RAID Levels Explained: RAID 0, 1, 5, 6, and 10

RAID levels define how multiple drives combine to balance speed, redundancy, and usable capacity in a single storage array. RAID, short for redundant array of independent disks, uses striping, mirroring, and parity to spread or duplicate data across two or more drives, and each level sets a different tradeoff between performance, fault tolerance, and the minimum number of drives required.

What Is RAID?

RAID is a storage technology that combines multiple physical drives into one logical unit. RAID (redundant array of independent disks) distributes data across two or more drives using striping, mirroring, or parity to improve speed, protect against drive failure, or both.

• One volume: a RAID controller or software layer presents the whole array to the operating system as a single drive.
• Three jobs: a level can raise throughput, add fault tolerance, or do both, depending on how it spreads data.
• Where it sits: RAID is a secondary storage layer that raises availability and speed beyond a single drive. The concept comes from a 1987 UC Berkeley paper.

What Are Striping, Mirroring, and Parity?

Striping, mirroring, and parity are the three data-distribution methods that define every RAID level. Striping splits data across drives for speed, mirroring duplicates data for redundancy, and parity stores calculated recovery information that rebuilds data after a failure.

What Is RAID 0?

RAID 0 stripes data across drives for maximum speed with no redundancy. RAID 0 needs a minimum of 2 drives, provides 100% usable capacity, and offers zero fault tolerance, so any single drive failure loses the entire array.

• Speed: two drives roughly double sequential read and write throughput because the controller works both in parallel.
• Capacity: 100% (n drives) , nothing is reserved for redundancy.
• Risk: failure risk rises with every drive added, since one failure destroys all data on the array.

Best for: scratch disks, video-editing caches, and reproducible temporary data where speed matters and the content can be regenerated.

What Is RAID 1?

RAID 1 mirrors identical data across drives for redundancy. RAID 1 needs a minimum of 2 drives, provides 50% usable capacity, and tolerates the failure of all but one drive in the mirror set.

• Protection: a two-drive array survives one failure with no data loss, because the surviving drive holds a complete copy.
• Performance: reads can speed up (the controller reads from either drive); writes match a single drive.
• Capacity: 50% , you pay one drive’s worth of space for the mirror.

Best for: operating-system drives, small-business servers, and any data that needs continuous availability with a simple two-drive setup.

What Is RAID 5?

RAID 5 stripes data with distributed single parity for a balance of capacity and redundancy. RAID 5 needs a minimum of 3 drives, provides usable capacity of (n-1) drives, and tolerates a single drive failure, rebuilding the lost data from parity.

• Capacity: (n-1) drives , a four-drive array gives three drives of usable space.
• Write penalty: every write recalculates parity, so writes are slower than a mirror.
• Rebuild caution: a rebuild reads every remaining drive in full; on large drives an unrecoverable read error (URE, roughly 1 per 12.5 TB on consumer disks) can fail the rebuild outright.

Best for: file servers and NAS units that need capacity efficiency with single-drive protection, using smaller or enterprise-rated drives.

What Is RAID 6?

RAID 6 extends RAID 5 with double parity for tolerance of two drive failures. RAID 6 needs a minimum of 4 drives, provides usable capacity of (n-2) drives, and tolerates the simultaneous failure of any two drives.

• Capacity: (n-2) drives , a six-drive array gives four drives of usable space.
• Safer rebuilds: the second parity block protects against a second drive failing (or a URE) during a long rebuild.
• Cost: double parity adds more write overhead than RAID 5.

Best for: large arrays of high-capacity drives where rebuild times are long and a concurrent second failure is a real risk , the modern default over RAID 5 on big disks.

What Is RAID 10?

RAID 10 combines mirroring and striping (1+0) for both speed and redundancy. RAID 10 needs a minimum of 4 drives, provides 50% usable capacity, and tolerates one drive failure per mirror set.

• Layout: drives are mirrored in pairs, then striped across the mirrors , a four-drive array is two mirrored pairs.
• Performance: high read and write speed plus redundancy, with no parity-calculation penalty.
• Fast rebuilds: recovery copies from the surviving mirror instead of recalculating parity.

Best for: databases and transactional workloads that need both high performance and strong fault tolerance, accepting the 50% capacity cost.

How Do You Calculate Usable Capacity?

Usable capacity follows a fixed formula per level based on drive count. RAID 0 yields n drives, RAID 1 yields n/2, RAID 5 yields (n-1) drives, RAID 6 yields (n-2) drives, and RAID 10 yields n/2 of the raw total. Arrays assume equal-size drives; a mixed-size array limits every drive to the smallest member.

• RAID 0 = n drives , the full raw total, nothing reserved.
• RAID 1 = n/2 drives , half, because every block is duplicated.
• RAID 5 = n-1 drives , one drive’s worth reserved for distributed parity.
• RAID 6 = n-2 drives , two drives’ worth reserved for double parity.
• RAID 10 = n/2 drives , half, from mirroring inside striped pairs.

What Is the Difference Between Hardware and Software RAID?

Hardware and software RAID differ in where the array logic runs. Hardware RAID uses a dedicated controller card with its own processor and cache, while software RAID runs the array logic on the host CPU through the operating system.

Both present the array to the operating system as one volume. The choice comes down to budget and CPU headroom versus the throughput and write protection of a dedicated controller.

Why Is RAID Not a Backup?

RAID is not a backup because it protects against drive failure, not data loss. RAID keeps the array available when a drive fails, but it does not protect against accidental deletion, file corruption, ransomware, or controller failure, since those errors hit every drive at once.

• RAID handles: a single (or double) physical drive failure without downtime.
• RAID does not handle: deletion, corruption, ransomware, controller failure, fire, theft, or operator error.
• Do both: run RAID for uptime and a separate backup for data protection, and pick the array around the right underlying drives for endurance.

RAID Levels Comparison

The table compares the five common RAID levels across the attributes that drive selection:

Last Thoughts on RAID Levels

RAID levels balance speed, redundancy, and usable capacity across multiple drives. RAID 0 maximizes speed, RAID 1 and RAID 10 mirror for simple redundancy, and RAID 5 and RAID 6 use parity to tolerate one or two drive failures with better capacity efficiency. The minimum drive count, fault tolerance, and capacity formula together guide the right level for a workload, and because RAID protects uptime rather than data, a separate backup remains essential.

Continue with how to choose a storage drive, compare HDD and SSD options, and use the computer hardware guide as the central reference.

Key Takeaways:

• RAID combines drives using striping, mirroring, and parity to balance speed, redundancy, and capacity.
• RAID 0 maximizes speed with no redundancy, needing 2 drives and risking total loss on one failure.
• RAID 1 and RAID 10 mirror data, giving 50% usable capacity and fast, simple redundancy.
• RAID 5 and RAID 6 use parity, tolerating one and two drive failures with (n-1) and (n-2) capacity.
• Hardware RAID offloads to a controller, while software RAID runs on the host CPU at lower cost.
• RAID is not a backup, because deletion, corruption, and ransomware propagate across every drive at once.

Frequently Asked Questions (FAQs)