Donald K. Burleson
Oracle Disk Shadowing or
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
Oracle Redundant Arrays of Inexpensive
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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