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RAID OVERVIEW

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What is RAID when used in Disaster Recovery as part of Defense In Depth (DID)?

RAID (Redundant Array of Independent Disks) is an acronym first used in a 1988 paper by Berkeley researchers Patterson, Gibson and Katz. It described array configuration and applications for multiple inexpensive hard disks, providing fault tolerance (redundancy) and improved access rates. RAID provides a foundation for physical data reliability on individual servers and forms a component part of security and disaster recovery DID.

RAID provides a method of accessing multiple individual disks as if the array were one larger disk, spreading data access out over these multiple disks, thereby reducing the risk of losing all data if one drive fails, and improving access time. Typically the RAID is used in large file servers, transaction of application servers, where data accessibility is critical, and fault tolerance is required.


RAID Level Best Applications Description Benefits Drawbacks
RAID 0 (JBOD) Performance without data redundancy. Where loss of data is not critical Raid_0.jpg (6172 bytes)
Striped
Data
Disks
Striping of data across multiple drives Performance No data redundancy
RAID 1 Online transactional processing. Small random requests; reads or writes Raid_1.jpg (4456 bytes) Data Disk;
Mirrored Disk
Data duplicated on two drives Read or write transaction performance. Redundancy More Expensive. than RAID 0 Twice the number of drives needed.
Data Disk;
Mirrored Disk
RAID 3 Visual imaging. Performance-driven applications with large, sequential blocks of data Raid_3-4.jpg (7386 bytes)
Striped
Data
Disks;

Dedicated
Parity Disk

Data striped in bytes across multiple drives. Dedicated parity drive High sustained data transfer rate. No performance degradation after drive failure. Redundancy Lower performance with small, random requests
RAID 4 Visual imaging. Performance-driven, single or multi-threaded applications Raid_3-4.jpg (7386 bytes)
Striped
Data
Disks;

Dedicated
Parity Disk

Data striped in blocks across multiple drives. Dedicated parity drive High sustained data transfer rate. No performance degradation after drive failure. Redundancy. Lower performance with small, random requests
RAID 5 Databases. Small random requests; read-intensive Raid_5.jpg (7460 bytes)
Striped
Data
Disks;
Rotating
Parity
Data written in blocks across all drives; parity distributed among all drives Good read performance with small, random I/O requests. Redundancy 50% performance loss with drive failure. Poor large request and write performance.

Conclusions:

The choice of RAID type varies based on the server requirements with RAID 5 being the most popular for Database servers requiring a minimum of 3 drives and mirrored or duplexed drives being the most popular for workgroup servers. Growth of capacity per disk drive is growing exponentially as single drive capacity exceeds 40 gigabytes and beyond over time. Compared to the size of most databases and size of user files combined, most small to medium companies will find the RAID 1 solution combined with good backup software to be the solution of choice providing a best practices combination of reliability, performance, cost, and speed of data access.

RAID  forms one component in good design for data redundancy with a tape backup, backup server, or application mirror as an integral part of Disaster recovery and Defense In Depth (DID) Security design.

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