Storage Solutions RAID & Direct Attached Disk Systems

RAID is an acronym first used in 1987 to describe a redundant array of inexpensive disks, a technology that allowed computer users to achieve high levels of storage reliability from low-cost and less reliable PC-class disk-drive components, via the technique of arranging the devices into arrays for redundancy. More recently, marketers representing industry RAID manufacturers reinvented the term to describe a redundant array of independent disks as a means of dissociating a "low cost" expectation from RAID technology.

"RAID" is now used as an umbrella term for computer data storage schemes that can divide and replicate data among multiple hard disk drives. The different schemes/architectures are named by the word RAID followed by a number, as in RAID 0, RAID 1, etc.  RAID's various designs involve two key design goals: increase data reliability and/or increase input/output performance. When multiple physical disks are set up to use RAID technology, they are said to be in a RAID array. This array distributes data across multiple disks, but the array is seen by the computer user and operating system as one single disk.

When tuning any system, you can address speed, price, or quality; and of these three, you can optimise for only two. Thus, cheap, fast systems will be of low quality; fast, good systems will never be cheap; and inexpensive, good systems are never fast. RAID storage systems combine many smaller, inexpensive disks to form larger, logical drives. Different RAID configurations can provide more storage, faster performance, or improved reliability, depending on your needs. RAID levels 1 to 5 were originally described in a paper published at Berkeley University in 1988 by researchers Patterson, Gibson and Katz. RAID levels 0 and 0+1 were added by the computer industry. There is not an optimum RAID level - you just have to consider whether you want to trade speed for security of data.
There are number of different RAID levels (most common in bold):

Level 0 -- Striped Disk Array without Fault Tolerance: Provides data striping (spreading out blocks of each file across multiple disk drives) but no redundancy. This improves performance but does not deliver fault tolerance. If one drive fails then all data in the array is lost.

Level 1 -- Mirroring and Duplexing: Provides disk mirroring. Level 1 provides twice the read transaction rate of single disks and the same write transaction rate as single disks.

Level 2 -- Error-Correcting Coding: Not a typical implementation and rarely used, Level 2 stripes data at the bit level rather than the block level.

Level 3 -- Bit-Interleaved Parity: Provides byte-level striping with a dedicated parity disk. Level 3, which cannot service simultaneous multiple requests, also is rarely used.

Level 4 -- Dedicated Parity Drive: A commonly used implementation of RAID, Level 4 provides block-level striping (like Level 0) with a parity disk. If a data disk fails, the parity data is used to create a replacement disk. A disadvantage to Level 4 is that the parity disk can create write bottlenecks.

Level 5 -- Block Interleaved Distributed Parity: Provides data striping at the byte level and also stripe error correction information. This results in excellent performance and good fault tolerance. Level 5 is one of the most popular implementations of RAID.

Level 6 -- Independent Data Disks with Double Parity: Provides block-level striping with parity data distributed across all disks.

Level 0+1 A Mirror of Stripes: Not one of the original RAID levels, two RAID 0 stripes are created, and a RAID 1 mirror is created over them. Used for both replicating and sharing data among disks.

Level 10 A Stripe of Mirrors: Not one of the original RAID levels, multiple RAID 1 mirrors are created, and a RAID 0 stripe is created over these.

Level 7 - A trademark of Storage Computer Corporation that adds caching to Levels 3 or 4.

RAID S - EMC Corporation's proprietary striped parity RAID system used in its Symmetrix storage systems.

The biggest concern with software RAID is that almost the entire configuration burden is left with the system administrator. A reliable RAID volume is not assembled by slapping together just any set of sub disks; you need to carefully select and combine sub disks to ensure that you are not saturating controllers or inadvertently creating single points of failure in your disks subsystem. For example, any two plexes can be associated to create a mirrored volume, but if the sub disks in the first plex are from the same physical device as those in the other, the loss of that device will ruin both plexes and the mirrored volume will fail.
Software RAID is less expensive than a hardware solution, but you'll make up the difference in administrative costs. Still, it can be a good solution, especially for small installations. If you