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Oracle Performance Tuning and Optimization
(Publisher: Macmillan Computer Publishing)
Author(s): Edward Whalen
ISBN: 067230886x
Publication Date: 04/01/96

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Size the Volume Properly

Make 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 let’s 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 15.1 The I/Os Generated by RAID Fault Tolerance

RAID Level I/Os per Write I/Os per Read

RAID-0 (no fault tolerance) 1 1
RAID-1 (mirroring) 2 1
RAID-5 (distributed data guarding) 4 (2 Reads, 2 Writes) 1

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.

Table 15.2 I/Os by RAID Level at 125 Reads per Second and 125 Writes per Second

RAID Level Writes/sec Reads/sec Total/sec

RAID-0 (no fault tolerance) 125 125 250
RAID-1 (mirroring) 250 125 375
RAID-5 (distributed data guarding) 250 125 + 250 675

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).

Table 15.3 Number of Disk Drives Required

RAID Level I/Os per second Disk Drives Required

RAID-0 (no fault tolerance) 250 5
RAID-1 (mirroring) 375 7.5 round to 8
RAID-5 (distributed data guarding) 675 13.5 round to 14

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).

Table 15.4 Disk Space by RAID Level

RAID Level Disk Drives Derived Disk Space Provided

RAID-0 (no fault tolerance) 5 5 gigabytes
RAID-1 (mirroring) 8 4 gigabytes
RAID-5 (distributed data guarding) 14 13 gigabytes

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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