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Oracle Tips by Burleson |
OCP Instructors Guide for
Oracle DBA Certification
Chapter 8 -
Miscellaneous Oracle Information
Monitoring Space
Utilization and Performance Statistics Historically
select tspace, fileid, run_date, maxspace, minspace, avgspace,
totspace from space_tracker
order by tspace, fileid, run_date;
Latches and Enqueues
What exactly is a latch? Oracle support
describes latches as "low level serialization mechanisms used to
protect shared data structures in the SGA. The implementation of
latches is operating system dependent, particularly in regard to
whether a process will wait for a latch and for how long." Its job
is to prevent multiple processes from executing the same piece of
code at the same time.
Enqueues, another locking mechanism employed by
Oracle, allow several processes to share a single database
resource. A process that wants to read a table issues a read table
lock that allows other processes to access the table is a good
example of an enqueue. An enqueue allows multiple processes to
store the lock mode to provide varying degrees of sharing. Enqueues
do not rely upon the database’s internal locking mechanism to keep
track of the locked resource, they instead use the operating
system's locking mechanism to provide the sharing functionality.
If a process has to wait because of an incompatible enqueue, the
operating system will place the requesting process on a wait queue.
The above text is
an excerpt from:
OCP Instructors Guide for Oracle DBA Certification
A Study Guide to Advanced Oracle Certified Professional Database
Administration Techniques
ISBN 0-9744355-3-8
by Christopher T. Foot
http://www.rampant-books.com/book_2003_2_OCP_print.htm
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