In the selection process, NDM searches the device table from the beginning, i.e. in the sequence defined at hardware generation time, and selects the first suitable free device (BEST-GENERATED-DEVICE). As a result, the first devices in the table are the ones more often selected and – particularly in connection with robot-operated magnetic tape archive systems such as ROBAR which always follow the mount proposal of NDM – are the ones subject to greater wear than the devices at the end of the table.
The “wrap around” function is provided for users interested in ensuring all their tape devices suffer the same amount of wear. This function can be activated with the MODIFY-MOUNTPARAMETER NEXT-TAPE-MOUNT=LEAST-RECENTLY-USED-DEVICE command. From the suitable free devices in the device table, NDM then selects the one which has remained unused for longest.
The MODIFY-MOUNT-PARAMETER ...,NEXT-TAPE-MOUNT=*BY-CONTROLLER command is offered for users who are interested in a device selection which is optimized for input/output throughput. A device can be selected from the suitable free devices on whose controller the fewest devices are in use. This ensures even distribution of the devices used over the controllers and channel paths available. Device selection in NDM only takes into account the local device assignments. The IORM function DDAL (see "IORM: Control of I/O resources") is used to extend optimization to all the VM2000 guest systems of a system.
The default setting of the device selection mechanism is NEXT-TAPE-MOUNT= *BESTGENERATED-DEVICE, i.e. NDM searches the device table from the beginning in the order specified during hardware generation and selects the first suitable free device.
Device selection in accordance with storage locations
Storage location management has been developed in NDM, in particular to support robot-controlled archives in BS2000 (software product ROBAR, see section "Archiving systems").
Volumes and the devices on which they are to be mounted can be assigned to storage locations in BS2000. Declaring storage locations enables volumes and devices from a precisely defined set to be selected and reserved.
Tape devices are assigned to storage locations by using the ADD-DEVICE-DEPOT command. 1024 tape devices are permitted per storage location. Assignment of a VSN to a storage location is managed by the software product MAREN. The storage locations set down there must match the specifications made by the operator with the ADD-DEVICE-DEPOT command.
Each storage location is identified in the MAREN catalog with its name (max. 8 characters), its type (REMOTE or LOCAL) and its operating mode.
The effects of the ADD-DEVICE-DEPOT command on device selection in NDM are explained below in the description of the basic principles. This description makes a distinction between two different types of allocation request (tape device request and tape volume request).
Device selection for tape device requests
There are three types of request that can be issued by a user for a tape device:
Request via the mnemonic device name (MN)
The user explicitly requests a certain device via its mnemonic device name; no selection takes place.
Request with specification of storage location
The user requests a tape device by specifying a device or volume type together with a storage location.
If the request is made using the SECURE-RESOURCE-ALLOCATION command, NDM only takes the specified storage location into account when making the selection. If no device is available there the user’s job is rejected.
If the request is made via an internal interface used by ARCHIVE, the caller must determine the storage location beforehand using MAREN. If this storage location is known to NDM, i.e. was set up by means of the ADD-DEVICE-DEPOT command, NDM selects a tape device from this storage location. Otherwise, NDM accesses the set of devices that supports the specified type but is not assigned to any specific storage location. This set of devices is called the “restpool”.
Request without specification of storage location
The user requests a tape device by specifying a device or volume type but without specifying a storage location.
If the request is made using the SECURE-RESOURCE-ALLOCATION command with the operand TYPE=TAPE-C4,...,LOCATION=*USER-DEF, for example, NDM attempts, via MAREN, to determine a default storage location for the specified volume type or a volume type supported by the specified device type. If this storage location is known to NDM, a tape device is selected from it. Otherwise, NDM accesses the “restpool”.
If the request is made using the SECURE-RESOURCE-ALLOCATION command with the operand LOCATION=*NONE, the MAREN call is omitted and the “restpool” is accessed directly.
Implicit device selection for tape volume requests
There are two types of request that can be issued by a user for a tape volume:
Request with specification of VSN
If the VSN is specified in the request, the storage location defined for this VSN in MAREN is used. If there is no entry for this VSN in MAREN, then MAREN will return either a storage location determined via the MAREN exit routine or a default storage location.
Request without specification of VSN
The tape volume request is made without specifying a VSN.
In this case, NDM attempts, via MAREN, to determine a default storage location for the specified volume type or a volume type supported by the specified device type. If this storage location is known to NDM, a tape device is selected from it. Otherwise, NDM accesses the “restpool”.