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In 3, we covered the basics of RAID technology. If you re not familiar with the concepts behind RAID, now would be a good time to go back and review them. In this section, we ll cover the details of RAID implementations as they affect SQL Server 7. The three main issues to consider are fault tolerance, performance, and disk space usage. Table 5-3 shows how these parameters are affected at various RAID levels. In addition to the standard levels of RAID, additional levels that involve the combination of multiple methods may be implemented. These are often referred to by means of notation such as RAID 5+0 or RAID 50 (two RAID 5 arrays that are mirrored). These im-
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RAID Level RAID 0 (Disk Striping) RAID 1 (Disk Mirroring) RAID 5 (Disk Striping with Parity)
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Minimum Drives 2 2
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Write Performance Negligible Increased
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FaultTolerant No Yes
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Disk Space Lost None 50% of total space Size of one disk in the array
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Increased
Decreased
Table 5-3.
SQL Server 7 Performance at Various RAID Levels
plementations combine various RAID levels in order to increase fault tolerance and/or performance and to decrease lost disk space. Keep in mind, however, that none of these solutions is likely to provide all three benefits, so there will be tradeoffs involved. Determining which tradeoffs are appropriate is the job of the systems administrator. In general, it is recommended that SQL Server use RAID for both fault tolerance and increased performance. These benefits are well worth the cost (loss of usable disk space), because fixed storage costs are rapidly decreasing. Although the Windows NT OS can support software-based RAID implementations, there are several potential drawbacks: w s v Overall system performance will decrease because the CPU is used to handle all parity calculations. Modifications to the disk configurations might require server reboots, which can cause downtime. Software-based disk mirroring (RAID 1) can cause downtime if the primary hard disk fails.
On the plus side, however, software-based RAID can be an inexpensive alternative to hardware-based solutions, especially when disk I/O performance is not critical. For more information on implementing Windows NT/Windows 2000 RAID features, see 3. Hardware-based RAID solutions are much more flexible, and offer additional advantages: w Increased performance Hardware RAID controllers include their own CPU that is dedicated to managing the calculations necessary for creating parity information. This offloads much of the duty from the main CPU, which can then work on other tasks, such as processing SQL Server queries.
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Support for more storage Because hardware RAID controllers are often used in mission-critical environments and for applications that require large amounts of storage, they are designed to support many SCSI devices. Although a single SCSI channel is limited to attaching seven devices (in addition to the controller itself), multiple SCSI channels can be put together to support increasingly large arrays. Hot-plug capability Some RAID controller and chassis configurations allow systems administrations to add and remove drives from a server without first powering it off. This can greatly increase uptime in systems that require the periodic addition of storage space. It also allows for quickly and easily replacing failed disks within an array. Hot spares Sometimes, having a RAID controller automatically use a spare disk is preferable to replacing a disk that has failed. The standby disk is not included as part of an array; it exists solely to take over the functions of a failed one. Such a hot spare is good protection against a complete data loss, and alleviates any possible need for immediate human intervention. Configuration flexibility Modern RAID controllers allow users to dynamically reconfigure certain disk configurations without the loss of data. For example, systems administrators can add a drive to a stripe set with parity (RAID 5) without having to back up all the data, reformat the drive array, and then restore the data. Not only is this convenient, but it also prevents potentially costly server downtime.
The potential drawbacks to adopting these solutions are the initial expense and the training that you may need in order to configure them properly. However, most organizations will find that the increased performance and flexibility of hardware RAID are well worth the costs. RAID solutions are available from many hardware vendors.
NOTE: The issues that surround write-caching controllers apply to hardware-based RAID solutions as well. Before implementing a backup scheme that includes database servers or write caching, be sure that you have a firm understanding of the tradeoffs involved.
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