System Architecture 2-51
Shared-Memory Internal Tables
The minimum value of DBSPACES is 1, representing the root dbspace.
OnLine Latch Table
The latch table is not a table in the same sense as other tables, but it functions
in a similar manner to track all latches in the OnLine system. Refer to
page 2-41 for a detailed discussion of how OnLine uses latches to control and
manage modifications to OnLine shared memory.
The number of latch entries is equal to the number of shared-memory
resources configured for the OnLine system. As OnLine administrator, you
cannot modify the number of latches.
You can obtain information about latches from the tbstat -s output. You can
use the user process information from the tbstat -u output to determine if a
user process is holding a latch.
OnLine Lock Table
The lock table represents the pool of available locks. Each entry in the lock
table represents one lock. A single transaction can own multiple locks. A
single user process is limited to 32 concurrent table locks. The lock table
includes an associated hash table.
When an entry in the lock table is used, a lock is created. The information
stored in the table describes the lock. The lock description includes the
following four items:
■ The address of the user process that owns the lock
■ The type of lock (exclusive, update, shared, byte, or intent)
■ The page and/or rowid that is locked
■ The tblspace where the lock is placed
The number of entries in the lock table is equal to the maximum number of
locks in this OnLine system, specified as LOCKS in the configuration file. Each
entry in the lock table occupies 32 bytes.
Refer to IBM Informix Guide to SQL: Tutorial for an explanation of locking and
SQL statements. Refer to page 2-38 for a detailed explanation of the effect of
locks on buffer management. Refer to page 7-88 for information on
monitoring locks with tbstat -k.
