About scan hosts on UNIX/Linux
Scan hosts are a component of the NetBackup Shared Storage Option.
Each shared drive has a host that is identified as the scan host. A scan host is the host from which the automatic volume recognition process (avrd) scans unassigned drives. (The robotic daemons scan assigned drives.) A scan host must have data path access to the drive.
The EMM database contains the shared drive information; that information includes the scan host. Media servers receive drive status information from the EMM server.
How the scan host is determined
EMM determines scan hosts; a scan host may be different for each shared drive. The first host in the environment to come online with a non-zero scan ability factor is the initial scan host for its drives.
To configure the scan ability factor of media servers, use the nbemmcmd command. For more information, see NetBackup Commands Reference Guide.)
The scan host can change
A scan host is assigned for a shared drive until some interruption occurs.
For example, if one of the following occurs, EMM chooses a new scan host:
The socket connection, the host, the drive, the drive path, or the network goes down.
The drive is logically placed in the Down mode.
The scan host temporarily changes to hosts that request tape mounts while the mount is in progress. Scan host changes occur so only one host at a time has access to the drive path.
Drive paths for the scan host
If a drive has multiple paths that are configured on the selected scan host, EMM selects a scan path as follows:
Shared tape drive polling
For shared tape drives, only the scan host polls drives until a mount request is received from NetBackup. During a mount request, NetBackup uses the host that requests the mount to poll the shared drive.
This design enables NetBackup to support Dynamic Loop Switching or SAN zones. Each tape drive needs to be detected only from a single host. Each tape drive can potentially have its own scan host that switches dynamically to process errors and continue availability. A central device arbitrating component manages scan host assignments for shared drives. The arbitrating component also provides a network drive reservation system so that multiple NetBackup media servers can share a drive.
Polling a shared tape drive allows dynamic loop switching and reduces the number of device accesses and reduces CPU time. However, it cannot detect connectivity breaks (for example, discontinuity in the Fibre Channel fabric) until I/O occurs.
See About the Enterprise Media Manager on UNIX/Linux