System Architecture 2-55
Shared-Memory Buffer Pool
The last available entry slot in the user table is always reserved for a
tbmonitorprocess, regardless of whether any other tbmonitor processes are
currently running.
The number of entries in the user table is equal to the maximum number of
users permitted on this OnLine system, specified asUSERS in the configu-
rationfile. If an additional user process attempts to attach to shared memory,
an error is returned.
Each entry in the user table occupies 102 bytes.
Theupper limit for the maximum number of users (specified as USERS in the
configuration file) is 1000. The lower boundary for the value ofUSERS is the
number of page cleaners (specified asCLEANERS in the configuration file)
plus 4, plus 1 if mirroring is enabled. The four minimum user entries are
reserved fortbinit,tbundo,tbmonitor, and an entry for an administrative
user.
Shared-Memory Buffer PoolThe OnLine buffer pool contains two types of buffers:
■Regular buffers
■Big buffers
If data pages are modified, entries are made in two other shared-memory
buffersthat function solely to ensure the physical and logical consistency of
OnLine data:
■Physical log buffer
■Logical log buffer
The functions of the physical and logical log buffers are described on
page 2-63 andpage 2-66, respectively.