Does RAID 1 have fault tolerance?

RAID 1, also known as disk mirroring, is a level of RAID that provides fault tolerance by writing identical copies of data to paired drives. This redundancy allows for continued availability and integrity of data if one drive fails. So the quick answer is yes, RAID 1 does provide fault tolerance through disk mirroring.

What is RAID 1?

RAID 1 is a RAID configuration that utilizes disk mirroring to duplicate all data from one hard drive to a second hard drive in real-time. This means any writes to the primary disk are instantly copied to the secondary disk. The two disks contain identical data but are treated by the system as a single logical drive.

The main purposes of RAID 1 are to provide fault tolerance and improve read performance. If one disk fails, the system can instantly switch to the other disk without any loss of data or interruption in service. Read performance is improved by distributing reads across both disks.

Key characteristics of RAID 1:

  • Requires at least 2 physical disks
  • Data is mirrored from one disk to another
  • Provides fault tolerance with no single point of failure
  • Improves read performance by spreading reads across disks
  • No parity calculation overhead like in RAID 5
  • 50% storage efficiency as the capacity is equal to a single disk

RAID 1 is one of the most popular RAID levels and provides a simple way to add redundancy to storage. It requires no parity calculation overhead like RAID 5. However, the tradeoff is lower storage efficiency as the total capacity is equal to a single disk.

How does RAID 1 provide fault tolerance?

RAID 1 delivers fault tolerance through disk mirroring. With two identical copies of all data, RAID 1 can survive a single disk failure without any data loss or system downtime. If one disk fails, the system seamlessly fails over to the surviving disk.

Some key ways RAID 1 provides fault tolerance include:

  • Disk mirroring – All data is written twice so there are always two copies
  • Redundant hardware – At least two disks are required so there is no single point of failure
  • Automatic failover – The system instantly switches to the surviving disk if one disk fails
  • Easy recovery – The failed drive can be replaced and rebuilt with the mirror copy

RAID 1 is able to withstand a single disk failure with zero downtime. Compare this to RAID 0 which has zero redundancy – the failure of any single disk would lead to complete data loss and system downtime. The mirroring and redundancy of RAID 1 provide much greater fault tolerance.

Rebuilding after a failed disk

When a disk does eventually fail in a RAID 1 array, rebuilding the data is straightforward. The failed drive simply needs to be replaced with a new blank drive which will be rebuilt using the data from the surviving mirrored disk. The rebuild process copies all the data from the intact drive to the new replacement drive. This restores redundancy and protection against a future second disk failure.

RAID 1 vs RAID 0 – Which is more fault tolerant?

RAID 0 and RAID 1 take very different approaches to managing multiple disks that highlight the fault tolerance difference:


  • Splits and stripes data across two or more disks
  • Provides improved performance
  • No redundancy – loss of any disk causes complete failure
  • Very poor fault tolerance


  • Mirrors disks by duplicating data
  • Slower write performance than RAID 0
  • Can survive the loss of a single disk
  • Excellent fault tolerance

While RAID 0 offers faster performance, it has zero fault tolerance. RAID 1 provides fault tolerance through redundancy but has slower writes. For mission critical data that needs redundancy, RAID 1 is the clear winner for fault tolerance.

RAID 10 – Mirroring RAID 1 arrays

RAID 10 takes things a step further by mirroring two RAID 1 arrays together. This provides the redundancy of RAID 1 along with the performance benefits of RAID 0 striping:

  • Create two separate RAID 1 arrays, each with mirrored disks
  • Stripe data across the two RAID 1 arrays
  • Fast performance from the RAID 0 striping
  • Fault tolerance from the RAID 1 mirroring

RAID 10 requires a minimum of 4 disks. If a disk fails in one of the RAID 1 arrays, the mirror in that array will take over just like with a standard RAID 1 setup. This provides fault tolerance up to two drive failures – the loss of both disks in a single mirrored array.

The combination of redundancy and striping makes RAID 10 one of the top choices for both performance and reliability. However, 50% of the total storage capacity is lost to redundancy just like with RAID 1.

RAID 1 in the real world

In practice, RAID 1 is a simple and effective way to provide fault tolerance for home computers, servers, and mission critical systems:

  • Operating system drives – Mirroring the boot drives safeguards against downtime from a failed OS drive.
  • Business servers – RAID 1 protects against data loss and improves uptime.
  • Transactional databases – The data redundancy guarantees transactions can be committed.
  • Virtualization – Hypervisors store virtual machine files on mirrored arrays.
  • Critical infrastructure – Power plants, hospitals, emergency systems often utilize RAID 1.

Any application that requires maximum uptime and reliability is a good candidate for deployment on RAID 1 arrays. The cost is simply having double the number of disks.

RAID 1 best practices

To maximize performance and fault tolerance with RAID 1, it’s important to follow some best practices:

  • Use identical disks from the same manufacturer and model
  • Configure separate RAID 1 arrays for operating system vs data
  • Use higher capacity disks to reduce the number of arrays
  • Spread RAID 1 disk pairs across separate controllers
  • Monitor disk health to promptly replace failed drives

Careful configuration and maintenance of the RAID 1 arrays will optimize both the performance and redundancy capabilities.

What are the downsides of RAID 1?

RAID 1 is simple to implement and delivers excellent fault tolerance. However, there are some downsides to the approach:

  • 50% storage efficiency – Array capacity is equal to a single disk
  • Slower write performance than RAID 0 due to mirroring writes
  • Requires at least two disks – increased hardware cost
  • Doesn’t fully utilize disk I/O bandwidth

The redundancy comes at the cost of reduced storage capacity and slower write speeds. RAID 1 is also pricier to implement since it requires a minimum of two disks versus a single disk.

When is RAID 1 inappropriate to use?

While RAID 1 is great for mission critical data, it may be overkill in certain use cases:

  • Low I/O web servers
  • High capacity video storage
  • Big data analytics clusters
  • Backup repositories

For large storage pools where fault tolerance is less critical, alternatives like RAID 5 and RAID 6 provide more efficient capacity utilization. RAID 1 makes less sense when data redundancy is not a priority.


RAID 1 delivers simple, effective fault tolerance by mirroring disks and removing single points of failure. The redundancy comes at the cost of halved storage capacity and slower writes. But for mission critical systems and data that require maximum uptime, RAID 1 remains a popular choice.

The mirroring capability of RAID 1 allows it to recover seamlessly from disk failures. Combined with hot spares and monitoring, downtime can be minimized. For these reasons, RAID 1 continues to be widely used in servers, virtualized infrastructure, and other technologies requiring high availability.