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Cache configuration. The database cache is the chunk of volatile memory (RAM) where the engine keeps the most recent data for fast retrieval. This improves performance as opposed to disk reads. There're two types of cache settings:
- The database cache itself, measured in pages, is the size of the buffer kept by the engine as long as there's at least one attachment to that database. Each database kept open by the engine has its separate cache.
- The client map size, measured in pages, is the size of the communication buffer for each IB client and is only meaningful for local clients, namely, clients that are in the same machine than the database. Therefore, this setting is important for the DBA working locally with the engine and in standalone, single-user applications using IB as a silent companion.. Also, this is only meaningful in platforms where a local connection is possible, for example, in Windows NT (through IPC).
A page is defined as a memory unit, not necessarily the same than the operating system's page size in memory or to the file system's cluster size or block size on a hard disk's partition. Since IB's roots are in the years when a workstation with 4 MB RAM was considered a luxury and small IBM mainframes exhibited 16 MB RAM, the default page size has been set to 512 KB, so the engine allocates small chunks on disk space and small chunks on RAM.
There are different levels of specifications for the database cache, always in number of database pages, from the most generic to the most specific:
- The engine itself comes with a predefined default, 256 pages of 512 KB each one. Those values are hard coded in the engine. It's the global setting for the cache size used for each database touched by the engine. Remember each database when opened gets its own cache in RAM.
- In the configuration file, (ibconfig in Windows and isc_config in UNIX), those global settings can be changed. In fact, the configuration file comes with the default values commented with "#" before.
- Using gfix or an equivalent utility, the setting is for a specific database. This overrides the prior engine's cache size for this database. If the database setting is left as zero, the engine's default applies.
- At connection time, using the parameters in the API, an specific attachment can be made with an specific cache size that overrides the global setting (but not the database setting, if the db setting is greater than zero). This value only is honored if it's greater than the value being used by other connections already active on the same database.
The facts are complicated by the presence of two IB architectures in some platforms since IB4.2 on Windows introduced the Super Server design. In Classic, since each connection is handled by a separate process, each connection has its own cache that must be kept consistent with other cooperating server processes working on the same database. Conversely, Super Server handles all connections inside one process with multiple threads, so one database gets effectively one cache, since threads by design share the address space of the owning process.
Usually, the manual known as "Operations Guide" describes in details the available settings for tweaking and configuring IB. Also, see this article on cache settings priority.
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