REDUNDANT ARRAY OF INEXPENSIVE DISCS RAID

What is RAID ? RAID is an acronym for Redundant Array of Independent Drives (or Disks), also known as Redundant Array of Inexpensive Drives (or Disks) The various types of RAID are data storage schemes that divide and/or replicate data among multiple hard drives

Why use RAID? Improve Reliability Improved Performance Fault Tolerance Improved Availability Higher Data Security

Key Terms Mirroring - the copying of data to more than one disk Striping - the splitting of data across more than one disk Parity - a redundancy check that ensures that the data is protected without having to have a full set of duplicate drives. Duplexing - an extension of mirroring that is based on the same principle as that technique expect it goes one step further in that it also duplicates the hardware that controls the two hard drives (or sets of hard drives).

About Single Level There are 6 Levels of RAID Each level, in use on their own is known as Single Level When two levels are combined for usage (ex. 1+0, 0+1) they are referred to as Multi Level (or Nested)

RAID 0 A striped set of at least two disks without parity The data is broken down into blocks and each block is written to a separate disk drive Best performance is achieved when data is striped across multiple controllers with only one drive per controller

Advantages of RAID 0 I/O performance is greatly improved by spreading the I/O load across many channels and drives No parity calculation overhead is involved Very simple design Easy to implement

Disadvantages of RAID 0 Not a "True" RAID because it is NOT fault-tolerant The failure of just one drive will result in all data in an array being lost Should never be used in mission critical environments

RAID 1

RAID 1 Advantages High data availability and high I/O rate (small block size). Improves read performance - twice the read transaction rate of single disks, same write transaction rate as single disks 100% redundancy of data means no rebuild is necessary in case of a disk failure, just a copy to the replacement disk Simplest RAID storage subsystem design – easy to maintain

RAID 1 Disadvantages Expensive due to the extra capacity required to duplicate data. Overhead cost equals 100%, while usable storage capacity is 50%. May not support hot swap of failed disk when implemented with software. Use hardware implementation.

RAID 1 Uses Accounting Payroll Financial Any application requiring very high availability

RAID 2 Level 2 is the "black sheep" of the RAID family, because it is the only RAID level that does not use one or more of the "standard" techniques of mirroring, striping and/or parity. RAID-2 is similar to RAID-4, but stores error correcting code (ECC) information instead of parity. Since all modern disk drives incorporate ECC under the covers, this offers little additional protection. RAID-2 can offer greater data consistency if power is lost during a write, however, battery backup and a clean shutdown can offer the same benefits.

RAID 2 Level 2 is the only RAID level that is not used today. It is expensive and often requires many drives. The controller required was complex, specialized and expensive. Most of all, level 2 was rendered obsolete by the use of ECC within a hard disk; essentially, much of what RAID 2 provides you now get for free within each hard disk, with other RAID levels providing protection above and beyond ECC. Due to its cost and complexity, level 2 never really caught on.

A STRIPED SET WITH A MINIMUM OF 3 DISKS AND DEDICATED PARITY. RAID 3 & 4

RAID 3 uses byte-level striping with a dedicated parity disk. is very rare in practice - one side-effect is that it generally cannot service multiple requests simultaneously

Disadvantages of RAID 3 Transaction rate equal to that of a single disk drive at best (if spindles are synchronized) Controller design is fairly complex Very difficult and resource intensive to do as a "software" RAID

RAID 4 uses block-level striping with dedicated parity disk similar to RAID 3 except that it stripes at the block level, rather than the byte level a minimum of 3 disks is required for a complete RAID 4 configuration.

Disadvantages of RAID 4 Complex controller design Worst Write transaction rate and Write aggregate transfer rate Difficult and inefficient data rebuild in the event of disk failure Block Read transfer rate equal to that of a single disk

RAID 5 Striped Set Minimum of 3 disks Spreads data over all drives If the drive goes down replacement is required

Advantages of RAID 5 Highest Read data transaction rate Medium Write data transaction rate Low ratio of ECC (Parity) disks to data disks means high efficiency Good aggregate transfer rate

Disadvantages of RAID 5 Disk failure has a medium impact on throughput Most complex controller design Difficult to rebuild in the event of a disk failure Individual block data transfer rate same as single disk

RAID 6 Striped Set Minimum of 4 disks Very important when it comes to large size drives and high availability systems Array will continue to operate if up to 2 drives fail Essentially an extension of RAID level 5 which allows for additional fault tolerance by using a second independent distributed parity scheme (dual parity

Advantages of RAID 6 Protects against multiple bad block failures while non-degraded Protects against a single bad block failure while operating in a degraded mode Perfect solution for mission critical applications

Disadvantages of RAID 6 More complex controller design Controller overhead to compute parity addresses is extremely high Write performance can be brought on par with RAID Level 5 by using a custom ASIC for computing Reed- Solomon parity Requires N+2 drives to implement because of dual parity scheme