Does RAID 1 need even number of drives?

What is RAID 1?

RAID 1, also known as disk mirroring, is a type of RAID (Redundant Array of Independent Disks) configuration that creates an exact copy of data on two or more disks ( The main purpose of RAID 1 is to provide redundancy in case one disk fails. Because it duplicates data across multiple disks, RAID 1 requires a minimum of two disks in order to function.

With RAID 1, any write operation is performed to both disks simultaneously. This provides fault tolerance and protects against data loss if one disk fails. The RAID 1 array can continue operating with data redundancy as long as at least one disk is functioning. Some key advantages of RAID 1 are increased read performance, easy recovery from disk failure, and minimum overhead for reconstructing a failed drive’s data from its mirror.

How Does RAID 1 Work?

RAID 1, also known as disk mirroring, is a storage configuration where data is written to two or more identical drives simultaneously. This creates an exact copy of the data across the drives (1). If one drive fails or becomes corrupted, the system can continue operating using the data from the other intact drive(s).

When data is written to a RAID 1 array, the RAID controller duplicates the write commands to each drive in the array. This results in an exact copy being written across all the drives. For example, in a 2-drive RAID 1 array, Drive 1 and Drive 2 will contain identical data. If Drive 1 fails, Drive 2 still has an intact copy of all the data (2).

A key benefit of RAID 1 is redundancy and fault tolerance. If a single drive fails, operations can continue uninterrupted using the remaining drive(s). This minimizes downtime and data loss. Additionally, the failed drive can be replaced and the RAID 1 array rebuilt using the data from the good drive(s) (3).

Does RAID 1 Require an Even Number of Drives?

RAID 1 does not actually require an even number of drives. It is possible to implement RAID 1 with an odd number of drives, such as 3, 5, 7, etc.

However, the benefits of RAID 1 are minimized when using an odd number of drives. RAID 1 provides redundancy by mirroring data across multiple drives. With an even number of drives, the redundancy is perfect – each drive has an exact duplicate. But with an odd number of drives, one drive will not have a duplicate mirror.

For example, in a 3-drive RAID 1 array, two of the drives contain identical data while the third drive contains unique unmirrored data. If one of the mirrored drives fails, the array still functions but is left with no redundancy for the unique third drive until the failed drive is replaced.

While possible, most experts recommend using an even number of drives in RAID 1 to maximize fault tolerance through full data mirroring across drives.

Ideal RAID 1 Configurations

The most common RAID 1 configuration is with two drives. With two drives, RAID 1 provides mirroring so that data is written identically to both drives. This allows for fault tolerance if one drive fails. While two drives is standard, RAID 1 can support up to 32 drives.

According to Seagate, “RAID 1 is most often implemented with two drives. Data on the drives is mirrored, providing fault tolerance in case of drive failure.”1 With two identically sized drives, the usable capacity in a RAID 1 configuration is equivalent to one disk drive.

The Oracle documentation notes that “If the RAID 1 array is built from different-sized disk drives, drive segment size is the size of the smallest disk.”2 Therefore, when using drives of different sizes, the total capacity available will be equal to the smaller drive.

While most common in pairs, RAID 1 can scale up to 32 drives. The usable capacity remains the same regardless of adding additional mirrored drives. The benefit is added redundancy and potential read performance gains.

Performance Impact of Odd vs Even Drives

In a RAID 1 array, data is mirrored between drives to provide redundancy. With an even number of drives, each drive has a dedicated mirror drive. However, with an odd number of drives, one drive will not have a mirror.

This can potentially impact performance in a few ways:

  • The unmirrored drive becomes a single point of failure. If that one drive fails, all data will be lost.
  • Read performance may suffer slightly since reads cannot always be balanced evenly across all drives. The unmirrored drive can become a bottleneck.
  • Write performance takes a bigger hit compared to read performance. With an even number of mirrored drives, writes can happen in parallel. But with an odd number, the unmirrored drive must handle all writes sequentially.

In summary, having an odd number of drives in a RAID 1 array can negatively impact performance compared to having an even number of mirrored drives. The magnitude depends on the workload, but writes will take the biggest performance hit from losing parallelization.1

Reliability Impact of Odd Drives

One of the key disadvantages of using an odd number of drives in a RAID 1 configuration is that the data on the unmirrored drive is not redundant. As this Tom’s Hardware forum post discusses, RAID 1 requires drives to be mirrored in pairs. With an odd number of drives, one drive will not have a mirrored partner drive.

This means that if that unpaired drive fails, the data on it will be lost. There is no backup copy of that data on another mirrored drive like there is for the rest of the array. As a result, a RAID 1 setup with an odd number of drives has lower reliability and fault tolerance compared to one with an even number of mirrored drives.

The unmirrored drive is a single point of failure. If it goes down, all the data on it will be inaccessible until it is replaced. This introduces a risk of downtime and potential data loss not present in a fully mirrored RAID 1 configuration. For maximum redundancy, it’s recommended to use an even number of drives in RAID 1.

When to Use Odd Number of Drives

There are some situations where using an odd number of drives in RAID 1 can make sense:

When adding to an existing array – If you already have an established RAID 1 array with an even number of disks, adding 1 additional disk can allow you to expand the array while keeping it in a mirrored configuration. The odd disk would be mirrored across the existing disks.1

Budget constraints – RAID 1 requires a minimum of 2 disks, so if budget is a concern, starting with 3 disks instead of 4 allows you to implement mirroring with fewer disks upfront. You can always add a 4th disk later to make it an even number if needed.2

While possible, odd drive RAID 1 configs are not ideal long-term solutions. The performance and reliability benefits of RAID 1 are maximized with an even number of disks. Odd drive counts should only be considered temporary or in specific edge cases.

Best Practices

When configuring RAID 1 arrays, it is generally recommended to use an even number of drives if possible. This allows for full disk mirroring in pairs. According to the Microsemi article, “RAID 1E is best suited for systems with three drives. For scenarios with four or more drives, RAID 10 is recommended.”

If using an odd number of drives is unavoidable, it is best to use at least 3 drives. As the HostDime article explains, “With only two disks, RAID 1 provides fault tolerance but no performance gains. Adding a third disk allows the array to handle some read requests in parallel for better speed.” This provides some redundancy, with one disk able to mirror data if another fails.

The LinkedIn article also advises initializing and formatting the array after configuring it, then testing it thoroughly. Following best practices helps ensure optimal performance and reliability from a RAID 1 array.

Alternatives to Odd RAID 1 Drives

While using an odd number of drives in RAID 1 is possible, it is not considered a best practice. There are some alternative RAID configurations that are better optimized for using an odd number of drives.

One popular alternative is RAID 10. This combines both striping and mirroring for improved performance and redundancy. RAID 10 requires an even number of drives, but they are grouped into mirrored pairs first. For example, with 5 drives you could have two mirrored pairs, and the remaining drive could be used for striping across the RAID 10 array for a total of 4+1 drives. This provides both speed and protection against drive failures.1

Another option is RAID 5. This uses striping with distributed parity, so it only requires a minimum of 3 drives. Parity allows the array to withstand a single drive failure by reconstructing the missing data. The tradeoff is slower write speeds due to parity calculations. With 5 drives, a RAID 5 array would use all disks for full capacity while still providing fault tolerance.1

In summary, both RAID 10 and RAID 5 provide alternatives to RAID 1 that can fully utilize an odd number of drives. RAID 10 optimizes for performance while RAID 5 offers maximum storage capacity and cost efficiency.


In summary, while RAID 1 can technically be configured with an odd number of drives, best practices recommend using an even number of drives for optimal performance and reliability. The key points are:

  • RAID 1 makes an exact copy of data across multiple drives for redundancy.
  • With an odd number of drives, one drive’s capacity will go unused which is inefficient.
  • Performance can suffer slightly with an odd number of mismatched drives.
  • Reliability decreases slightly with an odd number, as the array is more vulnerable to failure during rebuilds.
  • 2, 4, or 6 drives are recommended for best RAID 1 performance and efficiency.

In conclusion, I would recommend configuring RAID 1 with an even number of matched drives, ideally 2, 4, or 6 drives. The performance and reliability gains of even drives outweigh the flexibility of adding an odd spare drive. Unless you specifically need an extra hot spare, stick to pairs or quads for the most optimized RAID 1 implementation.