Hardware

Harddrives - RAID

Stands for Redundant Array of Independent Disks. There are a number of different standards for RAID the most common being:

RAID works by placing data on multiple disks and allowing input/output (I/O) operations to overlap in a balanced way, improving performance. Because the use of multiple disks increases the mean time between failures (MTBF), storing data redundantly also increases fault tolerance.

RAID arrays appear to the operating system (OS) as a single logical hard disk. RAID employs the techniques of disk mirroring or disk striping. Mirroring copies identical data onto more than one drive. Striping partitions each drive's storage space into units ranging from a sector (512 bytes) up to several megabytes. The stripes of all the disks are interleaved and addressed in order.

Benefits of RAID

Performance, resiliency and cost are among the major benefits of RAID. By putting multiple hard drives together, RAID can improve on the work of a single hard drive and, depending on how it is configured, can increase computer speed and reliability after a crash.

With RAID 0, files are split up and distributed across drives that work together on the same file. As such, reads and writes can be performed faster than with a single drive. RAID 5 arrays break data into sections, but also devote another drive to parity. This parity drive can see what is working when one nonparity drive fails, and can figure out what was on that failed drive. This function allows RAID to provide increased availability. With mirroring, RAID arrays can have two drives containing the same data, ensuring one will continue to work if the other fails.

Although the term inexpensive was removed from the acronym, RAID can still result in lower costs by using lower-priced disks in large numbers.

Comparison of RAID Technologies

Features Minimum # Drives Data Protection Read Performance Write Performance Read Performance (degraded) Write Performance (degraded) Capacity Utilization Typical Applications
RAID 0 2 No Protection High High N/A N/A 100% High End Workstations, data logging, real-time rendering, very transitory data
RAID 1 2 Single-drive failure High Medium Medium High 50% Operating System, transaction databases
RAID 1E 3 Single-drive failure High Medium High High 50% Operating system, transaction databases
RAID 5 3 Single-drive failure High Low Low Low 67%-94% Data warehousing, web serving, archiving
RAID 5EE 4 Single-drive failure High Low Low Low 50%-88% Data warehousing, web serving, archiving
RAID 6 4 Two-drive failure High Low Low Low 50%-88% Data archive, backup to disk, high availability solutions, servers with large capacity requirements
RAID 10 4 Up to one disk failure in each sub-array High Medium High High 50% Fast databases, application servers
RAID 50 6 Up to one disk failure in each sub-array High Medium Medium Medium 67%-94% Large databases, file servers, application servers
RAID 60 8 Up to two disk failure in each sub-array High Medium Medium Low 50%-88% Data archive, backup to disk, high availability solutions, servers with large capacity requirements

Summary

Types of RAID Description Advantages
Software-Based

Best used for large block applications such as data warehousing or video streaming. Also where servers have the available CPU cycles to manage the I/O intensive operations certain RAID levels require.

Included in the OS, such as Windows®, Netware, and Linux. All RAID functions are handled by the host CPU which can severely tax its ability to perform other computations.

Low price

Only requires a standard controller

Hardware-Based

Best used for small block applications such as transaction oriented databases and web servers.

Processor-intensive RAID operations are off-loaded from the host CPU to enhance performance.

Battery-back write back cache can dramatically increase performance without adding risk of data loss.

Data protection and performance benefits of RAID

More robust fault-tolerant features and increased performance versus software-based RAID

External Hardware Connects to the server via a standard controller. RAID functions are performed on a microprocessor located on the external RAID controller independent of the host.

OS independent

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