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Databases on RAID 5 devices

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We will be implementing an Oracle 8i database on an N-class HP, 12GB RAM and 8 CPUS (550mhz). There is no internal DASD for this server. It is all on either XP/250 or IBM shark DASD. There is RAID 5 technology. My question is should I still pay attention to spreading the database around multiple mount points? It seems like this may no longer be necessary. We are now given a "slice" of DASD from a remote source. It is then divided into...

logical devices on our server. I remember the days of being taught the "7 disk solution" or "12 disk solution" as taught by Oracle. I do not know if this still applies. Opinions?

This type of question is becoming more frequent, especially as hardware vendors sell larger and larger disk volumes.

First off, be wary of using RAID 5 for an Oracle database. RAID levels 3 and 5 employ a "write-penalty". It takes longer to write to RAID 5 volumes than it does to write to non-RAID volumes. This is because parity bits need to be computed and stored as well as storing your data. This can slow things down considerably for write-intensive database operations.

Some people will tell you to never use RAID 5 for an Oracle database. I have a database here where I am using RAID 5, but that data is read-mostly. Things I *never* put on RAID 5 are control files, online redo logs, temp tablespace, and the archive log destination. These objects are write intensive and RAID 5 will slow down database operations if these objects are on RAID 5 devices.

Now on to your question... One can just throw datafiles on a RAID 5 disk volume. Yes, RAID will take care of striping those datafiles. So your chances of an index and a table falling on the same volume are decreased. But those chances are not eliminated. You have no control over where the index and table will fall. To maintain that control, I suggest that you use multiple RAID 5 partitions. This way, you can be guaranteed of separating multi-concurrent objects (such as a table and its index).

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This was first published in February 2002

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