mdadm is Ubuntu 22.04’s command-line tool for creating and maintaining Linux software RAID (MD) arrays. A safe workflow is: install and verify mdadm, confirm which disks are empty, create the appropriate RAID level, monitor the initial resync, save the array definition in /etc/mdadm/mdadm.conf, and keep a tested backup outside the array. When a member fails, mark it failed, remove it, add a compatible replacement, and watch the rebuild. Capacity changes require an mdadm reshape first and a filesystem resize afterward.
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The commands below use an example array named data. Replace example devices such as /dev/sdb with the devices you have positively identified. Running a create, format, or superblock command on the wrong disk can destroy data.
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mdadm manages Linux kernel MD devices. In create mode it writes a RAID superblock to each component device, starts the array, and begins an initial resynchronization. The array can normally be used while that resync runs, although intensive workloads make the process slower and increase exposure while redundancy is being established.
RAID provides availability when a disk fails; it is not a backup. Accidental deletion, filesystem corruption, malware, theft, fire, and a failed reshape can affect every member. Keep a separate, tested backup before creating an array and before any replacement or reshape operation.
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The Ubuntu Jammy man page identifies the packaged release as mdadm 4.2-0ubuntu2 (Ubuntu/Canonical, accessed 2026). Command behavior can differ with later Ubuntu updates, so check the local man page with man mdadm when an option is unfamiliar.
Choose a RAID level before touching the disks
Usable capacity is based on the smallest member in a conventional array. Mixing substantially different sizes wastes the excess on larger disks. The failure counts below describe the normal layout, not every possible failure pattern.
| Level | Minimum disks | Approximate usable capacity | Simultaneous disk failures tolerated | Performance and rebuild considerations | Best fit |
|---|---|---|---|---|---|
| RAID0 | 2 | Sum of all members | 0 | Excellent sequential and parallel throughput; one failure loses the entire array and there is no rebuild protection. | Temporary or reproducible data where speed matters more than availability. |
| RAID1 | 2 | Capacity of one member (with equal-sized disks) | Usually 1 in a two-disk mirror | Reads can be served from either copy; writes go to both. Rebuilds are straightforward but copy the complete mirror. | Small arrays that prioritize simple redundancy and fast recovery. |
| RAID5 | 3 | (Number of disks − 1) × smallest disk | 1 | Parity provides good capacity efficiency. Writes incur parity work, and a second failure during rebuild is fatal to the array. | Capacity-conscious workloads with a single-disk failure requirement and a strong backup. |
| RAID6 | 4 | (Number of disks − 2) × smallest disk | 2 | Dual parity tolerates two failures, but writes and rebuilds require more computation and I/O than RAID5. | Larger arrays where rebuild exposure is a major concern. |
| RAID10 | 4, normally an even number | About half of raw capacity | At least one; more are possible if they are in different mirrors | Mirroring plus striping gives strong read/write performance and generally shorter, less parity-intensive rebuilds. The exact failure tolerance depends on which members fail. | Databases, virtual machines, and other workloads that value predictable I/O and redundancy. |
RAID level alone does not determine safety. Disk quality, controller or backplane behavior, workload, monitoring, and the time required to obtain a replacement all matter. For a home server, RAID1 or RAID10 is often easier to recover than parity RAID; for a larger set of disks, RAID6 can provide more failure margin than RAID5.
Install mdadm and verify the package
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Refresh Ubuntu’s package metadata:
sudo apt update -
Install mdadm:
sudo apt install mdadmUbuntu may ask whether arrays should be started automatically during boot. Automatic assembly is normally appropriate for a persistent data array; you can review the resulting configuration later.
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Confirm the installed version:
mdadm --versionOn the Jammy package documented above, the relevant version is
mdadm 4.2-0ubuntu2. If the command is missing, the installation did not complete successfully.
Identify unused disks without destroying existing data
Before creating anything, map Linux device names to physical disks. Device names can change after hardware or boot-order changes, so record model, serial, and size as well as the name.
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One free scan finds every outdated or missing driver and matches the right update for your exact hardware.Free scan · exact hardware matchlsblk -o NAME,SIZE,TYPE,FSTYPE,MOUNTPOINTS,UUID,MODEL,SERIALOnly select devices that you have confirmed are the intended members. Every selected disk or partition must be unused and unmounted. Check each candidate individually:
lsblk /dev/sdb
findmnt /dev/sdbIf a filesystem is mounted, unmount it before proceeding:
sudo umount /dev/sdb1The create operation writes RAID metadata and can destroy prior contents. Do not use a disk containing an operating-system partition, a swap area, or irreplaceable files unless you have deliberately backed it up and understand the consequence. For predictable replacement later, use the same partitioning scheme and partition type on every member; whole-disk members are also possible, but you must be consistent when adding replacements.
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Create the array
The general create syntax is:
sudo mdadm --create /dev/md/<name> --level=<level> --raid-devices=<n> <devices>For example, a four-disk RAID10 using whole disks could be:
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sudo mdadm --create /dev/md/data --level=1 --raid-devices=2 /dev/sdb /dev/sdcFor RAID5 or RAID6, specify at least three or four members respectively:
sudo mdadm --create /dev/md/data --level=5 --raid-devices=3 /dev/sdb /dev/sdc /dev/sdd
sudo mdadm --create /dev/md/data --level=6 --raid-devices=4 /dev/sdb /dev/sdc /dev/sdd /dev/sdeRead the confirmation carefully. mdadm will warn that the devices contain data or that the array is being created with a particular metadata format. Confirm only after checking the device list. The kernel starts the initial resync immediately; do not power off the machine or disconnect members during this operation.
Use partitions when your design requires them
If you created matching partitions, pass those partitions rather than the whole disks, for example:
sudo mdadm --create /dev/md/data --level=1 --raid-devices=2 /dev/sdb1 /dev/sdc1A replacement must then provide a compatible partition such as /dev/sdd1. A whole-disk array and a partition-member array are different layouts; do not casually substitute one for the other.
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Use /proc/mdstat for a quick, continuously updated view:
cat /proc/mdstatTypical output identifies the array, level, active members, recovery or resync percentage, estimated time, and current speed. A line showing all expected members in brackets and a state such as [UU] indicates that a two-member mirror currently has both members present. A missing position, such as [_U], indicates degradation.
For detailed state, run:
sudo mdadm --detail /dev/md/dataCheck these fields:
- State:
cleanmeans no pending synchronization is reported;activemeans the device is running.degraded,recovering, orresyncingrequires attention. - Raid Devices and Active Devices: compare the expected count with the active count.
- Failed Devices and Removed Devices: a nonzero value identifies a member that needs investigation.
- Update Time and events: useful when comparing member metadata during recovery.
- Rebuild or resync progress: avoid rebooting or removing hardware until the operation has completed unless the machine is in an emergency.
Run the detail command again after a resync finishes. A healthy array is not merely one that is mounted; all expected members should be active and the state should no longer be degraded.
Make the array assemble after reboot
Save the array’s identity in mdadm’s configuration and rebuild the initial RAM filesystem so early boot can discover it:
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sudo mdadm --detail --scanReview the output, then append the correct array definition:
sudo mdadm --detail --scan | sudo tee -a /etc/mdadm/mdadm.conf
sudo update-initramfs -uIf the file already contains an ARRAY line for the same array, edit the file instead of creating duplicate entries. Confirm the saved definition with:
grep -E '^(ARRAY|HOMEHOST|DEVICE)' /etc/mdadm/mdadm.confAfter the array is assembled, create a filesystem if it is new. This step destroys any remaining data on the assembled device:
sudo mkfs.ext4 /dev/md/dataCreate a mount point and mount it:
sudo mkdir -p /srv/data
sudo mount /dev/md/data /srv/dataFor persistent mounting, obtain the filesystem UUID and add that UUID to /etc/fstab rather than relying on a potentially changing device name:
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sudo blkid /dev/md/dataTest the entry before rebooting:
sudo mount -aA filesystem is a separate layer from RAID. mdadm can report a clean array while the filesystem still needs repair, and expanding the array does not automatically expand the filesystem.
Replace a failed member safely
Start by confirming the failure in detail output and in the kernel log. Do not remove a disk merely because one transient read error appeared; verify which physical device corresponds to the failed Linux name.
sudo mdadm --detail /dev/md/data
lsblk -o NAME,SIZE,MODEL,SERIAL,MOUNTPOINTSMark and remove the failed member
If mdadm has not already marked it failed, mark the exact member:
sudo mdadm /dev/md/data --fail /dev/sdbThen remove it from the active configuration:
sudo mdadm /dev/md/data --remove /dev/sdbFor a partition-member array, use the partition name, such as /dev/sdb1. If the device is physically dead, the fail step may report that it is already faulty; verify that the detail output now lists it as failed or removed.
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Install and prepare a compatible replacement
Choose replacement media at least as large as the member being replaced. Match the interface and, where practical, the sector size and partition layout. If the original member used a partition, copy the partition table to the new disk using a method appropriate to your environment, then verify the resulting partition size before adding it. Never copy a partition table without checking which disk is source and which is destination.
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After the replacement appears under the expected device name, confirm that it is not mounted and contains no data you need. For a whole-disk array:
sudo mdadm /dev/md/data --add /dev/sdbFor a partition-member array:
sudo mdadm /dev/md/data --add /dev/sdb1mdadm starts a rebuild when the member is accepted. Monitor both views:
watch cat /proc/mdstat
sudo mdadm --detail /dev/md/dataLeave the system powered and the replacement connected until recovery completes. During a rebuild, the array has less failure tolerance and performance may be reduced. Investigate any second error immediately rather than assuming the first rebuild can continue safely.
Degraded RAID5 or RAID6 after an unclean stop
The Linux kernel RAID documentation warns that parity can be untrustworthy when a degraded RAID5 or RAID6 array starts after an unclean shutdown. Do not treat a degraded start as proof that every block is correct. Review member event counters, identify the most recent consistent set, and secure a backup or disk image before making a risky assembly decision.
In some recovery cases the documented assembly operation requires an explicit --force option. That option overrides mdadm’s normal safety refusal; it is not a repair command and should not be used routinely. Use it only after establishing which members belong to the same array and understanding that parity may need to be rebuilt:
sudo mdadm --assemble --force /dev/md/data /dev/sdb /dev/sdc /dev/sdd /dev/sdeSubstitute the exact, verified member list and level for your array. If you cannot establish a consistent set, stop and restore from backup or obtain specialist recovery help. A forced assembly followed by writes can make later recovery harder.
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mdadm’s --grow operation can reshape supported arrays, including adding capacity or changing the number of active RAID devices. The exact command depends on the current level, metadata, layout, and whether a replacement has first been added as a spare. Read the Jammy mdadm manual for the operation that matches your array instead of copying a command for a different level.
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A typical capacity expansion sequence is:
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Take and verify a current backup. A reshape is a long, intensive operation; a power loss or second disk failure can leave a parity array unavailable.
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Add the new, compatible disk or partition as a spare:
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sudo mdadm /dev/md/data --add /dev/sdf -
Increase the array’s device count with the appropriate grow command. For example, a four-member target count would be expressed as:
sudo mdadm --grow /dev/md/data --raid-devices=4Use the actual target count and only after confirming that the array level supports the planned reshape.
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Monitor reshape progress:
cat /proc/mdstat
sudo mdadm --detail /dev/md/data -
After mdadm reports completion and the array is clean, expand the filesystem separately. For ext4:
sudo resize2fs /dev/md/dataOther filesystems have their own online or offline grow procedures; use the filesystem’s documentation and verify the mount state required by that tool.
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Check the resulting filesystem size and run an application-level sanity check before deleting the backup or considering the operation complete.
Do not assume that adding a disk alone increases usable space. Until the reshape and filesystem resize both finish, the extra media may be an unused spare or capacity that the filesystem cannot address.
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Stop and dismantle an array
To retire an array, first stop applications and unmount every filesystem that uses it:
sudo umount /srv/dataStop the MD device:
sudo mdadm --stop /dev/md/dataRemove or update its ARRAY entry in /etc/mdadm/mdadm.conf, then rebuild the initramfs if the configuration changed:
sudo update-initramfs -uOnly after confirming the correct member devices and preserving any needed data should you erase their mdadm metadata:
sudo mdadm --zero-superblock /dev/sdb /dev/sdcUse partition names if the array used partitions. Zeroing the wrong disk can make another array unassemblable, so verify model, serial, and mount status immediately before running this command. Stopping an array does not itself erase data; formatting or zeroing the wrong device does.
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Quick Recap
Troubleshooting checklist
mdadm: device or resource busy: find mounts and holders withfindmntandlsblk; stop services and unmount the filesystem before creating, stopping, or removing a member.- A disk is absent from
lsblk: check cabling, power, the enclosure or backplane, and kernel messages. mdadm cannot rebuild onto media the operating system cannot see. - The replacement is rejected as too small: compare exact usable sector counts, not just decimal capacity labels. A replacement must be at least as large as the member’s RAID partition.
- The array assembles under an unexpected name: inspect
sudo mdadm --detail --scanand/etc/mdadm/mdadm.conf; use the UUID-based array definition and update the initramfs. - The array remains degraded after adding a disk: check
mdadm --detailfor a rebuilding or spare state, and inspect/proc/mdstatfor progress or a new read/write error. - Filesystem size did not increase: mdadm growth and filesystem growth are separate. Run the filesystem-specific resize command only after the array reshape is complete.
- Performance drops during recovery: this is expected while parity or mirror data is copied. Avoid unnecessary heavy I/O, but do not interrupt recovery without understanding the failure risk.
Operational habits that prevent emergencies
- Record each member’s serial number, slot, partition layout, RAID level, filesystem, and mount point.
- Check
/proc/mdstatandmdadm --detailafter installation, replacement, reboot, and any storage alert. - Keep a compatible spare or a tested procurement path; rebuild exposure lasts until synchronization completes.
- Test restoring files from the independent backup. A clean RAID state is not evidence that a backup exists.
- Document the exact array UUID and the recovery procedure so another administrator can identify the right disk under pressure.
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