With the introduction of OpenStorage disks in NetBackup 6.5, the NetBackup media server may not be the host where backup images are stored. The function of media server is to move data between primary storage (NetBackup client) and secondary storage. The system which provides secondary storage is known as a storage server.
There could be multiple media servers that can send backup images to the same storage server. The access to storage server is controlled by credentials. Any media server that has credential to the storage server can store/retrieve backup images from the storage server.
When there are multiple media servers that can access the same storage server, NetBackup selects a media server based on media server load balancing strategy. This is explained in NetBackup 6.5 System Administrator's Guide I.
There are scenarios where the automatic selection of media server may be undesirable. During backups, it is possible to select a specific media server for an OpenStorage disk pool using storage unit configuration. However this level of control is not available for restores, source side of a duplications and source side of synthetics.
Veritas is working on developing specific features to address these limitations. For the time being, Veritas recommends the following point fixes to address these specific issues.
Issue: The NetBackup environment has OST-connected storage (OST appliance or PDDO) that can be accessed by multiple media servers and SAN media servers. By default restore is subject to media server load balancing logic and sometimes the SAN media servers are selected to read data. Some customers want NetBackup's legacy behavior where the NetBackup administrator can control the media server used for restores.
From NetBackup 6.5.4 onwards, it is possible to use the same media server that did the backup for restores as well. This can be achieved by creating a touch file USE_BACKUP_MEDIA_SERVER_FOR_RESTORE at /usr/openv/netbackup/db/config or Install_PathVeritasNetBackupdbconfig on master server. This configuration, if enabled, affects all media servers using disk pools. Also, if this configuration touch file is created, all FORCE_RESTORE_MEDIA_SERVER and FAILOVER_RESTORE_MEDIA_SERVER entries are ignored.
Issue: The NetBackup environment has media servers and storage servers at multiple sites connected by a Wide Area Network(WAN). Optimized duplications were done from local storage server to remote storage server using the media server available locally. If a restore is attempted from remote copy to a client at the same site, the data may travel across WAN to the media server at the local site.
Use bprestore command with '-disk_media_server' option to force the media server at the remote client to be selected for restores. For Oracle RMAN backups, use the send parameter NB_ORA_DISK_MEDIA_SERVER to specify the desired media server.
Issue: When multiple media servers have credentials to access a storage server, a media server is selected for performing image cleanup (nbdelete) based on media server load balancing. It is possible to have a remote media server that has credentials (connected by a WAN link) being selected to perform cleanup which would be extremely slow.
On NetBackup 7.0.1, it is possible to allocate specific media server to run image cleanup for specific disk media id (associated with a specific disk volume) using a configuration file named CLEANUP_MEDIASERVER_MAP located at /usr/openv/netbackup/db/config or Install_Path\Veritas\NetBackup\db\config on master server.
Issue: When multiple media servers have credentials to the same storage server and if a synthetic backup image needs to be created on the same storage server using one of those media servers, it is possible that the read side of synthetics is started on a different media server. This affects performance of synthetics, it would have been better to start the read side operation in the same media server where synthetic write is being performed.
From NetBackup 6.5.x and 7.x onwards, it is possible to configure NetBackup to assign a common media server for read and write of synthetics if the source and destination are the same diskpool. The following configuration setting can be used.
nbemmcmd -changesetting -common_server_for_dup required -machinename <name_of_the_host>
Please note: This setting will be applicable for both duplication and synthetic jobs.
Issue: During optimized duplications of a batch of images, the images after the first in the batch may not use the requested media server. Disk media id may get truncated for copies created via optimized duplications.
These are defects in NetBackup and were resolved in NetBackup 6.5.6 and NetBackup 7.0.1 releases.
Although Media Server Load Balancing is a great way to identify a media server that has the least load, it may not be the desired host for performing non-backup jobs under certain specific use cases. Veritas recommends using the point solutions listed above to use a specific media server. Work is in progress to improve the media server selection criteria for non-backup jobs.