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Ubuntu software RAID is Linux MD managed by mdadm. You can create an array in the Ubuntu Server installer or build and maintain one from the command line. Choose the RAID level first, back up anything important, partition every member consistently, and verify rebuild status after every change. RAID improves availability or throughput depending on its layout, but it is not a backup.
What Ubuntu RAID does
RAID (Redundant Array of Independent Disks) makes multiple physical drives act as one logical storage device. Ubuntu’s software implementation is Linux MD, administered with mdadm. The installer exposes common layouts through manual partitioning; the mdadm command-line interface gives you finer control over devices, spares, metadata and recovery.
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An array protects only against the failures its layout is designed to tolerate. It does not protect against accidental deletion, ransomware, filesystem corruption, theft, fire or a mistake made while repartitioning. Keep an independent backup, verify that it can be restored, and do not begin array work until that backup is current.
Choose the RAID level before touching a disk
The layout determines minimum drive count, usable capacity, failure tolerance and how exposed the array is during a rebuild. No cited Ubuntu documentation provides a universal performance ranking; workload, controller limits, filesystem, queue depth and drive type all matter.
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| Level | Minimum devices | Usable-capacity model | Failed-device tolerance | Operational characteristics | Boot suitability |
|---|---|---|---|---|---|
| RAID0 (striping) | 2 | Approximately the sum of member capacities, limited by the smallest members | None; one failed member loses the array | Data is striped across disks. There is no redundancy, so rebuild protection does not exist. | Not a safe choice for a stock Ubuntu /boot filesystem. |
| RAID1 (mirroring) | 2 | Approximately the capacity of the smallest member | One failed member in a two-device mirror | Each member holds a copy. Reads and writes depend on the workload and implementation; rebuilding reads the surviving copy and writes a replacement. | The Ubuntu guide states that stock Ubuntu bootloader support for soft RAID on /boot is limited to RAID1. |
| RAID5 (striping with distributed parity) | 3 | Roughly total member capacity minus one member’s worth for parity | One failed member | Parity updates add work to writes. A second failure before recovery completes makes the array unavailable. | Do not place /boot on RAID5 with the stock Ubuntu bootloader. |
| RAID6 (striping with dual parity) | 4 | Roughly total member capacity minus two members’ worth for parity | Two failed members | Dual parity increases write and rebuild work but gives more failure margin during recovery. | Do not place /boot on RAID6 with the stock Ubuntu bootloader. |
Use equal-size partitions when possible. If members differ, the array normally uses only the size available on the smallest participating partition, leaving unusable space on larger devices. Deliberately reserve separate partitions when the machine needs a non-array boot area, swap, or other metadata.
Prerequisites and safety checks
- Back up first. Creating an array usually destroys existing partition contents. Store a copy on independent storage and test a restore.
- Confirm physical identities. Examine model, serial, size, filesystem and mount point before selecting a member. Do not assume that
/dev/sda,/dev/sdband similar names will remain attached to the same disk. - Plan the partition map. Create matching member partitions, leaving space for boot, swap or other required partitions. A replacement disk must be able to receive a compatible partition.
- Use stable identifiers later. Filesystem UUIDs or labels belong in
/etc/fstab; array definitions belong in/etc/mdadm/mdadm.conf. - Check the platform. The Ubuntu Server installation ISO includes software-RAID options, but the machine must have enough physical drives for the selected level.
- Account for encryption and NVMe. Ubuntu 24.04 LTS receives security maintenance until 31 May 2029. Its release notes warn that TPM-backed full-disk-encryption kernel constraints can make the
vmdmodule required for NVMe RAID unavailable in some configurations. Validate this interaction before choosing that encryption path.
For a server, record each disk’s serial number, intended role and partition number. This simple inventory prevents the most dangerous replacement error: operating on a healthy member because device names changed after a reboot.
Configure an array in the Ubuntu Server installer
The installer is suitable when you are installing Ubuntu and want the partitioning and filesystems created in one transaction. The labels below follow the Ubuntu Server workflow; wording can vary slightly between installer releases.
- Boot the Ubuntu Server installation ISO and proceed until storage configuration.
- Choose manual partitioning. Select each intended disk and create equivalent partitions on the member drives. Do not assign a filesystem to a partition that is meant to become an MD member.
- Select Configure Software RAID and confirm the operation when prompted.
- Choose Create new MD drive.
- Select the RAID type. The Ubuntu guide lists two drives for RAID0 or RAID1, three for RAID5 and four for RAID6.
- Set the number of active devices and, if desired, spare devices. A spare remains available for an automatic or manual replacement but still needs compatible capacity.
- Select the member partitions. Recheck every disk and partition against your inventory before accepting.
- Return to the storage layout and assign a filesystem and mount point to the new MD device, such as an ext4 filesystem mounted at
/srv/data. Keep boot partitions separate unless you have deliberately designed a RAID1 boot arrangement. - Review the summary, confirm that no required partition or backup has been selected for destruction, and finish installation.
The installer creates the array, but you still need to verify it after the first boot. Check synchronization progress, confirm that all expected members are present, and test that the array assembles again after a controlled reboot before loading important data.
Build the array with mdadm
The command-line method is useful on an existing Ubuntu system or when you need precise control. The following sequence is a pattern, not a copy-and-paste recipe: replace every placeholder with the partitions you verified on the actual host.
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- Note:The eSATA port on this product does not support the use of a computer’s SATA-to-eSATA adapter. Hot-swapping is not supported. The computer’s eSATA port must support RAID functionality to properly access multiple drive bays via the eSATA port; otherwise, only one drive bay can be accessed.
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1. Install the management tools and inspect disks
sudo apt install mdadm
lsblk -o NAME,SIZE,TYPE,FSTYPE,MOUNTPOINTS
sudo blkidStop services and unmount anything that will be overwritten. Compare the output with your serial-number inventory. If a partition contains a filesystem or mount point you still need, stop and revise the plan.
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sudo mdadm --create /dev/md0 --level=1 --raid-devices=2 /dev/<partition1> /dev/<partition2>Change --level=1, the device count and member paths to match your design. RAID5 and RAID6 require at least three and four devices respectively. Never paste an example while leaving generic paths in place, and never add a disk from another array without checking its metadata and identity.
3. Watch initial synchronization
cat /proc/mdstat
sudo mdadm --detail /dev/md0/proc/mdstat reports active arrays and synchronization or recovery progress. mdadm --detail shows the level, state, event information and each member’s role. An array may be usable before synchronization finishes, but its redundancy is not fully established until recovery reaches completion.
4. Create a filesystem and mount it
sudo mkfs.ext4 /dev/md0
sudo mkdir -p /srv/raid
sudo mount /dev/md0 /srv/raidCreating a filesystem is destructive, so run mkfs only against the newly assembled MD device. Choose a filesystem and mount point appropriate for the host; the example uses ext4 only as a representative pattern.
Make assembly and mounting persistent
An array that works only until reboot is not a finished configuration. Save the array identity and ensure early boot can assemble it when required.
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- Up to 48TB (1) of max capacity on two 7200RPM Ultrastar Enterprise-class hard drives inside
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- Inspect the array details and capture its UUID and member information with
sudo mdadm --detail /dev/md0. - Ensure
/etc/mdadm/mdadm.confcontains an appropriateARRAYdefinition for the array. That file is the standard location documented bymdadm.conf(5). - Use the filesystem UUID or a deliberate filesystem label in
/etc/fstab, not a fragile/dev/sdXpath. Obtain the filesystem UUID withblkid. - Ensure the initramfs includes the array definition when the array is needed during boot. Rebuild the initramfs using your Ubuntu administration procedure after changing the configuration.
- Test the entry before relying on it: unmount and remount where safe, then reboot during a maintenance window and confirm that the array assembles and the mount appears at the intended path.
For a data-only array, a failed mount should not prevent the operating system from booting. For a root or boot array, validate the complete recovery path before deployment.
Verify health and monitor rebuilds
Check status immediately after creation, after every reboot and after every member replacement. A healthy two-device mirror commonly displays a status marker such as [UU]; a three-member healthy array can show [UUU]. A missing or failed member appears as a gap, for example [U_].
cat /proc/mdstat— quick view of arrays, degraded state and synchronization or recovery percentage.sudo mdadm --detail /dev/md0— authoritative member roles, state, events and device counts for the selected array.- System logs — correlate a failed member with I/O errors, link resets or other hardware symptoms before replacing anything.
- Alerts and scheduled checks — keep monitoring active on servers; a degraded array without notification can remain one failure away from data loss.
Do not reboot repeatedly during a long rebuild unless necessary. A second failure during recovery is more dangerous than a normal, fully synchronized array, particularly for layouts that tolerate only one failed device.
Replace a failed member safely
Replacement is a controlled sequence, not simply inserting a new disk. Confirm the failed partition first, then verify the replacement’s size, interface and partition layout.
- Run
sudo mdadm --detail /dev/md0and inspect logs to identify the failed member. Match the partition to your serial-number inventory. - If the member is still marked as faulty but present, mark it failed explicitly with the appropriate
mdadmoperation. Do not mark a healthy member just to test a command. - Remove the failed member with
mdadm --removeafter confirming its identity. Physically replace the disk if required. - Partition the replacement so the intended member partition is compatible in size, type and alignment. Copying a partition table without checking the target can overwrite unrelated data.
- Add the new partition with
mdadm --add. Use the exact array device and replacement partition, not the whole disk unless your design intentionally used whole-disk members. - Watch
/proc/mdstatandmdadm --detailuntil recovery completes. Keep the array under observation afterward to ensure the new member remains active.
A replacement from another array may contain old superblock metadata. Confirm UUIDs and clear stale metadata only after you have proved that the device is the intended replacement and contains nothing to preserve.
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Reassemble an existing array after discovery or migration
If an array was created previously but is not active after boot, first inspect devices and metadata rather than creating a second array. The documented scan operation is:
sudo mdadm --assemble --scanAfterward, inspect /proc/mdstat and the relevant mdadm --detail output. If assembly fails, check that all expected member partitions are visible, that the UUIDs match the saved array definition, and that no member was accidentally repartitioned. Do not force assembly with an incomplete or conflicting set of devices unless you understand the data-loss consequences.
Designing boot with RAID
Boot has stricter requirements than an ordinary data mount. The Ubuntu Community Help Wiki warns: “the /boot filesystem cannot use any softRAID level other than 1 with the stock Ubuntu bootloader.” If your data array uses RAID0, RAID5 or RAID6, place /boot on a separate arrangement that the firmware and bootloader can read, commonly a RAID1 design validated for the target machine.
Install boot code on every disk that must boot independently. The Ubuntu guide provides grub-install examples for additional disks; adapt the device names to your firmware mode and installation. Verify the result by temporarily removing or disconnecting one boot-capable disk only during a maintenance window, with a tested backup and a recovery console available.
Ubuntu also documents degraded boot configuration. Treat degraded boot as an emergency capability, not a substitute for replacing the failed member. Test it deliberately, record the recovery procedure and return the system to a fully redundant state.
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Maintenance checklist
- Keep an independent, restorable backup before partitioning, resizing or replacing members.
- Record disk serials, partition roles, array UUIDs, filesystem UUIDs and boot-disk locations.
- Check
/proc/mdstat,mdadm --detailand system logs on a schedule. - Alert on degraded, recovering, reshaping and failed states.
- Use replacement RAID hard drives with matched or greater usable capacity and a compatible interface; verify exact model compatibility, workload endurance and current pricing before purchase.
- After a replacement, wait for complete recovery and confirm the member returns to an active state.
- Rehearse restoring files from backup; RAID cannot recover data deleted or encrypted by an application or attacker.
- Retest assembly and boot after kernel, firmware or storage-controller changes.
Common mistakes and their fixes
Wrong disk selected
Stop immediately, unmount if possible and compare lsblk, blkid and hardware serials. Device-letter order is not a reliable identity. If data was overwritten, stop further writes and begin recovery from your independent backup.
Array is degraded after reboot
Check whether the member is missing, failed or merely not assembled. Compare the array UUID in /etc/mdadm/mdadm.conf with mdadm --detail, then use mdadm --assemble --scan only when the expected devices and metadata are present.
Filesystem will not mount
Confirm that the MD device is active, that the filesystem UUID in /etc/fstab is correct and that the mount point exists. Do not run filesystem repair against an inactive or wrong device.
Replacement will not add
Check that the replacement partition is large enough, has compatible partitioning and is not carrying stale metadata from another array. Recheck the target partition before issuing mdadm --add.
System will not boot from the array
Revisit the /boot RAID-level restriction, confirm boot code exists on every intended boot disk and test firmware mode consistently. A data array can be healthy while the machine still lacks a bootable path.
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