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System Architecture 2-55
Shared-Memory Buffer Pool
The last available entry slot in the user table is always reserved for a
tbmonitor process, 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 as USERS in the configu-
ration file. If an additional user process attempts to attach to shared memory,
an error is returned.
Each entry in the user table occupies 102 bytes.
The upper limit for the maximum number of users (specified as USERS in the
configuration file) is 1000. The lower boundary for the value of USERS is the
number of page cleaners (specified as CLEANERS in the configuration file)
plus 4, plus 1 if mirroring is enabled. The four minimum user entries are
reserved for tbinit, tbundo, tbmonitor, and an entry for an administrative
user.
Shared-Memory Buffer Pool
The 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
buffers that 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 and page 2-66, respectively.