RAID (Redundant Array of Independent Disks) is a data storage technology that combines multiple disk drive components into a logical unit. RAID provides increased storage performance and reliability through redundancy. Using RAID can protect your data from drive failures. The Mac Disk Utility application included with Mac OS allows you to manage RAID configurations.
Overview of RAID
RAID uses multiple disks to achieve different goals including increased data reliability, better performance, or both. The specific type of RAID configuration determines how the disks are used:
- RAID 0 (striping) – Data is split across multiple disks for faster reads and writes. There is no redundancy in RAID 0.
- RAID 1 (mirroring) – Disks are mirrored for complete data redundancy. If one disk fails, the data can still be accessed from the other disk.
- RAID 5 – Data is striped across disks like RAID 0, but parity information is also distributed across the disks to allow for data rebuild in case of failure.
- RAID 10 – Combines both RAID 0 striping and RAID 1 mirroring for speed and redundancy.
The core advantage of RAID is protection against disk failures. By using multiple disks, RAID can ensure continued access to data even if one of the drives fails. The specific RAID level determines the amount of redundancy and how many disk failures can be tolerated.
RAID in Mac Disk Utility
Mac Disk Utility provides a simple graphical interface to manage RAID configurations. The main features include:
- Create RAID sets – Combine multiple physical disks into a RAID 0, 1, or 10 set.
- Add disks to a RAID set – Expand storage capacity of an existing RAID configuration.
- Remove disks from a RAID set – Migrate data off a disk to remove it from a RAID.
- Rebuild degraded RAID sets – Repair RAID after a disk failure.
- Validate and repair RAID sets – Verify and fix RAID issues.
Disk Utility can work with the internal drive bays in Mac desktops like the Mac Pro, as well as external Thunderbolt RAID enclosures. This allows flexible RAID configurations using a mix of internal and external drives.
Creating a RAID set
To create a new RAID array in Disk Utility:
- Open Disk Utility (found in Applications > Utilities).
- Click the File menu and select “RAID Assistant” to start the RAID configuration wizard.
- Select your desired RAID type – RAID 0, 1, or 10.
- Select the physical disks to include in the RAID.
- Choose options like encryption and disk formatting.
- Click Create RAID Set to build the RAID.
The RAID volume will appear in Disk Utility after it finishes creating. You can manage it like any other disk.
Adding and removing disks
To expand a RAID set, click on the RAID volume in Disk Utility then choose Edit > Add Disk to RAID Set. Select a new disk to add to the array. The RAID will rebuild to expand capacity and redundancy.
To remove a disk, click the RAID volume and choose Edit > Remove Disk from RAID Set. Then select the disk you want to take out of the array. The data will migrate off that disk to the remaining disks in the background.
Repairing RAID issues
If Disk Utility reports an issue with a RAID set, you can try to repair it. With a redundant RAID type like 1, 5, or 10, you may be able to rebuild after a disk failure.
First, identify any failed disks causing degradation. Replace those disks if possible. Then select the degraded RAID volume and choose Edit > Repair RAID Set. Disk Utility will rebuild the set to restore redundancy and access to the full volume.
For soft RAID issues without disk failures, use Edit > Validate RAID Set to check for inconsistencies. Disk Utility can repair any logical issues to restore full RAID functionality.
The RAID type impacts performance characteristics. RAID 0 stripes data across multiple disks with no redundancy, which improves read and write speeds. But a disk failure results in total data loss. RAID 1 mirrors disks to protect data, but write speeds are slower compared to a single disk.
This table compares RAID performance:
|RAID Type||Read Speed||Write Speed||Redundancy|
|5||Fast||Medium||Single Disk Fault Tolerance|
RAID 0 has the best performance but no fault tolerance. RAID 10 balances speed and redundancy for more general use. The tradeoffs depend on your specific storage needs.
Choosing RAID Levels
Consider these factors when choosing a RAID level:
- Performance requirements – RAID 0 and 10 offer the best speed.
- Needed redundancy – RAID 1 and 10 provide full redundancy. RAID 5 has single disk fault tolerance.
- Number of disks available – RAID 10 needs a minimum of 4 disks. RAID 5 needs at least 3 disks.
- Planned capacity – RAID 0 and 10 provide less overall storage than combined disk capacity. RAID 1 halves available storage.
Also weigh the increased complexity vs. benefits when considering RAID. A single disk is simpler and may be adequate for some use cases.
These are suggested RAID scenarios for different needs:
- General storage: 2-4 disk RAID 10 – Fast and fully redundant.
- Photo/video editing scratch disks: 3+ disk RAID 0 – High speed for throughput.
- Backup storage: 2 disk RAID 1 – Mirroring for redundancy.
- OS boot drives: RAID 1 – Peace of mind from mirroring system disks.
Expanding RAID Capacity
One benefit of hardware RAID over a single disk is the ability to expand capacity. With Disk Utility’s RAID tools, there are a couple approaches to gain more storage:
- Add disks – Insert new drives and add them to the existing RAID set to gain space.
- Replace with larger disks – Swap out the disks with larger ones to grow capacity.
Adding new disks is simpler. But replacing with larger disks allows bigger incremental expansions in capacity. Either way, expanding the underlying disks translates to more space available in the RAID set.
RAID expansion steps
To expand a RAID set’s capacity:
- Obtain new drives that are equal or larger in capacity compared to the existing RAID disks.
- Install the new disks into open drive bays or enclosures.
- Open Disk Utility and select the RAID volume.
- Choose Edit > Add Disk to RAID Set and select the new disks.
- The RAID volume will rebuild and expand to utilize the additional storage.
Ideally use matching disks from the same vendor and model when expanding. Larger disks can be added but unused space may be wasted.
Dealing with Disk Failures
The main benefit of RAID is protection against disk drive failure. If a drive fails in a redundant RAID configuration, the data remains accessible from the other disks.
If Disk Utility reports a failed or failing drive in the RAID set, take these actions:
- Back up data if possible – Backup critical data in case multiple disks end up failing.
- Replace the failed drive – Swap in a new replacement disk.
- Repair the RAID set – Initiate rebuild through Disk Utility to restore redundancy.
- Restore backups – If rebuild fails due to additional disk issues, restore from backups.
With RAID 1 or 10, you can continue accessing data during the rebuild process after swapping the failed drive. RAID 5 will be degraded and inaccessible until finished rebuilding parity.
Avoiding full failure
Monitor Disk Utility regularly to check if disks are exhibiting signs of failure before they lead to bigger issues:
- Watch for increasing drive errors reported in Disk Utility.
- Keep an eye on the drive health meters for any degradation.
- Look for slower disk performance that could indicate problems.
- Listen for unusual drive noises which often precede failure.
Addressing disks that are starting to fail early on can help avoid a full RAID failure. Be proactive and preemptively swap out suspect drives.
Migrating Off of RAID
There are situations where you may want to discontinue using a RAID set and migrate back to standalone disks. For example:
- Transitioning to a new storage architecture
- Upgrading to larger capacity individual drives
- Scaling back redundancy for budget reasons
To migrate away from RAID:
- Backup all RAID data – Ensure backups exist first.
- Break the RAID set – Use Disk Utility to delete the RAID volume.
- Erase disks – Disk Utility can erase the RAID member disks.
- Reformat disks – Reformat the erased disks as independent volumes.
- Restore data – Migrate data back from the backups.
Traditional RAID makes migrating off arrays complex since data is spread across disks. Filesystem-based software RAID solutions like ZFS simplify migration.
Alternative Filesystem RAID Options
In addition to Disk Utility’s RAID management, there are also filesystems for Mac that support RAID:
- ZFS – Supports software RAID with redundancy and bit rot protection.
- APFS – Can be formatted with software RAID 0 striping.
- SoftRAID – Third party software RAID that works at the filesystem level.
These filesystems handle RAID at the software level for more flexibility. Features like storage pools and volume management can facilitate easier RAID administration and capacity expansion.
Comparing hardware vs. software RAID
|Attribute||Hardware RAID||Software RAID|
|Flexibility||Less flexible||More flexible|
|Cost||More expensive||Less expensive|
|Ease of expansion||Difficult||Easier|
Software RAID through something like ZFS provides a compelling option for many use cases. But hardware RAID can still outperform in speed-sensitive environments.
Uses Cases for RAID on Mac
There are a variety of scenarios where using RAID on a Mac can be beneficial:
- Video editing – Fast RAID 0 scratch disks improve editing performance.
- Photos – RAID 1 or 10 provides redundancy for irreplaceable photos.
- Backups – RAID protects against backup disk failures.
- Network Attached Storage – NAS appliances often use RAID for capacity and uptime.
- Boot drives – Mirror system drives with RAID 1.
- Servers – Fileservers and databases benefit from RAID’s performance and reliability.
RAID is most beneficial for I/O intensive workloads using large amounts of storage. It provides added speed and redundancy compared to single disks.
When not to use RAID
RAID has downsides as well. It may not be ideal in these cases:
- Low capacity storage needs – Single drives often suffice.
- No need for redundancy – Like secondary storage.
- Budget constraints – RAID has higher startup costs.
- Software can’t use RAID – Some backup software lacks RAID support.
Weigh the pros and cons versus alternatives like single larger drives or cloud storage when deciding if RAID fits your needs.
Key takeaways on RAID in Mac Disk Utility:
- RAID combines multiple disks for improved performance, redundancy, or both.
- Popular RAID types are 0, 1, 5, and 10 suited for different use cases.
- Disk Utility provides a simple interface to configure and manage RAID.
- RAID can expand storage capacity by adding more disks.
- For drive failures, RAID allows continued operation once the failed disk is replaced.
- Filesystem RAID options like ZFS provide an alternative to hardware RAID.
- Use RAID for mission critical data that needs speed, redundancy, or increased storage.
RAID can deliver valuable benefits like speed and reliability in the right situations. Mac Disk Utility gives easy access to RAID for both protection and performance.