Without continuous access to the application data, your
application servers cannot process client requests. The product supports
various methods of maintaining data availability.
If your data resides on IBM® i servers, you can use one of
the following options to achieve higher availability for your data:
Data replication
To ensure the availability
of all components of your application server environment, you can
use data replication to create a backup copy of your application data.
Data replication uses clustering, remote journaling, and third-party
data replication software to maintain two physically separate copies
of application data. Data replication can provide disaster recovery
capabilities for the databases because the systems in the cluster
can be geographically separated.
Clustering provides the basis
for communication between two or more servers. This communication
is necessary to back up your data on a physically separate machine.
Remote
journaling creates a copy of your application data to assure hot backup.
There are two types of remote journaling:
- Synchronous remote journaling, where data is written to both the
primary and backup databases simultaneously. This type of journaling
ensures that no entries are lost in the event of a system failure.
However, it can negatively impact performance, because it requires
the application to wait while data is written to both databases.
- Asynchronous remote journaling, where data is written to the primary
database directly from the application. This type of journaling, enables
the application to continue processing client requests while data
is copied to the backup system. It does not impact application performance,
but the most recent entries might be lost if a failure occurs.
Switchable disk
Switchable disk uses clustering,
local journaling, and Independent Auxiliary Storage Pool (IASP) to
ensure the availability of application data. The application data
is stored in an IASP, and if a failure occurs, the IASP can be switched
to another node.
The advantage of switchable disk is that no
data replication occurs. Therefore, you do not have to worry about
data synchronization. However, the database becomes a single point
of failure. In addition, the backup node must be physically close
to the primary node. Therefore, this configuration does not provide
disaster recovery.
Switchable disk uses the following components
and services:
- A server cluster. You must have the High Availability Switchable
Resources function (5761-SS1 or 5770-SS1 Option 41) installed on all
of the nodes in your cluster.
- Clustering. When you use switchable disk, clustering provides:
- The services required to switch the IASP.
- Wellness monitoring of backend database partitions.
- Local journaling. Switchable disk uses local journaling
to preserve database transaction boundaries.
Switchable disk is best utilized in a multiple cell topology.
that includes these features:
- All of the cells process client requests as described in multiple
cell topology.
- One of the cells hosts the primary database system, which receives
data from application servers on the other cells.
- The primary database system connects to an IASP.
- One of the other cells hosts the secondary database system, which
is inactive unless there is a failure in the primary system. If the
primary system fails, application data is directed to the secondary
database system in this other cell, and the IASP connects to the secondary
system.