Multiplexing control files
We currently run with Oracle9i (9.2.0) on Sun Solaris and all disks are on the SAN EVA technology. Recently a consultant told us that we don't have to multiplex the control file because we are on another type of storage (SAN EVA tech). Is it true? Is there possible logical corruption? The consultant told us that we just have to bring the backup of the control file if ever we have problem. What if we extent a tablespace on a new datafile and we have to bring back a backup that doesn't include that datafile? Isn't it better to have more than one copy of the controlfile with no regard to what technology we are using for data storage? Is there any doc concerning issues with that new technology?
Your consultant is giving you advice that many top-level DBAs would
simply ignore. Always let Oracle multiplex your control files for you.
Never rely on the hardware to make multiple copies for you. There are a
couple of reasons. First, if a SysAdmin or DBA accidentally deletes the
control file, the mirror copy is deleted as well. If Oracle maintained
multiplexed copies for you, you could simply replace the missing
control file with one of its copies. Second, if there is some sort of
corruption in the control file, it can propagate to the hardware
mirrored copy. It would be more difficult for Oracle multiplexed
control files to all get corrupted. Finally, restoring a backup control
file is a last resort in my book. When you restore from a backup
control file, your only option is to open the database with RESETLOGS.
This means that you should immediately shut down the database and take
a cold backup. If you restore your missing control file from another
existing control file, you can open the database normally and you do
not need to take an immediate backup.
The advice to rely on hardware or non-Oracle software to maintain a
mirror copy of your control files and online redo logs is bad advice in
my opinion. These files are not that big and multiplexing them through
Oracle can save you tons of heartache down the road. I always multiplex
my control files and online redo logs within Oracle.
This was first published in May 2004