Databases Reference
In-Depth Information
them. However, rebuilding control files can be difficult, introduces risk, and can be
easily avoided.
The location of the control files is defined, as previously mentioned, by the CON‐
TROL_FILES initialization parameter. You can specify multiple copies of control files
by indicating multiple locations in the CONTROL_FILES parameter for the instance,
as illustrated here:
control_files = (/u00/oradata/control.001.dbf,
/u01/oradata/control.002.dbf,
/u02/oradata/control.003.dbf)
This parameter tells the instance where to find the control files. Oracle will ensure that
all copies of the control file are kept in sync so all updates to the control files will occur
at the same time. If you do not specify this parameter, Oracle will create a control file
using a default filename or by leveraging Oracle Managed Files (if enabled).
Many Oracle Databases are deployed on some type of redundant disk solution such as
RAID-1 or RAID-5 to avoid data loss when a disk fails. (RAID is covered in more detail
in
Chapter 7
.) You might conclude that storing the control file on protected disk storage
eliminates the need for maintaining multiple copies of control files and that losing a
disk won't mean loss of the control file. But there are two reasons why this is not an
appropriate conclusion:
1. If you lose more than one disk in a
striped array
or
mirror-pair
, you will lose the
data on those disks. This type of event is statistically rare, but if it does occur, you
could be faced with a damaged or lost control file. As you would have your hands
full recovering from the multiple disk failures, you would likely prefer to avoid
rebuilding control files during the recovery process. Multiplexing your control files,
even when each copy is on redundant disk storage, provides an additional level of
physical security.
2. Redundant disk storage does nothing to protect you from the perpetual threat of
human error. Someone could inadvertently delete or rename a control file, copy
another file over it, or move it. A mirrored disk will faithfully mirror these actions,
and multiplexed control files will leave you with one or more surviving copies of
the control file when one of the copies is damaged or lost.
You do not need to be concerned with additional performance impact when writing to
multiple control files. Updates to the control files are insignificant compared to other
disk I/O that occurs in an Oracle environment.
