Databases Reference
In-Depth Information
determine what fraction of the data is active, and use only the active proportion in the
ratio calculation.
13.9
Strategies for Availability and Recovery
Before discussing the strategies for data recovery and availability it's worth pausing to
consider the degree to which this is at all needed. There are essentially two major sce-
narios. The first scenario concerns itself with the case that a database server fails for
any reason including hardware fault or software crash, and while that server is being
repaired we require a strategy that allows processing to continue. The second scenario
is concerned with possible geographic disaster, where entire neighborhoods or cities
can experience a serious disaster that would make the database unusable either
because it has been destroyed or because connectivity to the region has been disrup-
tive (a working database that nobody can connect to is less than useful). This would
include major disasters such as New York City during the September 11 attacks, or
the devastation caused by hurricane Katrina
5
that caused massive destruction.
The major strategies for database availability fall into six categories:
1.
Application controlled multicast.
2.
Replication.
3.
Transaction log in-memory shipping.
4.
Shared disk.
5.
Cluster failover.
6.
Flash copy or backup transfer.
With application-controlled standby server, the application applies all change in
the database to two or more database servers. We've called this “application-controlled
multicast” for want of a better term. As a result at any point in time there is more than
one database that has all the required data. Should one database server fail, the remain-
ing servers are available with all of the up-to-date data. The benefit of this strategy is
that it is entirely within the control of the application designer, and no dependency is
made on recovery features of the database server. However, there are several issues with
this strategy. For example:
5
The territory defined as a disaster zone by the United States government following Katrina cov-
ers an area of land approximately the size of the United Kingdom. Katrina caused mass destruc-
tion over much of the north-central Gulf Coast of the United States. Worst hit were New
Orleans, Louisiana, and coastal Mississippi.
