| Array |
| Two or more hard disk drives grouped together to appear as a single device to the host computer. |
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| Bootable Array Support |
| The ability to make the system boot from a RAID array instead of from a standalone (single) disk. |
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| Cache |
| A temporary, fast storage area that holds data from a slower storage device for quick access. Cache storage is normally transparent to the accessing device. |
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| Fault Tolerance |
| The ability of a system to continue to perform its functions even when one or more hard disk drives have failed. |
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| Hardware-based RAID |
| Processor-intensive RAID operations are off-loaded from the host CPU to enhance performance. |
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| Hot Spare |
| A spare hard drive which will automatically be used to replace the failed member of a redundant disk array. |
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| Hot Swap |
| The ability to remove a failed member of a redundant disk array and replace it with a good drive without bringing down the server or interrupting transactions that involve other devices. |
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| Mirroring (RAID 1) |
| Provides data protection by duplicating all data from a primary drive on a secondary drive. |
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| OCE (Online Capacity Expansion) |
| A process for adding storage capacity to an existing RAID array without having to take the server offline. |
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| RAID (Redundant Array of Independent Disks) |
A category of disk drives that employ two or more drives in combination for fault tolerance and performance. RAID disk drives are used frequently on servers but aren't generally necessary for personal computers.
There are number of different RAID levels:
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. |
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| Software-based RAID |
| Included in OSs 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. |
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| Striping |
| Spreads data evenly over multiple drives to enhance performance. Because there is no redundancy scheme, it does not provide data protection. |
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