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RAID Solutions

A Redundant Array of Individual Disks (RAID) is a simple technical concept that helps address two of the biggest storage challenges facing businesses today: data protection and data availability. Chances are you will not run into a corporation that doesn't put a high priority on protecting the information on their computers systems. Most businesses would suffer should their data or business applications become unavailable.

RAID systems are built around the concepts of mirroring, striping and parity. Striping is a method that allows multiple hard disk drives to be treated as one large disk. Every time a file is written to a RAID array, it is subdivided and recorded in part on each disk drive in the array.

When a RAID system uses mirroring, an exact duplicate of all information saved to the RAID set is duplicated onto a second drive. If one of the two drives in a mirror should fail, the other will be available to run applications, store information or recover files.

Parity is what enables RAID systems to deliver data protection and availability on a larger scale. When parity is used within a RAID array the data is striped on the hard disks along with some extra information that will allow the RAID system to recover all the data on the array should one drive fail. When combined with striping, a RAID system with parity can contain dozens of hard drives giving you potentially terabytes of fault tolerant and highly available storage space with enhanced performance characteristics.

The benefits of the ability to create fault tolerant drive arrays are obvious. The information stored on your computer systems is extremely valuable. In many cases a business who suffers a hard disk failure will be unable to deliver services to it's customers until that data can be restored from tape. If you have a RAID system in place, a hard disk failure will not stop your ability to do business. The faulty hard disk can be replaced later at a more convenient time without affecting your business' day-to-day operations.

Companies that value their data perform routine backups to tape. What they often do not consider is that a tape backup can only restore their data from the last time they performed a backup. What this means is that if you backup every night, and hard disk fails during the business day, all the data recorded on that disk on that day is lost. For some businesses, it could mean that all of the sales transactions for the day were lost. How do you tell a customer that the order they placed is gone? I'm sure that is not a conversation you want to have.

Although we have been talking about how RAID delivers data protection and availability, this is not the only benefit it brings to the table. A RAID array can also help to improve the performance of your computer systems. Every time you read or write information on a hard disk, you are limited by the speed at which that single disk can perform the operation. When you combine multiple disks into one RAID array, its ability to simultaneously read and write data using striping across all of the disks greatly enhances your performance.

Any company can benefit from the high performance, data protection and high availability that RAID delivers. If you see a use for RAID in your environment look at the resources listed below to learn more.

Advantages:

• Provides real-time data protection with uninterrupted access when a hard drive fails.
• Increases computer system uptime and network availability.
• Protects against data loss.
• Highly scalable. Add more drives as your storage needs grow.
• Components can be replaced on-line to simplify service.
• Can be bootable to provide protection for the operating system as well as data.
• Simple to implement.
• Improves systems performance and application responsiveness.
• The most inexpensive way to get data protection and availability.

Disadvantages:

• Proprietary disk drives in some systems limit your purchasing options.
• Still need tape backup for offsite storage and corruption protection.
• Corrupted or virus infected data will be mirrored as well.

How can RAID deliver a good return on your investment?

• RAID systems significantly reduce down time due to hard drive failure.
• RAID Arrays offer better performance compared to a JBOD (just a bunch of drives) configuration.
• Speed performance over JBOD (just a bunch of drives) configuration.
• High availability, increased performance if utilizing R0 or R1+R0, and inexpensive to implement with high data availability as the reward.
• First line of defense against drive failures - RAID redundancy recovers faster than restoring data from tape.

The Various Types of RAID

RAID stands for Redundant Array of Independent (sometimes Inexpensive) Disks. Using a RAID storage subsystem has the following advantages:

Provides disk spanning by weaving all connected drives into one single volume.
Increases disk access speed by breaking data into several blocks when reading/writing to several drives in parallel. With RAID, storage speed increases as more drives are added.
Provides fault-tolerance by mirroring or parity operation.
 

A few terms that you will need to understand include:

Data Striping. Data is split across multiple drives in a RAID array to form a single logical storage unit. Each drive's storage space is partitioned into stripes, ranging in size from one sector (512 bytes) to multiple megabytes. The stripes then are interleaved so that the logical storage unit is made up of alternating stripes from each drive.

Mirroring. Used in RAID levels 1 and 1+0 for data recovery. Data is duplicated through mirroring across two disks. If one drive fails, the data remains available on the other disk. It's sort of like low-end clustering.

Parity. Information Used in RAID levels 3, 4 and 5 for data recovery. In the event of a drive failure, parity information can be combined with the other remaining data to regenerate the missing information.

What are the RAID levels?
 

NRAID

Disk Spanning

Minimum Disks Required=1

Capacity=N

Redundancy=No

NRAID stands for Non-RAID. The capacity of all the drives is combined to become one logical drive (no block striping). In other words, the capacity of the logical drive is the total capacity of the physical drives. NRAID does not provide data redundancy.

JBOD

Single Drive Control

Minimum Disks Required=1

Capacity=1

Redundancy=No

JBOD stands for Just a Bunch of Drives. The controller treats each drive as a stand-alone disk, therefore each drive is an independent logical drive. JBOD does not provide data redundancy.

RAID 0

Disk Striping

Minimum Disks Required=2

Capacity=N

Redundancy=No

RAID 0 provides the highest performance but no redundancy. Data in the logical drive is striped (distributed) across several physical drives.

RAID 1

Disk Mirroring

Minimum Disks Required=2

Capacity=N/2

Redundancy=Yes

RAID 1 mirrors the data stored in one hard drive to another. RAID 1 can only be performed with two hard drives. If there are more than two hard drives, RAID (0+1) will be performed automatically.
 

RAID (0+1)

Disk Striping with Mirroring
Minimum Disk Required=4

Capacity=N/2

Redundancy=Yes

RAID (0+1) combines RAID 0 and RAID 1 - Mirroring and Striping. RAID (0+1) allows multiple drive failure because of the full redundancy of the hard drives. If there are more than two hard drives assigned to perform RAID 1, RAID (0+1) will be performed automatically.

IMPORTANT: “RAID (0+1)” will not appear in the list of RAID levels supported by the controller. If you wish to perform RAID 1, the controller will determine whether to perform RAID 1 or RAID (0+1). This will depend on the number of drives that has been selected for the logical drive.
 

RAID 3

Disk Striping with Dedicated Parity Disk
Minimum Disk Required=3

Capacity=N-1

Redundancy=Yes

RAID 3 performs Block Striping with Dedicated Parity. One drive member is dedicated to storing the parity data. When a drive member fails, the controller can recover/ regenerate the lost data of the failed drive from the dedicated parity drive.
 

RAID 5

Striping with Interspersed Parity
Minimum Disk Required=3

Capacity=N-1

Redundancy=Yes

RAID 5 is similar to RAID 3 but the parity data is not stored in one dedicated hard drive. Parity information is interspersed across the drive array. In the event of a failure, the controller can recover/regenerate the lost data of the failed drive from the other surviving drives.

RAID 6

Striping with Interspersed Parity
Minimum Disk Required=4

Capacity=N-1

Redundancy=Yes

RAID-6: This can be thought of as "RAID 5, but more". It stripes blocks of data and parity across an array of drives like RAID 5, except that it calculates two sets of parity information for each parcel of data. The goal of this duplication is solely to improve fault tolerance; RAID 6 can handle the failure of any two drives in the array while other single RAID levels can handle at most one fault. 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.

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