Apress.Expert.Oracle.Database.Architecture.9i.and.10g.Programming.Techniques.and.Solutions.Sep.2005

CHAPTER 5 ■ ORACLE PROCESSES 181
CTWR: Change Tracking Processes
This is a new optional process of the Oracle 10g database. The CTWR process is responsible for
maintaining the new change tracking file, as described in Chapter 3.
RVWR: Recovery Writer
This process, another new optional process of the Oracle 10g database, is responsible for
maintaining the before images of blocks in the Flash Recovery Area (described in Chapter 3)
used with the FLASHBACK DATABASE command.
Remaining Utility Background Processes
So, is that the complete list? No, there are others. For example, Oracle Data Guard has a set of
processes associated with it to facilitate the shipping of redo information from one database
to another and apply it (see the Data Guard Concepts and Administration Guide from Oracle
for details). There are processes associated with the new Oracle 10g Data Pump utility that you
will see during certain Data Pump operations. There are Streams apply and capture processes
as well. However, the preceding list covers most of the common background processes you
will encounter.
Slave Processes
Now we are ready to look at the last class of Oracle processes: the slave processes. There are
two types of slave processes with Oracle: I/O slaves and parallel query slaves.
I/O Slaves
I/O slaves are used to emulate asynchronous I/O for systems or devices that do not support it.
For example, tape devices (which are notoriously slow) do not support asynchronous I/O. By
using I/O slaves, we can mimic for tape drives what the operating system normally provides
for disk drives. Just as with true asynchronous I/O, the process writing to the device batches a
large amount of data and hands it off to be written. When the data is successfully written, the
writer (our I/O slave this time, not the operating system) signals the original invoker, who
removes this batch of data from its list of data that needs to be written. In this fashion, we
can achieve a much higher throughput, since the I/O slaves are the ones waiting for the slow
device, while their caller is off doing other important work getting the data together for the
next write.
I/O slaves are used in a couple of places in Oracle. DBWn and LGWR can make use of them to
simulate asynchronous I/O, and RMAN will make use of them when writing to tape.
Two parameters control the use of I/O slaves:
• BACKUP_TAPE_IO_SLAVES: This parameter specifies whether I/O slaves are used by RMAN
to back up, copy, or restore data to tape. Since this parameter is designed around tape
devices, and tape devices may be accessed by only one process at any time, this parameter
is a Boolean, and not the number of slaves to use, as you might expect. RMAN will
start up as many slaves as necessary for the number of physical devices being used.