This guide explains how to configure a software RAID 5 array on Gentoo Linux using mdadm.

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1. Install mdadm

emerge --ask sys-fs/mdadm

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2. Prepare Your Disks

Identify the drives, e.g., /dev/sdb, /dev/sdc, /dev/sdd.

(Optional but recommended) Clear old RAID superblocks:

mdadm --zero-superblock /dev/sdb /dev/sdc /dev/sdd

Partition each disk (recommended; use fdisk, gdisk, or parted). Mark partitions as type fd (Linux RAID autodetect) if using fdisk.

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3. Create the RAID 5 Array

Example with 3 disks/partitions:

mdadm --create /dev/md0 --level=5 --raid-devices=3 /dev/sdb1 /dev/sdc1 /dev/sdd1

• --level=5: RAID 5
• --raid-devices=3: Number of drives (min 3)
• Replace /dev/sdX1 with your actual partition names.

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4. Monitor Array Creation

Check progress:

watch cat /proc/mdstat

Wait for the sync to complete before storing important data.

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5. Create a Filesystem

Example for ext4:

mkfs.ext4 /dev/md0

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6. Mount the Array

mkdir /mnt/raid
mount /dev/md0 /mnt/raid

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7. Configure Array Auto-Assembly at Boot

a. Save array details:

mdadm --detail --scan >> /etc/mdadm.conf

b. Enable mdadm at boot

OpenRC (Gentoo default):

rc-update add mdadm boot

Systemd (if in use):

systemctl enable mdadm

c. Add to /etc/fstab for automatic mounting:

/dev/md0    /mnt/raid    ext4    defaults    0 0

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8. Verify Everything

cat /proc/mdstat
mdadm --detail /dev/md0

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References

• Gentoo Wiki: RAID
• Gentoo Wiki: mdadm
• mdadm man page

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