Raid Options

RAID (rād) short for Redundant Array of Independent (or Inexpensive) 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.

Why RAID?

RAID data recovery may seem like a bit of an oxymoron to most of us. After all, isn't that expensive RAID system we almost blew the whole IT budget on, supposed to eliminate the risk of data loss?

Well, not exactly. RAID systems may be fault-tolerant, but they are not fault proof. While most commercial RAID implementations can tolerate the loss of a single hard drive, if multiple hard drives fail, or other serious problems occur, data recovery from Backup may be necessary.

There are number of different RAID levels:

RAID	RAID Title	RAID Description
Level 0	Striped Disk Array without Fault Tolerance (Min 2 Disk)	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 (Min 2 Disk)	Provides disk mirroring. Raid 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 (Min Up to 14 Disk)	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 (Min 3 Disk)	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 (Min 3 Disk)	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 (Min 3 Disk)	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 (Min 4 Disk)	Provides block-level striping with parity data distributed across all disks.
Level 7	-	A trademark of Storage Computer Corporation that adds caching to Raid Levels 3 or 4
Level 0+1	A Stripe of Mirrors (Min 4 Disk)	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 (Min 4 Disk)	Not one of the original RAID levels, multiple RAID 1 mirrors are created, and a RAID 0 stripe is created over these.
Level 50	High I/O Rate & Data Transfer Performance (Min 6 Disk)	RAID 50 should have been called "RAID 03" because it was implemented as a striped (RAID level 0) array whose segments were RAID 3 arrays (during mid-90s)
RAID S	-	EMC Corporation's proprietary striped parity RAID system used in its Symmetrix storage systems. EMC Symmetrix series&emdash;the world’s leading high-end networked storage solutions. Powerful, scalable systems address the critical challenge of managing ever-escalating information growth. Optimize of storage resources, support multiple service levels, centralize management, and reduce total cost of ownership&emdash;while consolidating storage.

Good Web site for RAID Tutorial http://www.acnc.com/04_01_00.html

Striping is a technique which offers the best performance of any RAID configuration. In a striped array, data is interleaved across all the drives in the array.

Mirroring, whatever you write to one drive, gets written simultaneously to another. Thus, you always have an exact duplicate of your data on the second drive.

Parity is an error correction technique commonly used in certain RAID levels. It is used to reconstruct data on a drive that has failed in an array. These parity bytes are added up by the controller to equal either an even or an odd number. By analyzing this value, the controller can determine whether the information has been compromised in any way.

RAID Hardware Vs RAID Software

RAID Hardware is always a disk controller to which you can cable up the disk drives.

RAID Software is a set of kernel modules coupled together with management utilities that implement RAID in Software and require no additional hardware.

Pros and cons

Software RAID is more flexible than Hardware RAID. Software RAID is also considerably less expensive. On the other hand, a Software RAID system requires more CPU cycles and power to run well than a comparable Hardware RAID System. Also, because Software RAID operates on a partition by partition basis where a number of individual disk partitions are grouped together as opposed to Hardware RAID systems which generally group together entire disk drives, Software RAID tends be slightly more complicated to run. This is because it has more available configurations and options. An added benefit to the slightly more expensive Hardware RAID solution is that many Hardware RAID systems incorporate features that are specialized for optimizing the performance of your system.

Assess the needs of your system and then decide which RAID solution works best for you. Please do keep in mind however, that regardless of which RAID solution you choose, neither can provide against administrator (human) error. Therefore, frequent, regularly scheduled back ups of your system are highly recommended.