Appendix D. RAID (Redundant Array of Independent Disks)

What is RAID?

The basic idea behind RAID is 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 will appear to the computer as a single logical storage unit or drive.

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 increase bandwidth for reading 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.

Who Should Use RAID?

Anyone who needs to keep large quantities of data on hand (such as an average system administrator) would benefit by using RAID technology. Primary reasons to use RAID include:

Enhanced speed
Increased storage capacity using a single virtual disk
Lessening the impact of a disk failure

RAID: Hardware vs. Software

There are two possible RAID approaches: Hardware RAID and Software RAID.

Hardware RAID

The hardware-based system manages the RAID subsystem independently from the host and presents to the host only a single disk per RAID array.

An example of a Hardware RAID device would be one that connects to a SCSI controller and presents the RAID arrays as a single SCSI drive. An external RAID system moves all RAID handling "intelligence" into a controller located in the external disk subsystem. The whole subsystem is connected to the host via a normal SCSI controller and appears to the host as a single disk.

RAID controllers also come in the form of cards that act like a SCSI controller to the operating system but handle all of the actual drive communications themselves. In these cases, you plug the drives into the RAID controller just like you would a SCSI controller, but then you add them to the RAID controller's configuration, and the operating system never knows the difference.

Software RAID

Software RAID implements the various RAID levels in the kernel disk (block device) code. It offers the cheapest possible solution, as expensive disk controller cards or hot-swap chassis [1] are not required. Software RAID also works with cheaper IDE disks as well as SCSI disks. With today's fast CPUs, Software RAID performance can excel against Hardware RAID.

The MD driver in the Linux kernel is an example of a RAID solution that is completely hardware independent. The performance of a software-based array is dependent on the server CPU performance and load.

For information on configuring Software RAID in the Red Hat Linux installation program, refer to the Official Red Hat Linux Customization Guide.

For those interested in learning more about what Software RAID has to offer, here is a brief list of the most important features:

Threaded rebuild process
Fully kernel-based configuration
Portability of arrays between Linux machines without reconstruction
Backgrounded array reconstruction using idle system resources
Hot-swappable drive support
Automatic CPU detection to take advantage of certain CPU optimizations

RAID Levels and Linear Support

RAID supports various configurations, including levels 0, 1, 4, 5, and linear. These RAID types are defined as follows:

Level 0 — RAID level 0, often called "striping," is a performance-oriented striped data mapping technique. This means the data being written to the array is broken down into strips and written across the member disks of the array, allowing high I/O performance at low inherent cost but provides no redundancy. The storage capacity of a level 0 array is equal to the total capacity of the member disks in a Hardware RAID or the total capacity of member partitions in a Software RAID.
Level 1 — RAID level 1, or "mirroring," has been used longer than any other form of RAID. Level 1 provides redundancy by writing identical data to each member disk of the array, leaving a "mirrored" copy on each disk. Mirroring remains popular due to its simplicity and high level of data availability. Level 1 operates with two or more disks that may use parallel access for high data-transfer rates when reading but more commonly operate independently to provide high I/O transaction rates. Level 1 provides very good data reliability and improves performance for read-intensive applications but at a relatively high cost[2]. The storage capacity of the level 1 array is equal to the capacity of one of the mirrored hard disks in a Hardware RAID or one of the mirrored partitions in a Software RAID.
Level 4 — Level 4 uses parity[3] concentrated on a single disk drive to protect data. It's better suited to transaction I/O rather than large file transfers. Because the dedicated parity disk represents an inherent bottleneck, level 4 is seldom used without accompanying technologies such as write-back caching. Although RAID level 4 is an option in some RAID partitioning schemes, it is not an option allowed in Red Hat Linux RAID installations[4]. The storage capacity of Hardware RAID level 4 is equal to the capacity of member disks, minus the capacity of one member disk. The storage capacity of Software RAID level 4 is equal to the capacity of the member partitions, minus the size of one of the partitions if they are of equal size.
Level 5 — This is the most common type of RAID. By distributing parity across some or all of an array's member disk drives, RAID level 5 eliminates the write bottleneck inherent in level 4. The only performance bottleneck is the parity calculation process. With modern CPUs and Software RAID, that usually isn't a very big problem. As with level 4, the result is asymmetrical performance, with reads substantially outperforming writes. Level 5 is often used with write-back caching to reduce the asymmetry. The storage capacity of Hardware RAID level 5 is equal to the capacity of member disks, minus the capacity of one member disk. The storage capacity of Software RAID level 5 is equal to the capacity of the member partitions, minus the size of one of the partitions if they are of equal size.
Linear RAID — Linear RAID is a simple grouping of drives to create a larger virtual drive. In linear RAID, the chunks are allocated sequentially from one member drive, going to the next drive only when the first is completely filled. This grouping provides no performance benefit, as it is unlikely that any I/O operations will be split between member drives. Linear RAID also offers no redundancy and, in fact, decreases reliability —— if any one member drive fails, the entire array cannot be used. The capacity is the total of all member disks.