Device management supports and monitors disk and tape usage, executes mounting and remounting procedures and protects used tapes and disks from erroneous intervention.
For this purpose device management uses two permanent tasks, the DISK-MONITOR (DM) for disk monitoring and the TAPE-MONITOR (TM) for tape monitoring. Temporary tasks are started for functions which temporarily inhibit permanent tasks (e.g. tape positioning).
Mounting of a volume before it is used or in the case of an allocation request
If the user requests a volume, a mount message is issued at the console if the volume has up to now not been recognized as mounted (NKVD013/NKVT013; this depends on whether a disk (D) or a tape (T) is to be mounted).
If volume monitoring recognizes the mounting of the requested volume (by evaluating the activation interrupt from the device), the outstanding message is automatically answered by the system. The operator is therefore not required to respond to the mount message.
Remounting of volumes during use
If a (defective) device is detached by reconfiguration (DETACH command), the operator is requested by the volume monitor to carry out remounting (if a standby device is available).
If no standby device is available, a temporary overload is accepted, i.e. the system holds a renewed mount request until a device is available or can be attached (ATTACH command). However, the operator can terminate this wait state by explicit withdrawal of the volume (CANCEL function of the CHANGE-TAPE-MOUNT command).
Protection of volumes from illegal device intervention and repositioning of tapes
Each operator intervention on a tape or disk unit (unloading a tape, deactivating the device) with a volume that is in use results in a temporary I/O lock for the user.
If during an I/O request it is determined that the volume is no longer available, a request is issued for the operator to make the volume ready again (remount message).
After reassigning the volume, the I/O operations can be continued. With tapes, automatic repositioning takes place.
If hardware resets of the devices are reported to the monitors, this also leads to identification of the volumes and, in the case of tapes with PHASE=IN-USE, to repositioning.
(Automatic) removal of a volume overload
If a device is withdrawn from an allocated tape in USE-MODE=DMS (DETACH UNIT=..., FORCE=*Y) without a standby device being available, a temporary overload is created (SH-TAPE: output column ACTION=NO DEVICE), i.e. more volumes of this device type are in use (SHOW-TAPE-STATUS: column PHASE=IN-USE), than devices are available.
The operator can use the command CHANGE-TAPE-MOUNT ...,EXCHANGE=(...) to influence or determine which volumes should temporarily not be available. If a device of an appropriate type is free, the operator is automatically requested to mount the volume (by means of ACTION=NO DEVICE) by the volume monitor.