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RAID Tutorial
Example of a server with 6x drives on a RAID 5 array
A Webserver shown with 6x removable hard drives configured to RAID 5

For personal computers or low end workstations single hard drives are ideal. But in many high ending business computing functions, scientific data collection, or any mission critical computing application, single hard drives are not sufficient because of their limitations, such as capacity, performance and data risk. For these types of applications they require more storage, more performance, and a lower risk of data loss with redundant backups. In order the satisfy these higher end system storage needs, we use a storage technique called RAID or Redundant Arrays of Inexpensive Disks.

RAID is used to combine multiple hard drives into a single logical volume. Thus instead your computer seeing several hard drives, it only sees one. Hard drives can be arranged in multiple ways to produce different results for different server applications and needs. Advantages of this new storage subsystem is increased, capacity, data security, and performance, the tradeoff, however , is increased cost and complexity.

  • Higher Data Security: Through the use of redundant disks, most RAID levels provide protection for the data stored on the array. This means that the data on the array can withstand even the complete failure of one hard disk (or sometimes more) without any data loss, and without requiring any data to be restored from backup. This security feature is a key benefit of RAID. All RAID levels provide some degree of data protection, depending on the exact implementation, except RAID level 0.
  • Fault Tolerance: RAID arrays that include redundancy provide a much more reliable overall storage subsystem than can be achieved by a single disk. This means there is a lower chance of the whole system failing due to hard drive failures.
  • Improved Availability: Availability refers to access to data. Good RAID systems improve availability both by providing fault tolerance and by providing special features that allow for recovery from hardware faults without disruption.
  • Increased, Integrated Capacity: By coming multiple hard drives together into a larger array, you add their capacity together (though a percentage of total capacity is lost to overhead or redundancy in most implementations). This facilitates applications that require large amounts of contiguous disk space, and also makes disk space management simpler. Let's suppose you need 2TB of space for a large database. You could put four 500GB drives into the system, but then you'd have to find some way to split the database into five pieces, and you'd be stuck with trying to remember what was were. Instead, you could set up a RAID 0 array containing those five 500GB hard disks; this will appear to the operating system as a single, 2TB hard disk! All RAID implementations provide this "combining" benefit, though the ones that include redundancy of course "waste" some of the space on that redundant information.
  • Improved Performance: Last, but certainly not least, RAID systems improve performance by allowing the controller to exploit the capabilities of multiple hard disks to get around performance-limiting mechanical issues that plague individual hard disks. Different RAID implementations improve performance in different ways and to different degrees, but all improve it in some way.

RAID function is controlled by a RAID controller card with ports on it for hard drives, or rather being on a card, the RAID controller chip can be onboard a motherboard.

Different types of RAID are called levels, from RAID level 0 to RAID level 6. RAID redundancy and performance progressively increases with each level.

RAID Level 0

RAID Level 0 also known as striping (without parity), is the simplest of the RAID levels and offers maximum performance but redundandcy whatsoever . Files are broken into stripes of a size dictated by the user-defined stripe size of the array, and stripes are sent to each disk in the array. Giving up redundancy allows this RAID level the best overall performance characteristics of the single RAID levels, especially for its cost. For this reason, it is becoming increasingly popular by performance-seekers, such as gamers, creativity professionals working on high definition video and sound editing, or 3D model rendering.

Capacity is calcualted by Number of Drives x Capacity of Disk = Capacity of Array

Advantages

Read/Write performance inscreased by spreading the load acrros many channels adn drives

No parity calculation overhead is involved

Very simple design

Disadvantages

Not "TRUE" RAID, because there is no fault tolerance

The failure of just one drive will result in all data in an array being lost

After a certain amount of drives, performance does not increase significatly.

Requirements

RAID Level 0 requires a minimum of 2 drives to implement.

Applications

Any application which requires very high speed storage, but does not need redundancy.

  • Video Production and Editing
  • Image Editing
  • Pre-Press Applications
  • Any application requiring high bandwidth

 

RAID Level 1

RAID Level 1 also known as mirroring, is the most basic of the redunandt RAID levels. It works by duplicating data onto 2 hard drives. Thus creating a mirrored hard drive array, that has two copies of the data , just in case one of the hard drive fails. If either drive fails, the other continues to function as a single drive until the failed drive is replaced. RAID 1 is popular for those who require fault tolerance and don't need top-notch read performance. However RAID 1 does offer supperiour read performance than a single drive, by offering two reads at once (one per hard drive).

A variant of RAID 1 is duplexing, which duplicates the controller card as well as the drive, providing tolerance against failures of either a drive or a controller. However this method is less common that straight mirroring.

(2 x Capacity of Disk) - Capacity of Disk = Capacity of Array or simply Capacity of Disk = Capacity of Array

Advantages

One Write or two Reads possible per mirrored array

Twice the Read transaction rate of single disks, same Write transaction rate as single disks

No rebuild is necessary in case of a disk failure, just another hard drive needs to be installed to replace the bad one

Disadvantages

Highest disk overhead of all RAID types (100%) - inefficient, 1 of 2 disks is used for redundany

Storrage capacity limited to size of one hard drive.

Requirements

RAID Level 1 requires a minimum of 2 drives to implement, and can only have a maximum of 2 drives.

Applications

Applications which require redundancy with fast random writes; entry-level systems where only two drives are available.

  • SMB Domain Server
  • Payroll
  • Financial
  • Entry-level Web Server
  • Small File Server

RAID Level 5

One of the most popular RAID levels, RAID 5 stripes both data and parity information across three or more drives. Each entire data block is written on a data disk; parity for blocks in the same rank is generated on Writes, recorded in a distributed location and checked on Reads. Fault tolerance is maintained by ensuring that the parity information for any given block of data is placed on a drive separate from those used to store the data itself.

(N x Capacity of Disk) - Capacity of Disk = Capacity of Array

Advantages

Highest Read data transaction rate

Medium Write data transaction rate

Good aggregate transfer rate

Huge array capacity possibilites

Disadvantages

Disk failure can affect throughput

Most complex controller design

Individual block data transfer rate same as single disk

Requirements

RAID Level 5 requires a minimum of 3 drives to implement

Applications

Applications which require increased storage capacity and higher performance.

  • Database Servers
  • File & Application servers
  • Intranet servers
  • ERP Servers
  • Financial
  • DVR Servers

RAID Level 6

RAID 6 is the latest and newest RAID level that safegaurds against data loss even if two hard drives fail at onece. RAID 6 stripes blocks of data across an array of drive, but it calculates two sets parity information for every block. The goal is to have a safer data array that can withstand multiple consiecutive failures. Performance-wise, RAID 6 is generally slightly worse than RAID 5 in terms of writes due to the added overhead of more parity calculations, but may be slightly faster in random reads due to spreading of data over one more disk.

Capacity of Disk x (Number of Disks - 2) = Capacity of Array

Advantages

RAID 6 is essentially an extension of RAID level 5 which allows for additional fault tolerance by using a second independent distributed parity scheme (dual parity)

Data is striped on a block level across a set of drives, just like in RAID 5, and a second set of parity is calculated and written across all the drives; RAID 6 provides for an extremely high data fault tolerance and can sustain multiple simultaneous drive failures

Perfect solution for mission critical applications

Disadvantages

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

Requirements

RAID Level 6 requires a minimum of 4 drives to implement

Applications

Applications which require increased storage capacity and bullet-proof redundancy.

  • Database Servers
  • File & Application servers
  • Intranet servers
  • ERP Servers
  • Financial Servers
  • Healthcare
  • Government

RAID Levels 0+1 (01)

RAID 0+1, 01 also known as a mirrored stripedp array, RAID 01 combine the best feaqtures of struping and mirroring to make an array the has high performance throughtput and superior fault tolerance. RAID 01 is a mirrored array of two smaller stripped arrays. System performance during a drive rebuild is also better than that of parity based arrays, since data does not need to be regenerated from parity information, but copied from the other mirrored drive.

RAID 0+1 is NOT to be confused with RAID 10. A single drive failure will cause the whole array to become, in essence, a RAID Level 0 array

(N x Capacity of Disk) - Capacity of Disk = Capacity of Array

Advantages

RAID 0+1 is implemented as a mirrored array of two RAID 0 arrays

High I/O rates are achieved thanks to multiple stripe arrays

No rebuild is necessary in case of a disk failure

Excellent solution for sites that need high performance but are not concerned with achieving maximum fault tolerance

Disadvantages

Very expensive / High overhead

All drives must move in parallel to proper track lowering sustained performance

Very limited scalability at a very high inherent cost

Requirements

RAID Level 0+1 requires a minimum of 4 drives to implement

Applications

Applications which require increased storage capacity and higher performance.

  • Video Production and Editing
  • Image Editing
  • Pre-Press Applications
  • Any application requiring high bandwidth