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Creator QR in VS .NET 3: Some Basics

CHAPTER 3: Some Basics
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Figure 3 1. One path to redundancy is writing the same data to many drives.
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RAID for Performance
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You can also utilize RAID to improve performance. Knowing how to do that can be helpful when trying to performance tuning queries. It will also serve you well when you go through your initial checklist and start to gather baseline information on the configuration of your servers. SQL Server is very much an input/output (I/O) intensive system. That means that the faster you can read and write data to and from your disks, the faster your response times will be. You can use RAID to increase your I/O throughput by taking advantage of striping your data across several disks at once. For example, a traditional hard disk has a little arm inside of it that moves back and forth every time you want to read and write data. Well, let s say you had to do 100 read and 100 write operations, and each one took 1/100th of a second. How long would that take on one disk Two seconds. Now, let s say you configured RAID as shown in Figure 3 2.
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CHAPTER 3: Some Basics
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Figure 3 2. A RAID configuration to improve performance by striping
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Now you have two little arms moving back and forth across two disks at the same time. You would still have your two hundred operations to perform, but they would be done on two drives simultaneously. Each individual drive would get half the work. Your overall time would be reduced from two seconds to one second. Another method for improving performance would be caching. Most RAID controllers allow for some amount of data to be stored in cache. The idea would be to cache as many nonsequential requests as possible in order to batch them together and send them as a sequential request to the hard drives.
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RAID for Fault Tolerance
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RAID uses two methods to provide for fault tolerance: mirroring and parity. Mirroring is exactly what it sounds like: you have two (or more) copies of your data on two (or more) different disks. the scenario in Figure 3 1 is a mirroring scenario. NOTE: Mirroring is most commonly thought of in terms of two drives mirroring each other. However, there is nothing stopping you from mirroring across more than two drives. Parity occurs when you add extra data called parity data to one of your disks, which can then be used to reconstruct your real data should one disk fail. The advantage of the parity approach is that you save on disk space. Figure 3 3 shows how one byte of parity data can protect two bytes of real data.
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Figure 3 3. One byte of parity data can protect two bytes of real data.
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CHAPTER 3: Some Basics
The parity approach to RAID is a good thing, unless you are concerned about performance. There s a negative impact on performance, because writing that extra bit of parity information each and every time will add additional overhead. Let s examine several different RAID levels that use different combinations of mirroring and parity to achieve protection and speed. I don t discuss every possible RAID level, just the ones you are likely to encounter as a DBA.
RAID 0
This level is commonly called disk striping, and the result is often referred to as a striped set of disks. Your data gets divided into chunks (or blocks) and distributed across all the disks in the array in a fixed order. Figure 3 4 illustrates the approach.
Figure 3 4. An example of a RAID 0 configuration
RAID 0 will increase performance as the operations can all be performed at the same time independently from one another, as noted earlier. The drawback is that any piece of data is on only one disk, because the data was divided across all of the disks evenly. In the event of a drive failure, all of the data that was on that disk is permanently lost. In fact, it s worse: lose one drive and you ve effectively lost all data in the array. The only way to recover from a drive failure in a RAID 0 array is to restore the entire array from the last backup. Therefore, RAID 0 does not make a good choice for SQL Server storage. It is worth noting that RAID 0 does not provide fault tolerance, nor does it provide redundancy.
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