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4.13 General database tuning
This section provides general tuning hints for databases.
4.13.1 Providing adequate statistics for optimization
Databases usually offer a wide variety of available choices when determining the best
approach to access data. Statistics, which describe the shape of the data, are used to guide
the selection of a low-cost data access strategy. Statistics are maintained in tables and
indexes. Examples of statistics include the number of rows in a table and the number of
distinct values in a certain column.
Gathering statistics can be expensive, but fortunately, for many workloads, a set of
representative statistics allows for good performance over a large span of time. Refreshing
statistics periodically if the data population shifts dramatically might be necessary.
4.13.2 Placing database log files on a fast disk subsystem
Databases are designed for high availability, transactional processing, and recoverability. For
performance reasons, changes made to table data might not be written immediately to disk.
These changes can be recovered if they are written to the database log. Updates are made to
database log files when the log buffer fills, at transaction-commit time, and for some
implementations after a maximum interval of time.
As a result, database log files might be heavily used. More important, the log-writes hold
commit operations pending, meaning that the application is synchronously waiting for the
write to complete. Therefore, the performance of write access to the database log files is
critical to overall system performance. For this reason, we suggest that database log files be
placed on a fast disk subsystem with write-back cache.
4.13.3 Placing logs on a separate device from the table space containers
A basic strategy for all database storage configurations is to place the database logs on
dedicated physical disks, ideally on a dedicated disk adapter. This placement reduces disk
access contention between I/O to the table space containers and I/O to the database logs
and preserves the mostly sequential access pattern of the log stream. Such separation also
improves recoverability when log archival is employed.
4.13.4 Providing sufficient physical memory
Accessing data in memory is much faster than reading it from disk. Because 64-bit hardware
is readily available and memory prices continue to fall, the sensible approach is to provision
enough memory to avoid most disk reads in steady state for many performance-critical
workloads.
Be careful to avoid virtual memory paging in the database machine. The database manages
its memory with the assumption that it is never paged and does not cooperate well if the
operating system swaps some of its pages to disk.
80 IBM Business Process Manager V8.0 Performance Tuning and Best Practices