Step-by-Step Guide to Building a Software RAID 5 on Linux with mdadm

Software RAID 5 Implementation

RAID 5 is a storage solution that balances performance, data safety, and cost. It combines the striping of RAID 0 with parity like RAID 1, offering read speeds similar to RAID 0, higher space utilization than RAID 1, and moderate write overhead.

1. Partition

Use two partitions on /dev/sda (e.g., /dev/sda8 and /dev/sda9) together with two whole disks /dev/sdb and /dev/sdc. Remember to set the partition type ID to fd (Linux RAID autodetect).

2. Create RAID 5

# mdadm -C /dev/md5 -a yes -l 5 -n 3 -x 1 /dev/sda8 /dev/sda9 /dev/sdb /dev/sdc

Options: -l specifies the RAID level (5). -a yes automatically creates the device node. -c sets the chunk size. -x defines the number of spare disks.

3. Create filesystem # mkfs.ext4 /dev/md5 4. Mount

# mkdir /mnt/raid5

# mount /dev/md5 /mnt/raid5

5. Generate configuration file

# cat /etc/mdadm.conf

ARRAY /dev/md5 metadata=1.2 spares=1 name=centos7.3.local:5 UUID=5ec4115c:ab1e25c8:ff36d8a2:74f6fd8a

Check RAID status:

# cat /proc/mdstat

Personalities : [raid0] [raid6] [raid5] [raid4]

md5 : active raid5 sdb[4] sdc[3](S) sda9[1] sda8[0]

Detailed info: # mdadm -D /dev/md5 Shows three active devices and one spare.

6. Disable and enable

# umount /mnt/raid5

# mdadm -S /dev/md5

# mdadm -A /dev/md5

Verify that the array restarts correctly.

7. Simulate /dev/sdc failure # mdadm /dev/md5 -f /dev/sdc 8. Remove /dev/sdc # mdadm /dev/md5 -r /dev/sdc The disk is hot‑removed from the array.

9. Add a disk back to the RAID # mdadm /dev/md5 -a /dev/sdc The disk appears as a spare again.

10. Expand the array from 3 to 4 members # mdadm -G /dev/md5 -n 4 -a /dev/sdd Run a filesystem check and resize:

# e2fsck -f /dev/md5

# resize2fs /dev/md5

Final status shows four active devices and one spare.