RAID Overview

From Knowledge76

Contents 1 Overview 2 RAID Concepts 2.1 Arrays 2.2 Mirroring 2.3 Striping 2.4 Distributed Parity 2.5 Hot Swap 3 RAID Levels 3.1 RAID 0 3.2 RAID 1 3.3 RAID 5 3.4 RAID 10 3.5 RAID 50 3.6 Hot Spare

Overview

System76 Servers offer many RAID techniques to provide data integrity, fault-tolerance, throughput, or additional capacity. This document can help you decide which version of RAID is best suited for your needs.

RAID Concepts

Arrays

In the storage industry, the term “array” is used to describe two or more disk drives that appear to the operating system as a single unit.

Mirroring

Mirrored arrays write data to paired drives simultaneously. If one drive fails, the data is preserved on the paired drive. Mirroring provides data protection through redundancy.

Striping

Striping across disks allows data to be written and accessed on more than one drive, at the same time. Striping combines each drive’s capacity into one large volume. Striped disk arrays achieve highest transfer rates and performance at the expense of fault tolerance.

Distributed Parity

Parity works in combination with striping on RAID 5 and RAID 50. Parity information is written to each of the striped drives, in rotation. Should a failure occur, the data on the failed drive can be reconstructed from the data on the other drives.

Hot Swap

The process of exchanging a drive without having to shut down the system. This is useful when you need to exchange a degraded drive or a bad drive in a redundant array.

RAID Levels

RAID 0

RAID 0 provides striping, but no mirroring or redundancy of any kind. Striped disk arrays achieve high transfer rates because they can read and write data on more than one drive simultaneously. RAID 0 requires a minimum of two drives. When drives are configured in a striped disk array, large files are distributed across the multiple disks using RAID 0 techniques. Striped disk arrays give exceptional performance, particularly for data intensive applications such as video editing, computer aided design and geographical information systems. RAID 0 arrays are not fault tolerant. The loss of any drive results in the loss of all the data in that array, and can even cause a system hang, depending on your operating system. RAID 0 arrays are not recommended for high availability systems unless additional precautions are taken to prevent system hangs and data loss.

RAID 1

RAID 1 is also known as a mirrored array. Mirroring is done on pairs of drives. Mirrored disk arrays write identical data to two drives using RAID 1 algorithms. This gives your system fault tolerance by preserving the data on one drive if the other drive fails. Fault tolerance is a basic requirement for mission critical systems like web and database servers. RAID 1 requires two identical disk in your server system.

RAID 5

RAID 5 combines striping data with parity to restore data in case of a drive failure. This array type provides performance, fault tolerance, high capacity, and storage efficiency. RAID 5 requires a minimum of three drives. Parity information is distributed across all drives rather than being concentrated on a single disk. This avoids throughput loss due to contention for the parity drive.

RAID 10

This array is a combination of RAID 1 with RAID 0. Striped and mirrored arrays for fault tolerance and high performance. RAID 10 requires a minimum of four drives to use both RAID 0 and RAID 1 techniques.

When drives are configured as a striped mirrored array, the disks are configured using both RAID 0 and RAID 1 techniques, thus the name RAID 10. A minimum of four drives are required to use this technique. The first two drives are mirrored as a fault tolerant array using RAID 1. The third and fourth drives are mirrored as a second fault tolerant array using RAID 1. The two mirrored arrays are then grouped as a striped RAID 0 array using a two tier structure. Higher data transfer rates are achieved by leveraging TwinStor and striping the arrays.

In addition, RAID 10 arrays offer a higher degree of fault tolerance than other types of RAID levels (such as RAID 1 and RAID 5), since the array can sustain multiple drive failures without data loss. For example, in a twelve drive RAID 10 array, up to six drives can fail (half of each mirrored pair) and the array will continue to function. Please note that if both halves of a mirrored pair in the RAID 10 array fail, then all of the data will be lost.

RAID 50

This array is a combination of RAID 5 with RAID 0. This array type provides fault tolerance and high performance. RAID 50 requires a minimum of six drives. Several combinations are available with RAID 50. For example, on a 12-port controller, you can have a grouping of 3, 4, or 6 drives.

A grouping of 3 means that the RAID 5 arrays used have 3 disks each; four of these 3-drive RAID 5 arrays are striped together to form the 12-drive RAID 50 array.

In addition, RAID 50 arrays offer a higher degree of fault tolerance than other types of RAID levels (such as RAID 1 and RAID 5), since the array can sustain multiple drive failures without data loss. For example, in a twelve drive RAID 50 array, up to one drive in each RAID 5 set can fail and the array will continue to function. Please note that if two or more drives in a RAID 5 set fail, then all of the data will be lost.

Hot Spare

A Hot Spare is a single drive, available online, so that a redundant array can be automatically rebuilt in case of drive failure. In other words if one drive in your array is damaged the Hot Spare will be used to automatically rebuild the array without data loss or server down time.