NDM offers the operator a wide range of control capabilities for volumes. Since these parameter settings are not dependent on the online status (and consequently not on dismounting the corresponding volumes), abnormal system behavior can result from not observing the current parameter settings (for example, this may result in the inability to allocate a disk even though it has already been mounted; if allocation of the disk is to be permitted anew, the operator has to modify the appropriate parameter value). All the values can be queried using information functions.
Other unexpected reactions can occur if the state of the hardware does not match the state of the software. (For example if a tape is mounted on a device in the ATTACHED state and the tape controller is not connected to the Server Unit, the tape cannot be allocated.)
The following sections describe the most important aspects of system behavior, including an explanation of the relevant situation in the SHOW output. Information is also given on how these situations can be dealt with.
The following situations are described:
Mount message in spite of volume being mounted
It may happen that a mount request is output at the console even though the operator has already mounted the volume requested. This may have the following causes:
Several volumes with the same VSN are online (SHOW-DISK-STATUS, SHOW-TAPE-STATUS). The operator must decide which volume is to be allocated.
The device on which the volume has been mounted is reserved by another task (SHOW-DEVICE-STATUS). The volume must be remounted.
The configuration state of the device (it is explicitly or implicitly detached) prevents allocation:
Controller or device is detached (SHOW-DEVICE-STATUS),
Path is (partially) removed (SHOW-DEVICE-CONFIGURATION).
Remount volume or attach device/path (ATT/INC).
The hardware state of the device prevents allocation:
Device is not yet activated (disk),
Tape is not yet positioned to BOT,
Controller is not attached.
The activation interrupt for the mounted volume was not provided by the hardware. Reading the VSN can be enforced by CHECK-DISK-/CHECK-TAPE-MOUNT.
The mounted volume has a different VSN from the one specified by the user (SHOW-DEVICE-STATUS, SHOW-DISK-STATUS, SHOW-TAPE-STATUS).
Other causes in the case of disk requests:
The device type specified by the user does not match the type of private disk mounted (SHOW-DISK-STATUS VOL=vsn,INF=*PAR). Reject the mount message or, if required, assign a second disk with the same VSN but with another device type.
VOLIN: UNIT specification requires a device different from the device on which the disk has been mounted. To continue, the volume must be remounted or the mount message must be rejected and the user be made to release the device.
The requesting task has reserved the device on which the disk is mounted by means of /SECURE-RESOURCE-ALLOCATION UNIT=mn and wishes to reserve the disk for DMS operation (USE=*DMS). Since disks with UNIT requests are only reserved for the usage mode SPECIAL, the mount message must be rejected and the user must be requested to release the device.
A disk cannot be reserved by other systems on the basis of its SVL allocation: For a description of the possible reactions see section "SVL allocation by other systems").
A disk is to be reserved which was used during the last session as a DRV disk and the DRV subsystem is not (yet) loaded. As a reaction the DRV subsystem must either be loaded and the mount message rejected to enable the user to make a new allocation attempt or the disk must be forced to operate in SRV mode by means of a positive response to the mount message.
Other causes in the case of tape requests:
MODIFY-MOUNT-PAR ALLOC=*NO is set (SHOW-MOUNT-PAR): A reply to the mount message is required.
The volume type specified by the user is not supported by the device on which the volume is mounted (SHOW-RESOURCE-ALLOCATION, SHOW-DEVICE-CONFIGURATION). The tape must be remounted.
A different device from the one on which the tape was mounted was explicitly reserved by the user (SECURE-RESOURCE-ALLOCATION UNIT=mn). The SECURE-UNIT request of the user is mandatory: the tape must be remounted on the specified device.
SVL allocation by other systems
Even if a disk has already been logged as online, a user’s allocation request can result in a mount message for this disk. This is the case if an inconsistency is detected between the valid SYSTEM-ALLOCATION of the disk and its actual SVL allocation by other systems. The mount message, which is preceded by a notice indicating this discrepancy, is designed to request the operator to make a decision or a response.
The possible operator responses and actions are explained below. The starting point for the considerations is the output of SHOW-DISK-STATUS VOL=vsn,INF=*ALL
All systems stored in the SVL (SYSTEMS) are no longer active:
The systems entered can be removed with UNLOCK-DISK VOL=vsn,SYS-ID=(...). There is then an automatic reply to the mount message and the allocation is thus accepted.Systems entered are still (to some extent) operating with the disk and the disk is to be allocated as system-exclusive by the home system:
The mount message must be rejected.Systems entered are still (to some extent) operating with the disk and the disk is to be allocated as system-shareable by the home system:
SVL-ALLOC=EXCL:
The mount message must be rejected as the disk cannot be allocated concurrently by another system.SVL-ALLOC=SHARE und alle Systeme aktiv:
16 external systems (in the case of shared disks) are stored in the SVL: Procedure as described in 3a)SVL-ALLOC=SHARE and a part of the system entered are no longer active: the inactive systems can be removed as described in paragraph 1 above.
For other procedures see 3b).
Permanent hardware error for allocated devices
If a tape or disk unit reserved by a volume which is mounted and used by one or more users constantly reports hardware errors (INOP,...), the following responses are possible:
The volume is a fixed disk:
If the hardware error cannot be corrected, the remount message must be rejected if the disk is a private disk (implicit volume cancel); if it is a public disk, BS2000 operation comes to a standstill.
The volume is a tape:
The affected volume is to be mounted on another device (command CHANGE-TAPE VOLUME=vsn,ACTION=*MOVE). If there is still a free device available, the system proposes a standby device to the operator by means of the remount message. Rejection of the command can have the following causes:
Volume allocation was preceded by SECURE-RESOURCE-ALLOCATION UNIT=mn. Consequently the volume is forced to remain allocated to the device while it is being used by the allocating task. If the hardware error cannot be corrected,
TSN.Nmust be entered in response to the remount message. This results in the volume being cancelled.No further standby device is available.
In each case, the device must be detached with DETACH-DEVICE UNIT=mn, FORCE=*YES.
Detachment of the device can have the following effects:
The volume is implicitly canceled if it has a non-standard label, or if the allocation is preceded by SECURE UNIT=*mn.
The volume is placed in the action state “NO DEVICE”, i.e. as soon as a device becomes free it is allocated to the private volume.
When detaching devices with replaceable volumes, waiting for a suitable standby device is supported in all phases of use, i.e. both in the IN-USE state and during loading (MOUNT or PREMOUNT state) and for pure device type allocations, e.g. after SECURE-RESOURCE-ALLOCATION DEVICE=(TYPE=TAPE-C4,NUMB=1).
The “NO DEVICE” action state may occur for volumes in use or during loading immediately following DETACH UNIT=mn,FORCE=*YES if no suitable standby device is available. A “free” device may be available in the sense that although a volume is not currently assigned to it, a type allocation exists for it. This allocation is suppressed.
The relevant task waits - during the loading phase for the volume linked to this type allocation - for a suitable standby device to become free.
Volumes in the IN-USE state displace volumes in the PREMOUNT state. Displacement of a volume in the MOUNT state is performed automatically only for manually operated devices. In the case of devices supported by ROBAR, a mount operation cannot be aborted once it has been started, i.e. the remount/recover routine is terminated with NO DEVICE and the mount operation is completed (attributes of ROBAR).
When a device becomes free as a result of ATTACH-DEVICE or the end of allocation of a another volume, the volume to be assigned is selected according to the phase (the sequence being: IN-USE before MOUNT before PREMOUNT) if more than one volume which, on the basis of the type and depot allocation, could be processed on this device is in the NO DEVICE state.
Private disk permanently locked by another system
A disk cannot be allocated if another system is stored in the SVL of the disk as a VTOC lock holder (see the VTOC-SYS field of SHOW-DISK INF=*SYS). The operator is informed of this state in the following cases:
An allocation request by the user results in message
NDV0002being output (the disk is permanently locked by another system).An implicit UNLOCK for the home system ID when the disk is mounted, or an explicit UNLOCK (UNLOCK-DISK VOL=vsn,SYS-ID=sys-id) also results in message
NDV0002.SET-DISK-PARAMETER VOL=vsn,ASS=*OPER is rejected with a reference to the VTOC lock holder.
It must be ensured that the system entered as the VTOC lock holder is no longer working with the disk and the lock is not retained because of a malfunction (e.g. system crash) or due to the cancellation of the disk. Only in such a case can the lock holder be removed; In all other cases the operator must wait until the lock is released by the allocating system, otherwise side effects may occur.
For a) and b) the lock holder can be removed by entering tsn.F in response to message NDV0002.
For c) the removal of the allocating system must be initiated explicitly with UNLOCK-DISK VOL=vsn,SYS-ID=sys-id. This request also results in message NDV0002, to which the operator must respond with tsn.F, as described above.
Checking for free disks
A check is carried out to determine whether a disk is free, for example, when it is to be initialized. A disk is not allocated by users if the SHOW-DISK command returns the following information:
PHASE=ONLINE
PHASE=IN-USE and VOL-A=FREE for disks with usage mode USE=DMS; In this case, the disk is allocated only by its ASSIGN-TIME and can be released by SET-DISK-PARAMETER VOL=vsn,ASS=*USER[,USER=*NO].
SECURE deadlock situations
The messages NKS0022 and NKS0054 inform the operator of deadlock situations recognized during SECURE processing of the tasks specified. These two messages refer to the following deadlock situations:
NKS0022- Deadlock due to resources which the tasks can retain after SECURE processing:The deadlock can only be resolved by canceling one or more tasks.
Example
TASK 11.
TASK 2
/CREATE-FILE FILE-NAME=DAT1,-
SUPPORT=PRIVATE-DISK(VOLUME=-
VOL1,DEVICE-TYPE=DEV1)/ASSIGN-SYSLST TO-FILE=DAT1:/SEC-RES DISK=(VOL=VOL2,-
TYPE=DEV2,-
ALLOC=EX),-
WAIT=....../CREATE=FILE FILE-NAME=DAT2,-
SUPPORT=PRIVATE-DISK(VOLUME=-
VOL2,DEVICE-TYPE=DEV2)/ASSIGN-SYSLST TO-FILE=DAT2:/SEC-RES DISK=(VOL=VOL1,-
TYPE=DEV1,-
ALLOC=EX),-
WAIT=.......Both task 1 and task 2 reserve for themselves a private disk in task-shareable mode by opening a private disk file (ASSIGN-SYSLST). The SECURE request of both tasks for task-exclusive allocation of the disk allocated as task-shareable by the other task results in both tasks waiting for the disk to be released.
Solution:
This classic deadlock situation can only be resolved by canceling one of the two tasks.NKS0054- Deadlock due to collectingIf a collector task is selected during the current session, the following deadlock situation can occur:
Figure 4: Deadlock situation because of collecting
Task 1 allocates disk DISK01 with
/CREATE-FILE FILE-NAME=abcand/ASSIGN-SYSLST TO-FILE=abcand waits for resources to be released by another task with/SECURE-RESOURCE-ALLOCATION FILE=(NAME=FILE1,ALLOC=EX).Task 2 is the collector task which waits for the disk to be released by task 1 with
/SECURE-RESOURCE-ALLOCATION DISK=*PAR(VOL=DISK=01,TYPE=D3435,ALLOC=EX),TAPE=(VOL=TAPE01,TYPE=TAPE-C4),WAIT=...Task 3 waits in the secure queue for the tape to be released by task 2 during the allocation of the file FILE1
/SECURE-RESOURCE-ALLOCATION
TAPE=(VOL=TAPE01,TYPE=TAPE-C4).Solution:
In this case the establishment of task 3 as collector task would cause tape TAPE01 to be released by task 2 ( task 2 loses its collector attribute). Task 3 therefore receives the resources it requested and leaves the secure queue. After task 3 has released the resources, the request of task 1 can be met, and after that task’s resources have been released, the request of task 2.
Changes to USER-ALLOCATION
The value of USER-ALLOC for a disk determines whether the disk can be reserved by the user. If “NO” is set, all allocation requests are rejected without any indication for the operator. It is therefore important for the operator to know in which cases (implicit) parameter modification occurs.
SET-DISK-PARAMETER VOL=vsn,USER=....
If USER-ALLOC=*STD is defined for a disk, the applicable actual value is that set with SET-DISK-DEFAULTS USER=.... . Thus if the default value changes, this private disk is also affected.
When a disk is canceled with CHANGE-DISK-MOUNT VOL=vsn,ACTION=CANCEL then USER-ALLOC=*NO is set.
Response to a remount message with
tsn.N(rejection) results in implicit cancellation of the disk (see point 3 above).
Information on reservations
Task-specific
The command SHOW-RESOURCE-ALLOCATION (SH-RES) provides information on the following resource allocations:
unit reservations
device type reservations
tape/disk reservations (explicitly via SECURE-RESOURCE-ALLOCATION or implicitly by opening or reserving files on private volumes)
System-global
SHOW-DEVICE-STATUS UNIT=*SEL-TYPE(ATTR=*FREE[,TYPE=xxx] is used to output a list of all devices (of a specific type) which have not yet been allocated explicitly (by SECURE-RESOURCE-ALLOCATION UNIT=mn) or implicitly (by allocation of a volume mounted on them);; This list, however, does not reflect the actual number of devices that can still be allocated: there are reservations which have not yet been allocated to a device (unit), e.g. type reservations by means of the SECURE-RESOURCE-ALLOCATION command.
An overview of the actual number of devices reserved, the type of reservation and the number of devices of a certain type which can still be allocated is provided in the output for the command SHOW-DEVICE-STATUS INF=*SUM[,TYPE=xxx].
SHOW-DEVICE-STATUS UNIT=*SEL-TYPE(TYPE=xxx), INF=*TASK shows the allocating or reserving tasks and the number of devices they occupy/reserve.
Proposals for performance-enhancing NDM parameter settings
Private disks used in DMS mode are repeatedly reallocated and released when files are processed by DMS at command level (COPY-FILE, DELETE-FILE, ADD-FILE-LINK, CREATE-FILE). If task allocation and release results in SVL reallocation or release of the private disk, SVL management actions (entry/removal of the system ID in the SVL of the disk) must be effected in addition to DMS input/output. If, however, the disk remains allocated after it has been released by the user (owing to continued valid reservation by another user or ASSIGN-TIME=*OPERATOR), the considerable overhead for SVL management is no longer required. A private disk used for DMS mode should therefore remain allocated through its ASSIGN-TIME until it is no longer needed and is specifically released by the operator. This can be effected by means of
SET-DISK-PAR VOL=vsn,ASS=*OPER
SET-DISK-DEFAULTS ASS=*OPER (valid for all disks with ASS=*STD)
If SPECIAL applications are required, b) is not recommended as an alternative, since disks mounted for SPECIAL usage later on are automatically allocated and must be explicitly released by the operator before being processed by the SPECIAL application (with SET-DISK-PAR VOL=vsn,ASS=*USER).
For data center operation where the SPOOLOUT is to be stored externally on tape, MODIFY-MOUNT-PAR UNLOAD=*NO (presetting) should be set, since otherwise the relevant tape is unloaded and made ready again by the operator each time a SPOOL file is rolled out.