Chapter 11: Redundant Array of Independent Disks (RAID)

This chapter introduces the concept of RAID, which allows an administrator to combine multiple small, inexpensive disk drives into an array to accomplish performance or redundancy goals not attainable with one large and expensive drive. This array of drives appears to the computer as a single logical storage unit or drive. RAID can be managed at either the hardware or software levels. This chapter deals primarily with software RAID.

What Is RAID?

RAID is a method in which information is spread across several disks, using techniques such as disk striping (RAID Level 0), disk mirroring (RAID level 1), and disk striping with parity (RAID Level 5) to achieve redundancy, lower latency and/or increased bandwidth for reading from or writing to disks, and maximize the ability to recover from hard-disk crashes.

The underlying concept of RAID is that data may be distributed across each drive in the array in a consistent manner. To do this, the data must first be broken into consistently sized chunks (often 32K or 64K in size, although different sizes can be used). Each chunk is then written to a hard drive in RAID according to the RAID level used. When the data is to be read, the process is reversed, giving the illusion that multiple drives are actually one large drive.