Tivoli Storage Manager backup concepts relevant to cluster environments
One of the biggest challenges when providing backup and recovery in a cluster environment is determining where backup operations should be performed in the context of the cluster's resources. Clusters are designed to offer high-availability to applications and users. A user of a mail or database application does not need to know which host physically owns a disk resource, but instead relies upon a virtualized connection to the cluster's resources. For example, any movement of resources within a cluster between hosts due to failover or load balancing is hidden from the end-user. A backup product can be placed within or external to the cluster's resource groups.
If the backup product is placed outside of the context of the cluster and its resource groups (for example, on a host that is not part of the cluster), it can map or mount the file systems and perform the backups relative to the mapped or mounted drives. The advantage to this model has already been demonstrated: As the disk resources move between the physical cluster hosts, the backup product retains the mapping of the file system. The big disadvantage of this method is that backup of data over network protocols such as Network File System or Common Internet File System is slower than backup of local file systems. This becomes a factor as data volumes grow and backup windows shrink. Another disadvantage is that you have no protection if the host you have chosen for backups fails!
The Tivoli Storage Manager Backup-Archive client is designed to manage the backup of cluster drives by placing the backup-archive client within the context of the cluster's resource groups. This gives the advantage of backing up data from local resources (as opposed to accessing the data across the network) to maximize the performance of the backup operation and to manage the backup data relative to the resource group. Therefore, the backup-archive client can always back up data on cluster resources, as if the data were local data, and maximize backup performance. This ensures that critical data is getting backed up across system failures.
The Tivoli Storage Manager Backup-Archive Client is not cluster aware, in that it does not use any API to determine the context under which it is running or provide any explicit mechanisms for failover or high-availability. The Tivoli Storage Manager Backup-Archive client does offer fault-tolerant scheduling capabilities which can be exploited to provide protection in cluster environments; this makes the Tivoli Storage Manager backup scheduling service highly available. For example, a schedule can be defined that performs an incremental backup of the /group1_disk1 file system. If the /group1_disk1 file system fails to another host in the cluster while the backup is being performed, the scheduled incremental operation is restarted from the other host, if the configuration steps outlined below are followed correctly. The progressive incremental backup methodology employed by Tivoli Storage Manager determines if files are already backed-up, and effectively continues processing the incremental backup from the point where the failover occurred.