Databases Reference
In-Depth Information
• Disk drives
• Disk controllers
• Flash memory
• CPUs
• Power supplies
• Cooling fans
• Network cards
Systems designed with this in mind include Oracle's engineered systems. For example,
the Oracle Exadata Database Machine and other Oracle engineered systems designed
to run the Oracle Database are pre-configured with redundant components to eliminate
single point of failure for any of the above.
Disk failure is the largest area of exposure for hardware since disks have the shortest
mean times to failure of any of the components in a computer system. Disks also present
the greatest variety of redundant solutions, so discussing that type of failure here should
provide a good example of how high availability can be implemented with hardware.
Disk Redundancy
Although the mean time to failure of an individual disk drive is very high, the ever-
increasing number of disks used for today's very large databases results in more frequent
failures. Protection from disk failure is usually accomplished using RAID (Redundant
Array of Inexpensive Disks) concepts. The use of redundant storage has become com‐
mon for systems of all sizes and types for two primary reasons: the real threat of disk
failure and the proliferation of packaged, relatively affordable RAID solutions.
RAID uses one of two concepts to achieve redundancy:
Mirroring
The actual data is duplicated on another disk in the system.
Striping with parity
Data is striped on multiple disks, but instead of duplicating the data itself for re‐
dundancy, a mathematical calculation termed
parity
is performed on the data and
the result is stored on another disk. You can think of parity as the sum of the striped
data. If one of the disks is lost, you can reconstruct the data on that disk using the
surviving disks and the parity data. The lost data represents the only unknown
variable in the equation and can be derived. You can conceptualize this as a simple
formula:
A + B + C + D = E
