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Oracle Performance Tuning and Optimization
Size the Volume ProperlyMake sure that you have enough disk drives to support the load you want to put on the logical volume. In Chapter 14, Advanced Disk I/O Concepts, you learned how many sequential and random I/Os per second your disks can support. Now use that information to determine how many disks you need in a disk array. Suppose that you want to support 250 random I/Os per second to your data files. These I/Os are split into 125 writes per second and 125 reads per second. Now lets analyze these requirements for several fault tolerant methods. Recall the data on RAID levels presented earlier in Table 15.1 (presented again here for your convenience).
Table 15.2 shows the number of I/Os per second generated for each RAID level when you run at 125 writes per second and 125 reads per second.
By using the value of 50 I/Os per second per drive, you can calculate the number of disk drives you need for each of these fault tolerance methods to achieve the required I/O rate (see Table 15.3).
As you can see from Table 15.3, even though RAID-5 is much more economical in terms of I/Os per second, you may need many more disk drives if you are in an I/O-bound situation. By configuring your system with RAID-5 so that you can achieve the desired number of I/Os, you also get the benefit of additional disk space because you will be using many more drives. table 15.4 looks at the same data in terms of disk space (assume that you are using 1-gigabyte SCSI disk drives).
As you can see, to provide for the extra overhead involved in RAID-5, you may have to purchase more disk drives than you really need for your configuration. Again, choose the RAID level that best meets your needs.
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