There will be two
sizes of I/O by Oracle: a single 8 KB data block and a 256 KB multiblock read (32 times
8 KB). Suppose you configure a four-disk array for use by Oracle with a chunk size of 64
KB so that the 256 KB of data will be spread across the four drives, with 64 KB on each.
Oracle and Parallelism | 169
Each 8 KB I/O will hit one spindle, as the 8 KB will lie within one 64 KB chunk.*
Striping can increase performance for small I/Os by maximizing concurrency: each
disk can service a different I/O. The multiblock I/Os of 256 KB may hit all four disks.
If the chunk size were 256 KB instead of 64 KB, on average each 256 KB I/O call
would hit one disk. In this case, the multiblock I/O will require fewer I/O calls with a
larger chunk size on the disks. In either case, a single disk will clearly satisfy singledata-
block I/O calls. Striping can increase I/O rates for large reads by driving
multiple disks with a single I/O call, as illustrated with a 64 KB chunk size and a
256 KB multiblock I/O.
Figure 7-4 illustrates the interaction of different-sized Oracle I/Os with arrays striped
using different chunk sizes.
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