Introduction to RAID Storage Systems

Understanding RAID (Redundant Array of Inexpensive Disks)

The basic idea of RAID is to combine multiple small, inexpensive disk drives into a group which yields performance exceeding that of one large, more expensive drive. This array of small drives is made to appear to the computer as a single virtual drive. In addition, the array can be made fault-tolerant by redundantly storing information in various ways.

Types of RAID

There are five types of array architectures, RAID-1 through RAID-5. Each type provides disk fault-tolerance and each offers different trade-offs in features and performance. In addition to these five redundant array architectures, it has become popular to refer to a non-redundant array of disk drives as a RAID-0 array.

Among the five types, only RAID-1, RAID-3 and RAID-5 are commonly used. RAID-2 and RAID-4 do not offer any significant advantages over these other types.

RAID Levels

Please click here to download illustration image files for all RAID levels 0 - 5. (70 KB)

RAID-1 vs. RAID-5

Two of the most popular RAID levels in use today are RAID-1 and RAID-5. What are the advantages and disadvantages of each?

Their relative performance can be seen clearly when the read and write bandwidths for both RAID-1 and RAID-5 arrays are plotted versus the number of drives in the array. In order to double the write performance of a single drive, a RAID-1 array would require 4 drives, and a RAID-5 array would require at least 7 drives. This makes RAID-1 the clear choice for performance in transaction processing and database environments. Conversely, RAID-5 is the best choice in read-intensive environments where maximum usable disk capacity is required. RAID-5 arrays provide greater storage efficiency than RAID-1 arrays. RAID-1 is the array of choice for performance-critical, fault-tolerant environments.

What RAID Provides

RAID technology does not prevent disk drive failures. However, RAID does provide insurance against such failures by enabling real-time data recovery without data loss. The benefits include:

1. Fault tolerance
2. High I/O performance
3. Mass data capacity
4. Configuration flexibility
5. Lower protected storage costs
6. Easy maintenance

The fault tolerance of arrays can also be significantly enhanced by choosing the right storage enclosure. Enclosures that feature redundant, hot-swappable drives, power supplies, and fans can greatly increase storage subsystem uptime. This is borne out by a number of widely accepted measures: Mean Time to Data Loss (MTDL), Mean Time of Data Availability (MTDA), Mean Time to Repair (MTTR), and Mean Time Between Failures (MTBF).

Hardware RAID vs. Software RAID

To make informed disk array purchase decisions, it is important to look beyond a discussion of the different RAID hardware levels. You must also understand the differences between hardware-based and software-based RAID implementations.

Software and Hardware RAID Definitions

Software and Hardware RAID Performance

Except for array functionality, hardware-based RAID schemes have very little in common with software-based implementations. The chart below summarizes the differences in performance. You can see that hardware-based arrays are likely to be the best choice for most applications.

In a forthcoming edition of IC News, we will publish a comprehensive product announcement about the new RAID-500 disk array for you.