What is RAID?
RAID, an acronym for Redundant Arrays of Inexpensive Disks, is away to virtualize multiple, independent hard disk drives into one or more arrays to improve performance, capacity and reliability (availability).The total array capacity depends on the type of RAID array you build and the number and size of disk drives. This total array capacity is independent of whether you use software or hardware RAID. The following sections look at the different implementations, the strengths and weaknesses and their impact to system performance and effectiveness in enhancing data availability.
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.
There are two possible RAID approaches: Hardware RAID and Software RAID.
1. 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.
2. 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 during installation, refer to the Software RAID Configuration.
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
- 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
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