Tutors on Authorization for access to individual objects , Access synchronization and Access prioritization and mount request queuing
It is currently still common to use authorization procedures for entire cartridges only and not for their components. However, in order to be prepared for future developments, such as the 1-terabyte tape cartridge, a management system should, even now, have appropriately detailed access protection for subdivisions of cartridges such as sides, partitions and volumes. All components of a cartridge are suitable for access control. The application purpose determines whether the user receives access to a side, a partition or a volume. An authorization is always applicable to all elements of the authorized object. If, for example, the right to access a cartridge is granted, this right also applies to all sides, partitions and
volumes of this cartridge. It is not only cartridges that should be provided with access control. For example, it is a good idea to restrict applications’ access to cartridges that are still available. To this end, the available cartridges are combined into one or more scratch pools. The applications
are then granted the right to access only certain scratch pools. Usually, access control is less important here than an optimal utilization of the free storage capacity. Drives are also suitable for access control. However, here, too, it is usually the optimal utilization of the drives that is sought. An allocation of drives to certain users or applications should, however, be possible. As a result, users can be granted exclusive access to drives. Naturally, the grouping of drives is again an option here for simplifying management and increasing drive utilization.
Access synchronization
As already mentioned several times, a library or a drive cannot receive and process several commands from various applications in parallel. Therefore, synchronization is required that serializes all commands received at the same time and forwards them to the drives one after the other. As a result of this functionality, devices can be used ‘quasi’ simultaneously, in the same way as operating systems allow a single processor to be
made available to several processes one after the other for a limited duration. This type of synchronization corresponds with dynamic tape library sharing and has already been described in Section 6.2.1.
Access prioritization and mount request queuing
Despite intelligent access control, there may be more mount requests than available drives at a certain point in time. Ideally, a system should collect requests into a request queue in this case (Figure 9.4). This queue can be available for each drive and also for each group of drives and collect all mount requests that cannot be carried out immediately. Once a drive becomes available again, the system can perform the next request and remove
this from the queue in question. In the search for the next request to be carried out, a scheduler can sometimes also evaluate the priority of requests and change their sequence accordingly. Request queues that are not bound to drives have the advantage that several free drives may be available for each new request that is taken out of the queue. The assignment of mount request and drive must be re-evaluated accordingly, taking into account utilization and priority in order to increase the utilization of the system as a whole. Depending upon realization, this can also lead to an application being again withdrawn from a drive, so that it has to interrupt the access to the tape. It should be possible to remove requests from a queue and change the priorities of requests via an administrative interface.
No comments:
Post a Comment