IBM 000-8697 사용자 설명서

IBM Informix OnLine Database Server Administrator’s Guide

OnLine requires a technique to ensure that all OnLine user processes find and
gain access to the same shared-memory segments. If two or more OnLine
database servers exist on a single

UNIX

 host machine, the shared-memory

segments associated with each OnLine server exist at different locations in
physical memory. The shared-memory segments for each OnLine server
must be uniquely identifiable to the database user processes.

The following list outlines the major steps that are completed when an
OnLine user process attaches to shared memory:

User process obtains SERVERNUM from configuration file.

User process calculates a shared-memory key value using
SERVERNUM.

User process requests a shared-memory segment using the shared-
memory key value. UNIX returns the shared-memory identifier for
the first shared-memory segment.

User process directs UNIX to attach the shared-memory segment to
its process space at SHMBASE.

If required, additional shared-memory segments are attached to be
contiguous with the first segment.

The tbinit daemon process creates the

 OnLine

shared-memory segments

during initialization. (Refer to

.) Associated with each shared-

memory segment are two pieces of information:

■

A shared-memory key value

■

A shared-memory identifier

When a user process directs the

UNIX

operating system to a shared-memory

segment, it defines the segment it needs using the shared-memory key value.
To attach to the segment, the user process must acquire from

UNIX

the shared

memory identifier that is associated with that segment. The

UNIX

 operating

system uses these identifiers to track each shared-memory segment.