A Redundant Array of Independent Disks (RAID) is an increasingly popular way of delivering fault-tolerant, high-performance disk configurations. There are a number of levels of RAID, and a number of factors to take into consideration when deciding on a RAID configuration, and the level of RAID to implement.
There are three levels of RAID commonly provided by storage vendors. They are:
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RAID 0 |
Sometimes referred to as striping disks. In this configuration, a logical disk is constructed from multiple physical disks. The data contained on the logical disk is spread evenly across the physical disk, hence, random I/Os are also likely to be spread evenly. There is no redundancy built into this configuration, so if a disk fails, it must be recovered from backup. |
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RAID 1 |
Referred to as disk mirroring. In this configuration, a logical disk is comprised of two physical disks. In the event that one physical disk fails, processing can continue using the other physical disk. Each disk contains identical data, and writes are processed in parallel so there should be no negative effects on write performance. Two disks are available for reads, so there can be an improvement in read performance. |
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RAID 5 |
A logical disk is comprised of multiple physical disks. Data is arranged across the physical devices in a similar way to disk striping (RAID 0). However, a certain proportion of the data on the physical devices is parity data. This parity data contains enough information to derive data on other disks should a single physical device fail. |
It is common to combine RAID 0 and RAID 1. Such striped and mirrored configurations offer protection against hardware failure, together with spread of I/O load.
Performance implications of RAID
Both RAID 0 and RAID 5 improve the performance of concurrent random reads by spreading the load across multiple devices. RAID 5, however, tends to degrade write I/O, because both the source block and parity block must be read and then updated.
Neither RAID 0 nor RAID 5 offer any performance advantages over single disk configurations when sequential reads or writes are being undertaken.
The performance of combined RAID 0 and RAID 1 for database files, and RAID 1 for redo logs is generally superior to any other configuration. It also offers full protection from media failure. RAID 5, however, requires less disk space than a RAID 0+1 configuration, and may provide acceptable performance in many circumstances.
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