Understanding and Designing Serviceguard Disaster Recovery Architectures

Figure 4 Logical Data Replication
Node 1 Node 2
Logical Replication
LAN
Advantages of using logical replication are:
• The distance between nodes is limited by the networking technology.
• There is no additional hardware needed to perform logical replication, unless you want to
boost CPU power and network bandwidth.
• You can implement Logical replication to reduce risk of duplicating human error. For example,
if a database administrator erroneously removes a table from the database, a physical
replication method duplicates that error at the remote site as a raw write to disk, but a logical
replication method can be implemented to delay applying the data at a remote site. Therefore
such errors will not be replicated at the remote site. This also means that administrative tasks,
such as adding or removing database tables, must be repeated at each site.
• With database replication you can roll transactions forward or backward to achieve the level
of currency desired on the replica, although this functionality is not available with file system
replication.
Disadvantages of logical replication are:
• It uses significant CPU overhead because transactions are often replicated more than once
and logged to ensure data consistency, with an exception of the simple database transactions
all transactions take significant CPU. It also uses network bandwidth, whereas most physical
replication methods use a separate data replication link. As a result, there may be a significant
delay in replicating transactions at the remote site, which affects data currency.
• If the replica takes over when the primary database fails and is corrupt, then the process to
restore the primary database to be used as the replica is complex. This often involves recreating
the database and doing a database dump from the replica.
• Applications must be modified frequently to work in an environment that uses a logical
replication database. Logical errors present in applications or in the RDBMS code itself that
may cause database corruption is replicated to remote sites. This is also an issue with physical
replication.
Ideal Data Replication
The ideal disaster recovery architecture is made up of the following combination:
• For performance and data currency—physical data replication.
• For data consistency—either a second physical data replication as a point-in-time snapshot
or logical data replication, which is used only in the cases where the primary physical replica
is corrupt.
Disaster Recovery Architecture Guidelines 13