Databases Reference
In-Depth Information
A
A Simple Performance
Model for Databases
Performance is your reality. Forget everything else.
—Harold Geneen
his appendix presents a simple performance cost model for evaluating physical
design methods and tradeoffs among various designs. The model includes estima-
tions for input/output (I/O) time and network delays.
T
A.1
I/O Time Cost—Individual Block Access
A block (or page) has been traditionally the basic unit of I/O from disk to fast memory
(RAM). It can range in size from 2 to 16 KB, although 4,096 bytes (4 KB) is the most
typical size in many systems. Blocks usually include many rows in a table, but occasion-
ally a large row can span several blocks. In recent years, prefetch buffers have been used
more often in operational systems to increase I/O efficiency. Prefetch buffers are typi-
cally 64 KB (in DB2, for instance). In some systems the disk track is the I/O transfer
size of choice.
Block access cost
= disk access time to a block from a random starting location
= average disk seek time + average rotational delay + block transfer.
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