Selecting a RAID Set-up level

There are two distinct types of drive array referred to, Physical and Logical i.e. A physical drive array is a group of physical hard disk drives. The physical hard disk drives are managed in partitions known as logical drives. A logical drive is a partition in a physical array of disks that is made up of contiguous data segments on the individual hard disk drives. A logical drive can consist of an entire physical hard disk drive array or a part of a hard disk drive array.

In selecting the physical hard disk drives for use in your RAID set-up you need to be aware that the final RAID system performance will be constrained by the slowest speed and smallest size of hard disk used.

Choose enterprise class high performance hard disk drives of matched type and capacity that the manufacturer recommends for use in a RAID and server application.

To ensure best performance for your specific application select your required RAID level when the system drive is created. The RAID level best suited for a hard disk drive array will be dependent on a number of factors:

• Number of physical drives in the array
• Capacity of the individual drives.
• Data redundancy needed.
• Hard Drive performance.

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RAID 0 – Data Striping

RAID 0 provides disk striping across all drives in the RAID array. RAID 0 does not provide any data redundancy, but does offer the best performance of any RAID level. RAID 0 breaks up data into smaller segments, and then stripes the data segments across each drive in the array. The size of each data segment is determined by the stripe size. RAID 0 offers high bandwidth.

RAID 0 Uses.

Ideal for applications that require high bandwidth but do not require fault tolerance.

Strong Points.

By breaking up a large file into smaller segments, the RAID controller can use both SAS drive and SATA drives to read or write the file faster. RAID 0 involves no parity calculations to complicate the write operation.

Weak Points.

RAID 0 is not fault tolerant. If a drive in a RAID 0 configured array fails then the virtual disk (all hard disk drives associated with the virtual disk) will fail.

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RAID 1 – Disk Mirroring/Disk Duplexing

In RAID 1, the RAID controller duplicates all data from one drive to a second drive. RAID 1 provides complete data redundancy, but at the cost of doubling the required data storage capacity. Table 2 provides an overview of RAID 1.

RAID 1 Uses.

Small data bases or any other application that requires fault tolerance but demands relatively small capacity.

Strong Points

Provides redundancy and fault tolerance with minimal capacity.

Weak Points

Requires twice as many disk drives and performance is badly impaired during drive rebuilds.

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RAID 5 – Data Striping with Striped Parity

RAID 5 includes disk striping at the block level and parity. Parity is the data’s property of being odd or even, and parity checking detects errors in the data. In RAID 5, the parity information is written to all drives. RAID 5 uses a form of strip with parity to maintain data redundancy. A minimum of three drives are required to build a RAID 5 array and they should be identical drives for the best performance

RAID 5 Uses.

RAID 5 is best suited for networks that perform a lot of small I/O transactions simultaneously.

Strong Points.

RAID 5 addresses the bottleneck issue for random I/O operations. Because each drive contains both data and parity, numerous writes can take place concurrently. RAID 5 implementations set-up includes a function called hot swap. This allows for drives to be replaced while the array is still functioning to either increase the drives capacity or to replace a damaged drive. The drive controller then takes time while the array is running to rebuild the data array across the drives. This is a valuable feature for systems that require 24×7 operations. A rebuild will generally be handled faster with a dedicated hardware RAID controller.

Weak Points

RAID 5 paired with large capacity drives are more susceptible to array failure due to longer rebuild times. During a rebuild losing another physical drive is catastrophic. RAID 6 provides protection against this issue.
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RAID 1 vs RAID 5

RAID 5

Needs 2 block reads and 2 block writes to write a single block , has lower storage costs better for low update rates and large amounts of data.

RAID 1.

better write performance, only requires 2 block writes, better for high update rates, needs typically 60% more disks

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RAID 6 – Distributed Parity and Disk Striping

RAID 6 is similar to RAID 5 (disk striping and parity), but instead of one parity block per stripe, there are two. With two independent parity blocks, RAID 6 can survive the loss of two disks in a virtual disk without losing data.

RAID 6 Provides high data throughput, especially for large files. Use RAID 5 for transaction processing applications because each drive can read and write independently. If a drive fails, the RAID controller uses the parity drive to recreate all missing information. Use also for office automation and online customer service that requires fault tolerance. Use for any application that has high read request rates but low write request rates.

RAID 6 Uses.

Use for data that requires a high level of protection from loss. Use for office automation and online customer service that requires fault tolerance. Use for any application that has high read request rates but low write request rates.

Strong Points

Provides data redundancy, high read rates, and good performance in most environments. Provides redundancy with lowest loss of capacity. If two drives in a RAID 6 virtual disk fail, two drive rebuilds are required, one for each drive. These rebuilds do not occur at the same time. The controller rebuilds one failed drive at a time. In the case of a failure of one drive or two drives in a virtual disk, the RAID controller uses the parity blocks to recreate the missing information. Provides the highest level of protection against drive failures of all of the RAID levels.

Weak Points

Not well suited to tasks requiring lot of writes. Suffers more impact if no cache is used (clustering). If a drive is being rebuilt, disk drive performance is reduced. Environments with few processes do not perform as well because the RAID overhead is not offset by the performance gains in handling simultaneous processes.

A RAID 6 virtual disk has to generate two sets of parity data for each write operation, which results in a significant decrease in performance during writes. Disk drive performance is reduced during a drive rebuild. Environments with few processes do not perform as well because the RAID overhead is not offset by the performance gains in handling simultaneous processes. RAID 6 costs more because of the extra capacity required by using two parity blocks per stripe.

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RAID 10 – Combination of RAID 1 and RAID 0

RAID 10 is a combination of RAID 0 and RAID 1. RAID 10 consists of stripes across mirrored drives. RAID 10 breaks up data into smaller blocks and then mirrors the blocks of data to each RAID 1 RAID set. Each RAID 1 RAID set then duplicates its data to its other drive. The size of each block is determined by the stripe size parameter, which is set during the creation of the RAID set. RAID 10 supports up to eight spans.

RAID 10 Uses.

Appropriate when used with data storage that requires 100 percent redundancy of mirrored arrays and that needs the enhanced I/O performance of RAID 0 (striped arrays). RAID 10 works well for medium-sized databases or any environment that requires a higher degree of fault tolerance and moderate to medium capacity.

Strong Points.

Provides both high data transfer rates and complete data redundancy.

Weak Points.

Requires twice as many drives as all other RAID levels except RAID 1.

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RAID 50 – Combination of RAID 5 and RAID 0

RAID 50 provides the features of both RAID 0 and RAID 5. RAID 50 includes both parity and disk striping across multiple arrays. RAID 50 is best implemented on two RAID 5 disk arrays with data striped across both disk groups.

RAID 50 breaks up data into smaller blocks and then stripes the blocks of data to each RAID 5 disk set. RAID 5 breaks up data into smaller blocks, calculates parity by performing an exclusive-or on the blocks and then writes the blocks of data and parity to each drive in the array. The size of each block is determined by the stripe size parameter, which is set during the creation of the RAID set.

RAID level 50 supports up to eight spans and tolerates up to eight drive failures, though less than total disk drive capacity is available. Though multiple drive failures can be tolerated, only one drive failure can be tolerated in each RAID 1 level array.

RAID 50 Uses

Appropriate when used with data storage that requires 100 percent redundancy of mirrored arrays and that needs the enhanced I/O performance of RAID 0 (striped arrays). RAID 10 works well for medium-sized databases or any environment that requires a higher degree of fault tolerance and moderate to medium capacity.

Strong Points

Provides both high data transfer rates and complete data redundancy.

Weak Points

Requires twice as many drives as all other RAID levels except RAID 1.

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