
System Architecture 2-51
Shared-Memory Internal Tables
The minimum value of DBSPACES is 1, representing the root dbspace.
OnLine Latch Table
Thelatch table isnota table inthesame sense asothertables, but itfunctions
in a similar manner to track all latches in the OnLine system. Refer to
page 2-41foradetaileddiscussionofhowOnLineuseslatchestocontroland
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
locksinthisOnLinesystem,specifiedasLOCKSintheconfigurationfile.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.
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