If dedupe is enabled on Client side, 1st backup will be all data, after that only unique data will be sent to media server.
If multiple clients are backed up to same media server, media server will globally dedupe as well.
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Extract from Dedupe Guide:
To enable Client Deduplication, set an attribute in the NetBackup master server
Client Attributes host properties.
To specify the clients that deduplicate backups
1 In the NetBackupAdministrationConsole, expand NetBackupManagement
> Host Properties > Master Servers.
2 In the details pane, select the master server.
3 On the Actions menu, select Properties.
4 On the Host Properties General tab, add the clients that use client direct to
the Clients list.
5 Select one of the following Deduplication Location options:
■ Always use themediaserver disables client deduplication. By default, all
clients are configured with the Always use the media server option.
■ Prefer to use client-side deduplication uses client deduplication if the
PureDisk plug-in is active on the client. If it is not active, a normal backup
occurs; client deduplication does not occur.
■ Always use client-side deduplication uses client deduplication. If the
deduplication backup job fails, NetBackup retries the job.
You can override the Prefer to use client-side deduplication or Always use
client-side deduplication host property in the backup policies.
