Understanding and Designing Serviceguard Disaster Recovery Architectures

dictates) or store it offline in a vault. If a disaster occurs at one site, the offline copy of data is used
to synchronize data at a remote site which functions in place of the failed site.
Data is replicated using physical offline backup, therefore data consistency is fairly high, barring
human error or an untested corrupt backup. However, data currency is compromised by the time
delay in sending the tape backup to a remote site.
Offline data replication is acceptable for many applications for which recovery time is not an issue
critical to the business. Although data replication might be performed daily or weekly, recovery
can might require a day to a week depending on the volume of data. Some applications, depending
on the role they play in the business, may need to have a faster recovery time, within hours or
even minutes.
Online Data Replication
Online data replication is a method of copying data from one site to another through a link. It is
used when very short recovery time, from minutes to hours, is required. To recover the use of a
system in a short time, the data at the alternate site must be replicated in real time to alternate site
disks.
Data can be replicated either synchronously or asynchronously. Synchronous replication requires
one disk write to be completed and replicated before another disk write can begin. This method
improves the possibility of keeping data consistent and current during replication. However, it
greatly reduces replication capacity, performance and, system response time due to increased
latency in data replication link.Asynchronous replication does not require the primary site to wait
for the completion of one disk write before beginning another. This can result in a data currency
issue, depending on the volume of transactions. An application that has very large volume of
transactions can get delayed by hours or days while replicating using asynchronous replication.
If the application fails over to the remote site, it will start up with data that is not current.
Currently, the two ways of online data replication are physical data replication and logical data
replication. Either of these can be configured to use synchronous or asynchronous writes.
Physical Data Replication
When a physical write is performed on a disk, it is replicated on another disk at another site. This
replication is not application dependant because the replication is a physical write to disk. This
allows each node to run different applications under normal circumstances. If the replicated data
is current and consistent, whenever a disaster occurs, an alternate node can take ownership of
applications and data.
As shown in Figure 3, physical replication can be done in software or hardware.
10 Disaster Recovery in a Serviceguard Cluster