Donald K. Burleson Oracle Tips

Using RAID with Oracle

RAID is the acronym for Redundant Arrays of Inexpensive Disks, and there are many different types or RAID (called RAID “levels”), and each has its own relative advantages and disadvantages. For Oracle9i databases, many of the RAID schemes do not posses the high performance required for an Oracle database, and moist Oracle professionals choose a RAID scheme that combines mirroring and block-level striping.

It is critical to note that the use of RAID does not guarantee against catastrophic disk failure. Oracle specifically recommends that all production databases be run in archivelog mode regardless of the RAID architecture, and that periodic Oracle backups should be performed. Remember that there are many components to I/O subsystems—including controllers, channels, disk adapters, SCSI adapters—and a failure of any of these components could cause unrecoverable disk failures of your database.

Below are the most commonly used RAID architectures for Oracle databases:

RAID 0

RAID 0 is commonly referred to as block-level striping. This is an excellent method for performing load balancing of the Oracle database on the disk devices, but it does nothing for high availability since none of the data is duplicated. Unlike manual datafile striping, where the Oracle professional divides an Oracle tablespace into small datafiles, with RAID 0, the Oracle datafile is automatically striped one block at a time across all of the disk devices. In this fashion, every datafile has pieces residing on each disk, and the disk I/O load will become very well balanced. 

RAID 1

RAID 1 is commonly called disk mirroring. Since the disks are replicated, RAID 1 may involve double or triple mirroring. The RAID 1 architecture is designed such that a disk failure will cause the I/O subsystem to switch to one of the replicated disks with no service interruption. RAID 1 is use when high availability is critical, and with triple mirroring, the mean time to failure (MTTF) for an Oracle database is measured in decades.

RAID 0+1 (RAID 10)

Raid 0+1 is the combination of block-level striping and disk mirroring. The advent of RAID 0+1 has made Oracle-level striping obsolete since RAID 0+1 stripes at the block level, dealing out the table blocks, one block per disk, across each disk device. RAID 0+1 is also a far better striping alternative since it distributes the load evenly across all of the disk devices, and the load will rise and fall evenly across all of the disks. This relieves the Oracle administrator of the burden of manually striping Oracle tables across disks.

RAID 5

Some of the newer hardware based Raid 5 storage does extremely well in performance in data warehouses. RAID 5 is a good approach for Oracle data warehouses where the load speeds are not important and where the majority of the system I/O is read-only activity.

If you like Oracle tuning, check out my latest book "Oracle Tuning: The Definitive Reference".  It's 980 pages of hard-core tuning insights, tips and scripts, and you can buy it direct from the publisher for 30%-off. Best of all, you get instant access to the code depot of Oracle tuning scripts.