Using RAID 5 With Your Mac

Fault Tolerant With Fast Read Times

RAID Server System
Mienny | Getty Images

RAID 5 is a striping RAID level designed to increase the speed of disk reads and writes. RAID 5 is similar to RAID 3 in that it uses a parity bit to help 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. Once the failed drive is replaced, the RAID 5 array can enter a data recovery mode, where the parity data in the array is used to rebuild the missing data on the newly installed drive.

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 following formula:

S = d * (n - 1)

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

Best Use for RAID 5

RAID 5 is a good choice for multimedia file storage. Its read speed can be very high, while the write speed is 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 will be defined by the smallest disk in the set (see formula above).

Due to the need to perform parity calculations and distribute the resulting calculation, RAID 5 is best when performed in hardware-based RAID enclosures. The Disk Utility app included with OS X 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.