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Donald K. Burleson

Oracle Tips  

Oracle Disk Shadowing or Mirroring

If you will have mission-critical applications that you absolutely cannot allow to go down, consider disk shadowing or mirroring. As the term implies, disk shadowing or mirroring is the process whereby each disk has a shadow or mirror disk to which data is written simultaneously. This redundant storage allows the shadow disk or set of disks to pick up the load in case of a disk crash on the primary disk or disks; thus the users never see a crashed disk. Once the disk is brought back online, the shadow or mirror process brings it back in sync by a process appropriately called resilvering. This also allows for backup since the shadow or mirror set can be broken (e.g., the shadow separated from the primary), a backup taken, and then the set resynchronized. I have heard of two, three, and even higher-number mirror sets. Generally, I see no reason for more than a three-way mirror as this allows for the set of three to be broken into a single and a double set for backup purposes. Shadowing or Mirroring is RAID1.

The main disadvantage to disk shadowing is the cost: For a 200-GB disk “farm,” you need to purchase 400 or more gigabytes of disk storage.

Oracle Redundant Arrays of Inexpensive Disks (RAID)

The main strength of RAID technology is its dependability. In a RAID5 array, the data is stored, as is parity data and other information about the contents of each disk in the array. If one disk is lost, the others can use this stored information to re-create the lost data. This makes RAID 5 very attractive. RAID 5 has the same advantages as shadowing and striping but at a lower cost. It has been suggested that if the manufacturers would use slightly more expensive disks (RASMED—redundant array of slightly more expensive disks) performance gains could be realized. A RAID 5 system appears as one very large, reliable disk to the CPU. There are several levels of RAID to date:

RAID0. Known as disk striping.

RAID1. Known as disk shadowing.

This is an excerpt by Mike Ault’s book “Oracle Administration & Management”.  If you want more current Oracle tips by Mike Ault, check out his new book “Mike Ault’s Oracle Internals Monitoring & Tuning Scripts” or Ault’s Oracle Scripts Download.




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