User-definable caches are a performance feature of Adaptive Server. This chapter addresses only the mechanics of configuring caches and pools and binding objects to caches. Performance information and suggested strategies for testing cache utilization is addressed in Chapter 5, “Memory Use and Performance,” in the Performance and Tuning Series: Basics.
Here are some general guidelines:
The size of the default data cache does not decrease when you configure other caches.
Make sure that your default data cache is large enough for all cache activity on unbound tables and indexes. All objects that are not explicitly bound to a cache use the default cache. This includes any unbound system tables in the user databases, the system tables in master, and any other objects that are not explicitly bound to a cache.
During recovery, only the default cache is active. Transactions logs are read the default cache. All transactions that must be rolled back or rolled forward must read data pages into the default data cache. If the default data cache is too small, it can slow recovery time.
Do not “starve” the 2K pool in any cache. For many types of data access, there is no need for large I/O. For example, a simple query that uses an index to return a single row to the user might use 4 or 5 2K I/Os, and gain nothing from 16K I/O.
Certain commands can perform only 2K I/O: certain dbcc commands, and drop table. dbcc checktable can perform large I/O, and dbcc checkdb performs large I/O on tables and 2K I/O on indexes.
For caches used by transaction logs, configure an I/O pool that matches the default log I/O size. This size is set for a database using sp_logiosize. The default value is 4K.
Trying to manage every index and object and its caching can waste cache space. If you have created caches or pools that are not optimally used by the tables or indexes bound to them, they are wasting space and creating additional I/O in other caches.
If tempdb is used heavily by your applications, bind it to its own cache. You can bind only the entire tempdb database—you cannot bind individual objects from tempdb.
For caches with high update and replacement rates, be sure that your wash size is large enough.
On multi-CPU systems, spread your busiest tables and their indexes across multiple caches to avoid spinlock contention.
Consider reconfiguring caches or the memory pools within caches to match changing workloads. Reconfiguring caches requires a restart of the server, but memory pool reconfiguration does not.
For example, if your system performs mostly OLTP (online transaction processing) during most of the month, and has heavy DSS (decision-support system) activity for a few days, consider moving space from the 2K pool to the 16K pool for the high DSS activity and resizing the pools for OLTP when the DSS workload ends.
