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Raid Explained

Raid Explained
Raid Explained

RAID is an acronym that was first defined by David A. Patterson, Garth A. Gibson and Randy Katz at the University of California to describe the phrase "Redundant Array of Inexpensive Disks". This technology allowed for more robust levels of storage reliability than could normally be expected from cheaper and in the early years of disk technology less reliable desktop storage.


As disk technology has matured so has the term and now days most people if asked would describe the acronym as a "Redundant Array of Independent Disks".

Raid arrays have evolved into common standards which are principally raid 0, raid 1, raid 5 and raid 10 as well as some more exotic standards. Raid 0 for example actually splits data across 2 drives and if one disk fails all data can be lost so it considered by many not to be a true raid at all as it offers no redundancy, but is a configuration that is often used for speed for high end disk intensive applications.

The need for raid 0 is actually reducing as hard disk data transfer speeds have increased anyway so it is no longer so important to risk speed versus redundancy but other forms of raid are still very much required for business critical servers require a high degree of redundancy in the event of failure or where a fast input/output of data is required.

Companies that operate systems requiring exceptionally fast access to data tend to use flash based drives which are hugely expensive for large amounts of data but the cost of solid state drives is falling sharply together with a corresponding increase incapacity.

Raid recovery from SSD's is an altogether different recovery challenge and should only be attempted by technicians conversant with flash memory recovery procedures.

The most common form of raid is most probably Raid 5, as with all raid recovery raid 5 data recovery should only be attempted by a knowledgeable technician well experienced in a wide variety of raid configurations and to strict procedures to ensure that the originating data remains intact at all costs...... read more about raid here


RAID Level 0 Striped Disk Array without Fault Tolerance: Provides data striping (spreading out blocks of each file across multiple hard 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.

RAID Level 1 Mirroring and Duplexing: Provides drive mirroring. RAID Level 1 provides twice the read transaction rate of single hard drive and the same write transaction rate as single disks. This offers this best RAID server data recovery.

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

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

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

RAID Level 5 Block Interleaved Distributed Parity: Provides data striping at the byte RAID Level and also stripe error correction information. This results in excellent performance and good fault tolerance. RAID Level 5 is one of the most popular implementations of RAID. Raid server data recovery at this RAID Level is often very successful due to the configuration.

RAID Level 6 Independent Data Disks with Double Parity: Provides block-RAID Level striping with parity data distributed across all disks.
RAID Level 0+1 A Mirror of Stripes: Not one of the original RAID 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.

RAID Level 10 A Stripe of Mirrors: Not one of the original RAID RAID Levels,sometimes referred to as Raid 1+0. multiple RAID 1 mirrors are created, and a RAID 0 stripe is created over these.

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

RAID Level 50 requires a minimum of 6 drives to implement RAID 5+0 (50): RAID 0 made up of RAID 5 arrays and should have been called "RAID 03" because it was implemented as a striped (RAID level 0) array whose segments were RAID 3 arrays. RAID 50 is more fault tolerant than RAID 5 but has twice the parity overhead. High data transfer rates are achieved thanks to its RAID 5 array segments. High I/O rates for small requests are achieved thanks to its RAID 0 striping

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

RAID-Z Sun's ZFS implements an integrated redundancy scheme similar to RAID 5 which it calls RAID-Z. RAID-Z avoids the RAID 5 "write hole"[4] by its copy-on-write policy: rather than overwriting old data with new data, it writes new data to a new location and then atomically overwrites the pointer to the old data. It avoids the need for read-modify-write operations for small writes by only ever performing full-stripe writes; small blocks are mirrored instead of parity protected, which is possible because the file system is aware of the underlying storage structure and can allocate extra space if necessary. There is also RAID-Z2 which uses two forms of parity to achieve results similar to RAID 6: the ability to sustain up to two drive failures without losing data

RAID 1+0 (10): A striped set of mirrored drives.(minimum four disks; even number of disks) provides fault tolerance and improved performance but increases complexity.

RAID 0+1: A mirrored set of striped drives.(minimum four disks; even number of disks) provides fault tolerance and improved performance but increases complexity.

RAID 0+5: RAID 5 made up of RAID 0 arrays.RAID 05 is a RAID 5 array comprised of a number of striped RAID 0 arrays

Drobo BeyondRAID Data Robotics, Inc. implements a storage technology that they call BeyondRAID in their Drobo storage device. While not a true RAID extension, it does provide for using up to 4 SATA hard drives in the device and consolidating them into one big pool of storage. It has the advantage of being able to use multiple disk sizes at once, much like a JBOD unit, while providing redundancy for all disks and allowing a hot-swap upgrade at any time. Internally it uses a mix of techniques similar to RAID 1 and RAID 5. Depending on the amount of data stored on the unit in relation to the installed capacity, it may be able to survive up to three drive failures, if the "array" can be restored onto the remaining good disks before another drive fails. The amount of usable storage in a Drobo unit can be approximated by adding up the capacities of all the disks and subtracting the capacity of the largest disk. For example, if a 500, 400, 200, and 100 GB drive were installed, the approximate usable capacity would be 500+400+200+100-(500)=700 GB of usable space. Internally the data would be distributed in two RAID 5-like arrays and one RAID 1-like set

RAID-K Kaleidescape's KSERVER-5000 and KSERVER-1500 use a proprietary RAID-K in their media storage units. RAID-K is similar to RAID 4 in using double parity, but RAID-K also uses another, proprietary method of maintaining fault tolerance. The system is easily modified, as users can expand the array simply by inserting additional hard disks. Additionally, if any hard disk is inserted with data already on it, the data is automatically added to the array instead of deleting the data, as many other RAID methods would require.

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