RAID Concepts and Terminology

Learn common RAID concepts and the terms used in RAID Manager and this user manual.

Key concepts

RAID (Redundant Array of Independent Disks) combines multiple physical drives into one logical storage unit (an array).
Different RAID levels determine how data is distributed across drives and how much protection you have if a drive fails. RAID levels are typically a tradeoff between three goals:
- Capacity—How much usable space you get.
- Performance—How fast data can be read and written.
- Protection—How many drive failures the array can tolerate before before there is potential for data loss.
Some RAID levels use striping with no redundancy to prioritize performance and capacity over data protection. There are also RAID levels that provide critical data protection using parity or mirrored copies of data.

Terminology

Configuration

Term	Meaning
Array	A combination of two or more physical drives presented to the operating system as a single volume. In many contexts, “array” is used to mean a virtual disk (vdisk). Note—Although an array is presented as a single volume, it can be partitioned by the operating system disk utility into multiple volumes, each of which may be formatted differently. Operating system disk utilities are Disk Utility (macOS) and Disk Management (Windows).
RAID	“RAID” contains the word “array,” and the two terms are often used interchangeably in user-facing documentation.
RAID level	The method used to distribute and protect data across the drives in an array (for example, RAID 0, RAID 1, RAID 5, RAID 6, RAID 10, RAID 50, RAID 60).
Striping	Data is split into blocks and written across multiple drives to improve performance. RAID 0 uses striping with no parity or redundancy.
Stripe	A single repeating data block layout pattern used by striped RAID.
Stripe size	The amount of data (typically measured in KB) written to one drive before the controller moves to the next drive in the array. Larger stripe sizes generally suit large, sequential transfers (video, audio, graphics), while smaller stripe sizes can suit smaller, mixed workloads.
Parity	Extra information calculated from data that enables recovery after a drive failure. RAID 5 uses one parity block (“P”) that rotates across drives; RAID 6 adds a second parity (“Q”) for additional protection.
Mirror / mirroring	Two drives contain identical data. With RAID 1, reads can be serviced from either drive; writes go to both.
Spare drive	A drive designated to take over for a failed drive so the device's system can immediately rebuild an array to maintain data redundancy. While a spare drive is very helpful for immediately replacing a failed drive, it remains in reserve and cannot be used to store data. As a result, a spare drive is optional and must be explicitly created. Spare—A spare drive dedicated to a single array. Global spare—A spare drive that can be used by any array on the device. Recommended for devices with multiple arrays.

Operations and health

Term	Meaning
Initialization	A process that prepares an array and can help prevent errors when handling data.
Background initialization	Background initialization is a check for media errors on the drives when creating an array. This check ensures that striped data segments are the same on all of the drives in the array.
Foreground initialization	An initialization that completes faster because it requires the device to be disconnected from the host. The device cannot be used for data operations during a foreground initialization.
Rebuild	The process of restoring redundancy after a drive failure. When a replacement drive takes over for a failed drive, the redundant data is rebuilt on the new drive. Array performance may be affected during a rebuild. Note—A rebuild can also occur if drives are swapped from their original bays. To avoid unnecessary rebuilds, do not move drives from their original bays.
Degraded	A condition where an array has reduced protection and may have reduced performance.
Consistency check	A maintenance operation that tests the integrity of parity data.

Operations such as initialization and rebuild can affect performance and data protection. Always back up important files before making changes to an existing array.

Summary of RAID levels

Use the table below as a quick overview of what each RAID level is designed to do. For more detailed descriptions of available RAID levels, see RAID Levels.

RAID level	Summary
RAID 0 (striping)	Stripes data across drives for performance and capacity with no redundancy.
RAID 1 (mirroring)	Writes identical data to two drives for protection.
RAID 5	Stripes data with rotating parity and survives one drive failure.
RAID 6	Stripes data with rotating parity and survives up to two drive failures.
RAID 10	A stripe of mirrored pairs.
RAID 50	A stripe of RAID 5 sets.
RAID 60	A stripe of RAID 6 sets.

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