:MIN RM SIZE RO TYPE MOUNTPOINT
vda 253:0 0 20G 0 disk
├─vda1 253:1 0 512M 0 part /boot
└─vda2 253:2 0 19.5G 0 part /
vdb 253:16 0 20G 0 disk
└─md0 9:0 0 20G 0 raid1
vdc 253:32 0 20G 0 disk
└─md0 9:0 0 20G 0 raid1

5. Then, in order to create an ext4 file system on RAID1 drive, we can run the following command:

# mkfs.ext4 /dev/md0

6. After that, we can create a backup directory and mount the RAID device to it:

# mkdir /backup
# mount /dev/md0 /backup/
# df -h

7. In order not to mount the device each time manually, make the following changes to fstab:

# nano /etc/fstab

/dev/md0 /backup ext4 defaults 1 2

How to view state or check the integrity of a RAID Array

1. Firstly, we can check data integrity in the array, use the following command:

# echo 'check' > /sys/block/md0/md/sync_action

2. Then we can view the output of the following file:

# cat /sys/block/md0/md/mismatch_cnt

If we get 0, our array is OK.

3. For stopping the check, we can run the following:

# echo 'idle' > /sys/block/md0/md/sync_action

4. To check the state of all RAIDs available on the server, use this command:

# cat /proc/mdstat
Personalities : [raid1]
md0 : active raid1 vdc[1] vdb[0]
20954112 blocks super 1.2 [2/2] [UU]

5. We can view more detailed information about the specific RAID using the following command:

# mdadm -D /dev/md0

6. After that, we can view the brief information using fdisk:

# fdisk -l /dev/md0

Disk /dev/md0: 21.5 GB, 21457010688 bytes, 41908224 sectors
Units = sectors of 1 * 512 = 512 bytes
Sector size (logical/physical): 512 bytes / 512 bytes
I/O size (minimum/optimal): 512 bytes / 512 bytes

How to Recover from a Disk Failure in RAID

If one of the disks in a RAID fails or gets damaged, we can replace it with another one.

We can use the following steps to recover a disk failure in RAID:

Firstly we need to find out if the disc is damaged and needs to be replaced. This can be done with the following command:

# cat /proc/mdstat

Personalities : [raid1]
md0 : active raid1 vdb[0]
20954112 blocks super 1.2 [2/1] [U_]

From the previous command, we can see that only one disk is active. Since [U_] means that a problem exists.

When both disks are healthy, the output will be [UU].

For getting more details about this we can use the following command:

# mdadm -D /dev/md0

/dev/md0:
Version : 1.2
Creation Time : Tue Dec 31 12:39:22 2020
Raid Level : raid1
Array Size : 20954112 (19.98 GiB 21.46 GB)
Used Dev Size : 20954112 (19.98 GiB 21.46 GB)
Raid Devices : 2
Total Devices : 2
Persistence : Superblock is persistent
Update Time : Tue Dec 31 14:41:13 2020
State : clean, degraded
Active Devices : 1
Working Devices : 1
Failed Devices : 1
State : clean, degraded

The last line shows that the one disk in the RAID is damaged.

In our case, /dev/vdc must be replaced.

To restore the array, we must remove the damaged disk and add a new one.

For removing the failed drive we can use the following command:

# mdadm /dev/md0 --remove /dev/vdc

And for adding a new disk to the array we can use the following command:

# mdadm /dev/md0 --add /dev/vdd

Disk recovery will start automatically after we add a new disk we can check this with the following command:

# mdadm -D /dev/md0
/dev/md0:
Version : 1.2
Creation Time : Tue Dec 31 12:39:22 2020
Raid Level : raid1
Array Size : 20954112 (19.98 GiB 21.46 GB)
Used Dev Size : 20954112 (19.98 GiB 21.46 GB)
Raid Devices : 2
Total Devices : 2
Persistence : Superblock is persistent
Update Time : Tue Dec 31 14:50:20 2020
State : clean, degraded, recovering
Active Devices : 1
Working Devices : 2
Failed Devices : 0
Spare Devices : 1
Consistency Policy : resync
Rebuild Status : 48% complete
Name : host1:0 (local to host host1)
UUID : 9d59b1fb:7b0a7b6d:15a75459:8b1637a2
Events : 42
Number Major Minor RaidDevice State
0 253 16 0 active sync /dev/vdb
2 253 48 1 spare rebuilding /dev/vdd
rebuild Status : 48% complete shows the current array recovery state.
spare rebuilding /dev/vdd shows which disk is being added to the array.
After rebuilding the array, check its state:
State : clean
Active Devices : 2
Working Devices : 2
Failed Devices : 0
Spare Devices : 0

How to add or remove disks to Software RAID on Linux

If we need to remove the previously created mdadm RAID device, we can use the following command to unmount it:

# umount /backup

Then run the following command:

# mdadm -S /dev/md0

mdadm: stopped /dev/md0

After destroying the RAID array, it will not be detected as a separate disk device:

# mdadm -S /dev/md0

mdadm: error opening /dev/md0: No such file or directory

We can scan all connected drives and recreate a previously removed (failed) RAID device according to the metadata on physical drives. For this we can run the following command:

# mdadm --assemble —scan

And if we want to remove an operable drive from an array and replace it, first tag the drive as a failed one using the following command:

# mdadm /dev/md0 --fail /dev/vdc

Then remove it using the following command:

# mdadm /dev/md0 --remove /dev/vdc

Finally, we can add a new disk, just like in case of a failed drive using the following command:

# mdadm /dev/md0 --add /dev/vdd

How to add a Hot-Spare Drive to an MDADM Array

We can add an extra hot-spare drive to quickly rebuild the RAID array if one of the active disks fails.

Firstly, add a free disk to the md device we want using the following command:

# mdadm /dev/md0 --add /dev/vdc

This disk will be shown as a spare disk while checking the status of RAID

To make sure that the hot-swap works, mark any drive as failed and check the RAID status using the following:

# mdadm /dev/md0 --fail /dev/vdb

After checking, we can see that the rebuilding of the array has started.

For adding an additional operable disk to the RAID, we must follow these two steps.

1. Add an empty drive to the array:

# mdadm /dev/md0 --add /dev/vdb

2. Now this disk will be displayed as hot-spare. To make it active, expand the md RAID device:

# mdadm -G /dev/md0 —raid-devices=3

Then the array will be rebuilt.

After the rebuild, all the disks become active:

Number Major Minor RaidDevice State
3 253 32 0 active sync /dev/vdc
2 253 48 1 active sync /dev/vdd
4 253 16 2 active sync /dev/vdb

How to remove an MDADM RAID Array

If we want to permanently remove our software RAID drive, use the following scheme:

# umount /backup – unmount the array from the directory
# mdadm -S /dev/md0 — stop the RAID device

Then clear all superblocks on the disks it was built of:

# mdadm --zero-superblock /dev/vdb
# mdadm --zero-superblock /dev/vdc

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Conclusion

In short, we saw the steps that our Support Engineers to Configure software RAID on Linux using MDADM.

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