Using RAID 5 With Your Mac

Store your data on a fault-tolerant RAID 5 array

An RAID array is a storage solution that combines multiple hard drives into one unit for the purpose of storing, backing up, and providing redundancy and security. RAID 5, with disk striping and parity, is ideal for use as a file-storage server or application server. This choice for Mac users requires a minimum of three drives and provides fault tolerance and good performance.

RAID Server System
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A redundant array of independent disks (RAID) protects against the failure of a single drive and provides improved performance and fast transfer rates by storing data across multiple disks.

Information in this article applies to RAID 5 with a Mac running macOS Catalina (10.15) through macOS Sierra (10.12).

RAID Features

There are several RAID levels including RAID 0, RAID 1, RAID 3, RAID 5, RAID 6, and RAID 10. Each RAID category possesses one or more of these features:

  • Disk striping refers to dividing data into blocks and writing the blocks across several storage devices.
  • Disk mirroring refers to duplicating data on two or more disks.
  • Parity bit calculates the data in two drives and stores the results on a third drive for the purpose of providing fault tolerance.

RAID 5 is a cost-effective option that provides excellent performance and redundancy in high-read environments.

About RAID 5 and the Mac

RAID 5 is a striping RAID level designed to increase the speed of disk reads and writes. Many Mac users opt for RAID 5 for multimedia file storage. Its read speed is fast, and the write speed is only slightly slower, due to the need to calculate and distribute the parity.

RAID 5 excels at storing large files, where data is read sequentially. Smaller, randomly accessed files have mediocre read performance, and write performance can be poor due to the need to recalculate and rewrite the parity data for each write operation.

Although RAID 5 can be implemented with mixed disk sizes, that isn't considered the preferred approach since the RAID 5 array size is defined by the smallest disk in the set.

Calculating RAID 5 Array Size

RAID 5 arrays use the equivalent of a drive for storing parity, which means the overall array size can be calculated using the formula:

S=d*(n-1)

The d is the smallest disk size in the array, and n is the number of disks that make up the array.

How RAID 5 Works

RAID 5 is similar to RAID 3 in that it uses a parity bit to ensure data integrity. However, unlike RAID 3, which uses a disk dedicated to storing the parity, RAID 5 distributes the parity to all drives in the array.

RAID 5 provides for drive failure tolerance, allowing any single drive in the array to fail without losing any data in the array. When a drive fails, the RAID 5 array can still be used to read or write data. After the failed drive is replaced, the RAID 5 array enters a data recovery mode, in which the parity data in the array is used to rebuild the missing data on the newly installed drive.

Software-Based vs. Hardware-Based Controllers

Due to the need to perform parity calculations and distribute the resulting calculation, RAID 5 is at its best when operating in a hardware-based RAID enclosure.

There are two types of RAID array controllers: hardware and software. Software-based controllers cost less and give the user flexibility when configuring the drives. Hardware-based controllers cost more but are recommended for complex arrays.

The Disk Utility app included with Macs doesn't support creating software-based RAID 5 arrays. However, SoftRAID, from third-party developer SoftRAID, Inc., can be used if a software-based solution is needed.