The Ultimate Guide to Understanding RAID

Data loss for anyone whether it is a business or individual ultimately means lost business or shattered memories respectively. Data is the most valuable asset of any business, and this can’t be understated, protection of business assets must always command the highest priority.

Backing up data will not be enough if you have no protection against online disk failure. Adding RAID to your storage configuration is the most cost effective and simplest ways to maintain access and data protection.

Whilst a number of companies offer RAID, not all RAID implementation are created equal. As an example there is Hardware RAID which is determined by a specific RAID controller which will probably have Cache memory as well to boost performance and possible battery back-up in case the controller fails. Hardware RAID has history and is very mature. Software RAID on the other hand is not totally new but the quality and reliability is dictated by the quality of the components used in the integration of the RAID components and the quality of software code driving the RAID implementation.

To choose the RAID level that’s right for you, begin by considering the factors below. Each one of these factors becomes a trade-off for another:

• Cost of disk storage
• Data protection (low, medium, high)
• Data Availability needed (low, medium, high)
• Performance Requirements (low, medium, high)

The Cost basically comes down to the trade-off between disk capacity and added data availability or performance. For example, RAID 1,10 and small disk counts of RAID 6 are costly in terms of lost disk space (50%), but high in data availability.

Performance also depends on the access pattern (random/sequential, read/write, long/short) and the numbers of users. This guide is intended to give an overview on the performance and availability of various RAID levels in general and may not be accurate in all user scenarios. This is meant only as a guide through the RAID jungle.

The following are commonly used RAID levels:

RAID	Min # Drives	Data Protection	Read Speed	Write Speed	Read Speed (degraded)	Write Speed (degraded)	Capacity Utilization	Typical Applications
RAID 0	2	No Protection	High	High	N/A	N/A	100%	High End Workstations, data logging, real-time rendering, very transitory data
RAID 0 Definition Advantages Disadvantages Striping without parity, improved performance, additional storage, no fault tolerance I/O performance is greatly improved by spreading the I/O load across many channels and drives (best performance is achieved when data is striped across multiple channels with only one drive per channel) No parity calculation overhead is involved Very simple design Easy to implement Not a “True” RAID because the failure of just one drive will result in all data in a virtual disk being lost Should not be used for critical data
RAID 1	2	Single-drive failure	High	Medium	Medium	High	50%	Operating System, transaction databases
RAID 1 Definition Advantages Disadvantages Mirroring without parity, fault tolerance for disk errors and single disk failures High performance up to twice the read transaction rate of single disks, and the same write transaction rate as single disks 100 percent redundancy of data means no rebuild of data is necessary in case of disk failure, just a copy to the replacement disk Typically supports hot-swap disks Simplest RAID storage subsystem design Highest disk overhead of all RAID types (100 percent) results in inefficient use of drive capacity Limited capacity since the virtual disk can only include two disk drives
RAID 5	3	Single-drive failure	High	Low	Low	Low	67% – 94%	Data warehousing, web serving, archiving
RAID 5 Definition Advantages Disadvantages Striping with distributed parity, improved performance, fault tolerance for disk errors and single disk failures Most efficient use of drive capacity of all the redundant RAID configurations High read transaction rate Medium-to-high write transaction rate Disk failure has a medium impact on throughput Most complex controller design Retrieval of parity information after a drive failure takes longer than with mirroring
RAID 6	4	Two-drive failure	High	Low	Low	Low	50% – 88%	High End Workstations, data logging, real-time rendering, very transitory data
RAID 6 Definition Advantages Disadvantages Striping with dual parity, fault tolerance for dual drive failures Can survive the loss of two disks without losing data Data redundancy, high read rates, and good performance Requires two sets of parity data for each write operation, resulting in significant decrease in write performance Additional costs because of the extra capacity required by using two parity blocks per stripe Retrieval of parity information after a drive failure takes longer than with mirroring
RAID 10	4	Up to one disk failure in each sub-array	High	Medium	High	High	50%	Fast databases, application servers
RAID 10 Definition Advantages Disadvantages Implements a 1+0 RAID Level, enabling block level and mirroring combined with striping, better performance, fault tolerance for disk errors and multiple drive failure (one drive failure per mirror set) RAID 10 has the same redundancy as RAID level 1 High I/O rates are achieved by striping RAID 1 segments Most expensive RAID solution Requires 2n where n > 1 disks Very limited scalability at a very high inherent cost
RAID 50	6	Up to one disk failure in each sub-array	High	Medium	Medium	Medium	67% – 94%	Large databases, file servers, application servers
RAID 50 Definition Advantages Disadvantages Combines multiple RAID 5 sets with striping, improved performance, fault disk errors and multiple drive failures (one drive failure per span) Allows creation of largest RAID groups, up to 256 drives (theoretical) High read transaction rate Higher degree of fault tolerance due to parity calculation being done for each RAID 5 subset Potential for faster read transaction rates over large RAID 5 virtual disks Medium-to-high write transaction rate Potential for faster read transaction rates over large RAID 5 virtual disks One of the more complex RAID implementations Less space efficient than RAID 5 since separate parity calculations are done for each RAID 5 subset Retrieval of parity information after a drive failure takes longer than using a mirrored solution
RAID 60	8	Up to two disk failures in each sub-array	High	Medium	Medium	Low	50% – 88%	Data archive, backup to disk, high availability solutions, servers with large capacity requirements
RAID 60 Definition Advantages Disadvantages Combines multiple RAID 6 sets with striping, improved performance, fault disk errors and multiple drive failures (two drive failures per span) Allows creation of largest RAID groups, up to 256 drives (theoretical) High degree of fault tolerance due to 2 parity calculations being done for each RAID 6 subset Medium-to-high write transaction rate One of the more complex RAID implementations Less space efficient than RAID 6 since separate parity calculations are done for each RAID 6 subset Retrieval of parity information after a drive failure takes longer than using a mirrored solution

Depending on how you implement RAID, the benefits include one or both of the following: