Abstract:
An automated memory cartridge system prioritizes requests for tape retrieval. The requests to transfer cartridges may be assigned a priority relating to the importance of the request. The system will recognize these requests and organize them so that higher priority requests will be executed ahead of other lower priority requests. This prioritization will be accomplished in such a manner which allows even very low priority requests to eventually be carried out.

Description:
A portion of the disclosure of this patent document contains material which is subject to copyright protection. The copyright owner has no objection to the facsimile reproduction by anyone of the patent disclosure, as it appears in the Patent and Trademark Office patent files or records, but otherwise reserves all copyright rights whatsoever. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates generally to electronic media storage and retrieval, and in particular, to the storage and retrieval of large amounts of data contained within magnetic tape cartridges. More specifically, the invention relates to an improved method and apparatus for prioritizing requests for data storage and retrieval of magnetic tape cartridges. 
     2. Description of the Related Art 
     In recent years, one method of electronic data storage has been the use of four-by-five inch, 200 megabyte tape cartridges. In the past, the storage and retrieval of such cartridges was for the most part carried out by human operators. Such human intervention not only decreased the reliability of tape selection and replacement, but also prolonged the time between when a data request was made and when that data was retrieved by loading a specific cartridge into an available tape drive. 
     Recent improvements have resulted in systems utilizing tape cartridges that incorporate magazine type automatic loaders which reduce the time that jobs wait for cartridges to be mounted and subsequently read. An example of an automated cartridge system (ACS) is taught by U.S. Pat. Nos. 4,945,429, 4,928,245, 4,932,826 and 4,864,511, all assigned to the Storage Technology Corporation, which are incorporated by reference herein. 
     FIG. 1 illustrates one such automated cartridge system. The automated cartridge system is made up of at least one library controller 121 (sometimes referred to as a library management unit (LMU)) and a plurality of library storage modules (LSM) 111, 112. 
     Each library controller 121 provides the interface between a host computer 101, 102 and the LSMs 111, 112. The library controller 121 interprets cartridge motion requests from the host computer 101, 102, relaying instructions to the appropriate LSMs 111, 112. 
     FIGS. 2-4 illustrate the top view and side view of an LSM 111, 112. Each LSM 111, 112 contains a plurality of storage cells 202 to store magnetic tape cartridges and a robot arm assembly 230 for handling the magnetic tape cartridges. The robot arm assembly 230 comprises a plurality of mechanisms all operating in conjunction to provide a movable arm 321 for retrieving tape cartridges from and returning tape cartridges to their individual storage cells 202. The robot arm 321 includes one or more hand assemblies 240 which performs the actual tape retrieval from, or replacement to, the individual storage cells 202. 
     Tape cartridges retrieved from the individual storage cells 202 are typically loaded onto tape drives 141, 142, 143, 144 so that the data contained on the tape cartridge can be read by the host computer 101, 102. One or more tape drives 141, 142, 143, 144 are attached to each of the LSMs 111, 112. The robot arm 321 loads a tape cartridge into the tape drive 141, 142, 143, 144 with the same motion as the replacement of a tape cartridge into one of the individual storage cells 202. 
     As illustrated in FIG. 5, tape cartridges are entered into and ejected from LSMs 111, 112 through the use of cartridge access ports (CAP) 160. 
     Tape cartridges may be transferred under robotic control from one LSM 111 to another LSM 112 through devices called &#34;pass-through-ports&#34; (PTPs) 150. As illustrated in FIG. 6, a PTP 150 is comprised generally of a plurality of storage cells 151, 152, 153, 154, called &#34;PTP slots&#34; adapted for rotation by a mechanism 155, called a &#34;PTP mechanism&#34; for transference between LSMs 111, 112. Through use of these PTPs 150, load sharing is accomplished. Tapes may be transported through PTP slots 151, 152, 153, 154 from one LSM 111 where they might be stored to another LSM 112 where there might be unused tape drives 141, 142, 143, 144. Thus, PTPs 150 allow for a more efficient use of available tape drives 141, 142, 143, 144. 
     In these systems, the task of transferring a cartridge from one location to another is initiated by a cartridge motion request from the host computer 101, 102 to the library controller 121. When the library controller 121 receives the request, it determines the proper path from the cartridge source to the cartridge destination. This path possibly includes source and/or destination storage cells 202, source and/or destination tape drives 141, 142, 143, 144, source and/or destination CAPs 160, and PTP slots 151, 152, 153, 154 for inter-LSM moves. The library controller 121 &#34;allocates&#34; available resources to cartridge motion requests to accomplish cartridge transfers. 
     The task of actually transferring a cartridge from one location within an LSM 111, 112 to another consists of two operations, a get and a put. In the get operation, the robotic means 230 retrieves the cartridge from a receptacle at a first location. In the put operation, the robotic means 230 moves the cartridge to its destination and inserts it into a receptacle at a second location. 
     Tape cartridge transfers along a path consist of the following: (1) path allocation; (2) source get; (3) one or more intermediate puts; (4) one or more intermediate gets; (5) destination put; and (6) path deallocation. 
     Path allocation consists of allocating for the duration of a tape cartridge transfer all PTP slots 151, 152, 153, 154 necessary for inter-LSM moves through the path, the source and/or destination tape drive 141, 142, 143, 144 and the source and/or destination CAPs 160. If the source tape drive 141, 142, 143, 144 or the source and/or destination CAP 160 are unavailable, the cartridge motion request is rejected. If any of the necessary PTP slots 151, 152, 153, 154 or tape drives 141, 142, 143, 144 are unavailable, the cartridge motion request, as described later, waits on a queue until said slot 151, 152, 153, 154 becomes available. 
     A source get consists of allocating the arm 321 and hand 240 of the robot mechanism 230 within the source LSM 111, 112 to retrieve a tape cartridge from the source location, whether it be a storage cell 202, tape transport 141, 142, 143, 144 or CAP 160. After the tape cartridge has been retrieved, the arm 321 is deallocated and is free to service other requests. However, the hand 240 remains allocated to the present request. 
     Intermediate puts and gets are repeated for intermediate LSMs 111, 112 along the selected path. An intermediate put always follows either a source get or intermediate get operation and, therefore, will have a hand 240 already allocated to the request. The intermediate put delivers a cartridge to a PTP slot 151, 152, 153, 154 in an LSM 111, 112. The intermediate put consists of a two step process. First, the PTP mechanism 155 in the intermediate LSM 111, 112 is allocated to the request to receive a cartridge and, second, the arm mechanism 321 within the intermediate LSM 111, 112 is allocated to move the cartridge to and insert it into the PTP slot 151, 152, 153, 154 in that LSM 111, 112. The arm 321, hand 240 and PTP mechanism 155 are deallocated upon completion of the intermediate put. 
     The intermediate get retrieves a cartridge from a PTP slot 151, 152, 153, 154. The intermediate get consists of a three step process. First, a hand 240 in the intermediate LSM 111, 112 is allocated to the request to retrieve the cartridge from a PTP slot 151, 152, 153, 154. Second, the PTP mechanism 155 is allocated to position the PTP slot 151, 152, 153, 154 in the intermediate PTP 150 for transference of the cartridge. Finally, the arm mechanism 321 within the intermediate LSM 111, 112 is allocated to retrieve a cartridge from the PTP slot 151, 152, 153, 154. The arm 321 and PTP mechanism 155 are deallocated upon completion of the intermediate get. The hand 240 remains allocated to the request. 
     Destination puts are similar to intermediate puts in that they always follow either a source get or intermediate get operation and, therefore, will have a hand 240 already allocated to the request. The destination put consists of allocating an arm mechanism 321 within the destination LSM 111, 112 to move a cartridge to and insert it into the destination storage cell 202, tape drive 141, 142, 143, 144 or CAP 160. Upon completion of the destination put, the arm 321 and hand 240 allocated to the request are deallocated. 
     When the tape cartridge has reached its destination location, all PTP slots 151, 152, 153, 154 for inter-LSM moves through the path, the source and/or destination tape drives 141, 142, 143, 144 and the source and/or destination CAPs 160 are deallocated. 
     As described above, the library controller 121 allocates available resources to cartridge motion requests to accomplish cartridge transfers. However, because the library controller 121 may be asked to execute a plurality of cartridge motion requests at any one time, cartridge motion requests compete for available resources. The library controller 121 manages the competition for available resources by allocating the available resources to the cartridge motion requests on a first-come first-served basis. Cartridge motion requests that request resources first are granted them first. A plurality of queues are implemented to handle the allocation of available resources: a PTP slot queue, a PTP mechanism queue, a drive queue, a hand queue and an arm queue. 
     When a new cartridge request is received, it is examined to determine if its destination is a tape drive 141, 142, 143, 144 and, if so, if the destination tape drive 141, 142, 143, 144 is available. If the destination tape drive 141, 142, 143, 144 is available, the drive is allocated to the request and processing of the request continues. If the drive 141, 142, 143, 144 is not available, the request is placed in a drive queue. The library controller 121 determines if a tape drive 141, 142, 143, 144 has become available. If a drive 141, 142, 143, 144 becomes available, the library controller 121 scans the drive queue from the oldest entry to the newest entry to determine if any requests in the drive queue need the available tape drive 141, 142, 143, 144. The available tape drive 141, 142, 143, 144 is allocated to the first request in the queue waiting on that tape drive 141, 142, 143, 144. 
     If the source location and the destination location are in different LSMs 111, 112, the library controller 121 determines if the necessary PTP slots 151, 152, 153, 154 are available. If the necessary PTP slots 151, 152, 153, 154 are available, the PTP slots 151, 152, 153, 154 are allocated to the request and the path for that particular request is granted, i.e., path allocation. Otherwise, the cartridge motion request is queued on the end of the PTP slot queue, implemented in the library controller&#39;s 121 computer memory. 
     Each time a PTP slot 151, 152, 153, 154 becomes available, the PTP slot queue is scanned from the oldest entry to the newest entry to determine which cartridge motion requests in the queue are waiting on that PTP slot 151, 152, 153, 154. The first request found for which all necessary PTP slots 151, 152, 153, 154 are available is granted the path and the necessary PTP slots 151, 152, 153, 154 are allocated to it, i.e., path allocation. 
     Once a path has been allocated to the cartridge motion request, the tape cartridge is transferred along the granted path through a succession of gets (i.e., a source get and a series of intermediate gets) and puts (i.e., a series of intermediate puts and a destination put). 
     The allocation of available resources to complete a get is as follows. The cartridge motion request is entered into the hand queue, organized from the oldest entry to the newest, to wait until a hand 240 becomes available. When a hand 240 becomes available, the hand queue is scanned from the oldest entry to the newest entry to determine which request in the queue is waiting on the available hand 240. The available hand 240 is allocated to the first request in the hand queue needing that hand 240. If a transfer between LSMs 111, 112 is involved, the cartridge motion request is entered into the PTP mechanism queue, organized from the oldest entry to the newest, to wait until a PTP mechanism 155 becomes available. When a PTP mechanism 155 becomes available, the PTP mechanism queue is scanned from the oldest entry to the newest entry to determine which request in the queue is waiting on the available PTP mechanism 155. The available PTP mechanism 155 is allocated to the first request in the PTP mechanism queue needing that hand 240. The request is then entered into an arm queue to wait until an arm 321 becomes available. When an arm 321 becomes available the arm queue is scanned from the oldest entry to the newest entry to determine which request in the arm queue is waiting on the available arm 321. The available arm 321 is assigned to the first request in the arm queue needing that arm 321. The request is then satisfied by moving the robotic arm 321 so as to move the hand 240 to the location of the receptacle containing the cartridge that is sought and then using the hand 240 to remove the cartridge from that receptacle. 
     At this point the get is complete. The arm 321 and PTP mechanism 155 are released for reallocation to the next request in the arm queue needing that arm 321 and the next request in the PTP mechanism queue needing that PTP mechanism 155. The hand 240 retains the cartridge it has retrieved and accordingly cannot be released. The request is then returned to the back of the arm queue to wait for an available arm 321 so it that it can complete its transfer through a put operation. 
     The put operation is essentially the same as that described in the previous paragraph. Upon completion of the get, the request is returned to the arm queue and, if necessary, the PTP mechanism queue, each queue being organized from the oldest entry to the newest, to wait until an arm 321 and PTP mechanism 155 necessary for the put become available. When a PTP mechanism 155 becomes available, the PTP mechanism queue is scanned from the oldest entry to the newest entry to determine which request in the queue is waiting on the available PTP mechanism 155. The available PTP mechanism 155 is allocated to the first request in the PTP mechanism queue needing that hand 240. When an arm 321 becomes available, the arm queue is scanned from the oldest entry to the newest entry to determine which request in the queue is waiting on the available arm 321. The available arm 321 is allocated to the first request in the arm queue needing that arm 321. When an arm 321 is allocated to the request, the arm 321 is moved so as to move the hand 240 and the cartridge contained therein to the receptacle at the destination location. The hand 240 then inserts the cartridge into the receptacle whereupon the put is complete. The PTP mechanism 155, arm mechanism 321 and hand 240 are then released for reallocation to the next request in the PTP mechanism queue, arm queue and hand queue, respectively. 
     In the operations described above, there exists one resource queue for each resource, i.e., PTP mechanism 155, drive 141, 142, 143, 144, hand 240 and arm 321, for the entire automatic cartridge system. Each time a resource becomes available, the appropriate queue is scanned from the oldest entry to the newest entry to determine which request in the queue is waiting on the available resource. 
     In an alternative implementation, there exists one resource queue for each resource for each LSM 111, 112 in the automated cartridge system. Each time a resource become available in an LSM 111, 112, it is allocated to the first element on the queue (in a first-come, first serve implementation, the first entry on the queue will always be the oldest request). This implementation eliminates the need to scan the queue to determine which request in the queue is waiting on the available resource. 
     As mentioned above, in these automated cartridge systems, cartridge requests are performed only on a first-come first-served basis. That is, all cartridge requests are forced to compete for the available resources on an equal footing, and those that request the resources first are granted them first. While this method of performing transfers operates as described, experience has shown that its performance could be improved. The operators of these systems are unable to modify the order in which the library controller 121 performs these requests. This prevents operators from prioritizing the requests, that is, compelling the ACS to perform an important cartridge request first, although they may have been sent to the system subsequent to less important cartridge transfer requests. 
     Therefore, there is a need for a method and apparatus for prioritizing requests for tape retrieval within an automated cartridge system (ACS). 
     SUMMARY OF THE INVENTION 
     In accordance with the present invention, there is provided an ACS which prioritizes requests for tape retrieval. The cartridge requests may be assigned a priority relating to the urgency of the request. The ACS will recognize these requests and organize them so that higher priority requests will be executed ahead of other lower priority requests. 
     It is another object of the present invention to carry out the prioritization in such a manner that even low priority requests will eventually be executed. To avoid the problem of lower priority jobs being held off indefinitely by a stream of incoming jobs with higher priorities, each time a mechanism is granted to a request, all lower priority requests waiting for that mechanism have their priority incremented by a predetermined amount. 
     It is another object of the present invention to provide the capability to execute certain requests only when no other requests are pending in the system. In accordance with this object of the invention, a special priority is assigned to these requests that inhibit their execution until no other requests are pending in the system. 
     It is another object of the present invention to provide the capability to execute certain requests before all current requests pending in the system. In accordance with this object of the invention, a super priority is assigned to these requests that is greater than any priority that may be assigned by the system. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     These and other objects, features and advantages of the inventions will be more readily apparent from the following detailed description of the invention in which: 
     FIG. 1 illustrates the automated magnetic tape cartridge library system in block diagram form; 
     FIG. 2 illustrates a top view of a library module; 
     FIG. 3 illustrates a cut away view of a library module with its associated robot arm mechanism and tape cartridge storage cell array; 
     FIG. 4 illustrates a perspective view of the robot arm mechanism; 
     FIG. 5 illustrates in plan view the access doors and cartridge access port shown in FIG. 2; 
     FIG. 6 illustrates in detail the pass through port shown in FIG. 2; 
     FIGS. 7a-1, 7a-2, 7b-1 and 7b-2 are flowcharts of an embodiment of the invention which provides for task optimization; 
     FIGS. 8a-1, 8a-2, 8b-1 and 8b-2 are flowcharts of an alternative embodiment of the invention illustrating incrementation of priorities of request in a queue; and 
     FIGS. 9a-1, 9a-2, 9b-1 and 9b-2 are flowcharts of an alternative embodiment of the invention illustrating incrementation of priorities of requests not selected for execution. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     FIGS. 7a-1, 7a-2, 7b-1 and 7b-2 are an embodiment of the invention for task prioritization in an ACS which may have a plurality of LSMs 111, 112 inter-connected by PTP mechanisms 155. In accordance with the invention, task prioritization is achieved by assigning a priority to each cartridge request. The priority of each request illustratively is a value from 0 to 99 with 99 being highest. 
     When a new cartridge request is received (step 10), it is examined to determine if its destination is a tape drive 141, 142, 143, 144 (step 11) and, if so, if the destination tape drive 141, 142, 143, 144 is available. If the destination drive 141, 142, 143, 144 is available, the drive 141, 142, 143, 144 is allocated to the request and processing of the request continues at step 15. If the drive 141, 142, 143, 144 is not available, the request is placed in a drive queue behind all requests of equal or greater priority and in front of all others (step 33). At step 47, the library controller 121 determines if a drive 141, 142, 143, 144 has become available. If a tape drive 141, 142, 143, 144 is now available, the library controller 121 determines if any requests in the drive queue need the available tape drive 141, 142, 143, 144 by scanning the drive queue from the highest priority request to the lowest priority request (step 67). The available tape drive 141, 142, 143, 144 is allocated to the first request in the queue waiting on that tape drive 141, 142, 143, 144 (step 87). 
     In alternative embodiment shown in FIGS. 8a-1, 8a-2, 8b-1 and 8b-2, to avoid the problem of lower priority requests being held off indefinitely, after the request is placed in the drive queue behind all requests of equal or greater priority and in front of all others (step 33), the library controller 121 increments the priority of all requests in the drive queue of priority lower than the newly entered request (step 107). 
     A further alternative embodiment is shown in FIGS. 9a-1, 9a-2, 9b-1 and 9b-2. Rather than increasing the priority after a new request has been entered in the drive queue of all requests in the hand queue with a priority lower than the newly entered request, to avoid the problem of lower priority requests being held off indefinitely, the library controller 121 increments after allocation of a drive 141, 142, 143, 144 the priority of all lower priority requests in the drive queue waiting for the allocated drive 141, 142, 143, 144 (step 127). 
     Referring back to FIGS. 7a-1, 7a-2, 7b-1 and 7b-2, after the destination drive 141, 142, 143, 144 has been allocated to the request, the request is examined to determine if a PTP slot 151, 152, 153, 154 is needed (step 15). All requests which need a PTP slot 151, 152, 153, 154 are placed in a PTP slot queue behind all requests of equal or greater priority and in front of lower priority requests (step 35). If PTP slots 151, 152, 153, 154 are available, the library controller 121 determines if any requests in the PTP slot queue need the available PTP slot 151, 152, 153, 154 by scanning the PTP slot queue from the highest priority request to the lowest priority request (step 70). When a request in the PTP slot queue is found for which all necessary PTP slots 151, 152, 153, 154 are available, the request is granted the path, i.e., the necessary PTP slots 151, 152, 153, 154 are allocated to it (step 90). 
     Referring again to FIGS. 8a-1, 8a-2, 8b-1 and 8b-2, to avoid the problem of lower priority requests being held off indefinitely, the library controller 121 increments the priority of all requests in the PTP slot queue of priority lower than the newly entered request (step 110). At step 50 the library controller 121 determines if any PTP slots 151, 152, 153, 154 are available. 
     In the further alternative embodiment shown in FIGS. 9a-1, 8a-2, 9b-1 and 9b-2, rather than increasing after a new request has been entered in the PTP slot queue the priority of all requests in the PTP slot queue with a priority lower than the newly entered request, to avoid the problem of lower priority requests being held off indefinitely, the library controller 121 increments after allocation of the PTP slots 151, 152, 153, 154 the priority of all lower priority requests in the PTP slot queue waiting for the allocated PTP slots 151, 152, 153, 154 (step 130). 
     Referring back to FIGS. 7a-1, 7a-2, 7b-1 and 7b-2, once the request is granted a path, the library controller 121 examines the request to determine if a hand 240 is needed (step 20). If a hand 240 is needed, the request is placed in a hand queue behind all requests of equal or greater priority and in front of all others (step 40). At step 55, the library controller 121 determines if a hand 240 has become available. If a hand 240 is now available, the library controller 121 determines if any requests in the hand queue need the available hand 240 by scanning the hand queue from the highest priority request to the lowest priority request (step 75). The available hand 240 is allocated to the first request in the queue waiting on that hand 240 (step 95). 
     As illustrated in the alternative embodiment shown in FIGS. 8a-1, 8a-2, 8b-1 and 8b-2, to avoid the problem of lower priority requests being held off indefinitely, the library controller 121 increments the priority of all requests in the hand queue of priority lower than the newly entered request (step 115). 
     Illustrated in the further alternative embodiment shown FIGS. 9a-1, 9a-2, 9b-1 and 9b-2, rather than increasing after a new request has been entered in the hand queue the priority of all requests in the hand queue with a priority lower than the newly entered request, to avoid the problem of lower priority requests being held off indefinitely, the library controller 121 increments after allocation of a hand 240 the priority of all lower priority requests in the hand queue waiting for the allocated hand 240 (step 135). 
     Referring back to FIGS. 7a and 7b, all requests which need a PTP mechanism 155 are placed in a PTP mechanism queue behind all requests of equal or greater priority and in front of all others (step 45). At step 60 the library controller 121 determines if an PTP mechanism 155 is available. If a PTP mechanism 155 is available, the library controller 121 determines if any requests in the PTP mechanism queue need the available PTP mechanism 155 by scanning the PTP mechanism queue from the highest priority request to the lowest priority request (step 80). The available PTP mechanism 155 is allocated to the first request in the queue waiting on that PTP mechanism 155 (step 100). 
     As illustrated in FIGS. 8a-1, 8a-2, 8b-1 and 8b-2, to avoid the problem of lower priority requests being held off indefinitely, the library controller 121 increments the priority of all requests in the PTP mechanism queue of priority lower than the newly entered request (step 120). 
     In the further alternative embodiment shown in FIGS. 9a-1, 9a-2, 9b-1 and 9b-2, rather than increasing after a new request has been entered in the PTP mechanism queue the priority of all requests in the PTP mechanism queue with a priority lower than the newly entered request, to avoid the problem of lower priority requests being held off indefinitely, the library controller 121 increments after allocation of a PTP mechanism 155 the priority of all lower priority requests in the PTP mechanism queue waiting for the allocated PTP mechanism 155 (step 140). 
     Referring again to FIGS. 7a-1, 7a-2, 7b-1 and 7b-2, all requests which need an arm 321 are placed in a wait arm queue behind all requests of equal or greater priority and in front of all others (step 30). At step 65 the library controller 121 determines if an arm 321 is available. If an arm 321 is available, the library controller 121 determines if any requests in the arm queue need the available arm 321 by scanning the arm queue from the highest priority request to the lowest priority request (step 85). The available arm 321 is allocated to the first request in the queue waiting on that arm 321 (step 105). 
     As illustrated in FIGS. 8a-1, 8a-2, 8b-1 and 8b-2, to avoid the problem of lower priority requests being held off indefinitely, the library controller 121 increments the priority of all requests in the arm queue of priority lower than the newly entered request (step 125). 
     In further alternative embodiment shown in FIGS. 9a-1, 9a-2, 9b-1 and 9b-2, rather than increasing after a new request has been entered in the arm queue the priority of all requests in the arm queue with a priority lower than the newly entered request, to avoid the problem of lower priority requests being held off indefinitely, the library controller 121 increments after allocation of an arm 321 the priority of all lower priority requests in the arm queue waiting for the allocated arm 321 (step 145). 
     In the operations described above, there exists one resource queue for each resource, i.e., PTP mechanism 155, drive 141, 142, 143, 144, hand 240 and arm 321, for the entire automatic cartridge system. Each time a resource becomes available, the appropriate queue is scanned from the oldest entry to the newest entry to determine which request in the queue is waiting on the available resource. 
     In an alternative implementation, there exists one resource queue for each resource for each LSM 111, 112 in the automated cartridge system. Each time a resource become available in an LSM 111, 112, it is allocated to the first element on the queue (with task prioritization, the first entry on the queue will have the highest priority). This implementation eliminates the need to scan the queue to determine which request in the queue is waiting on the available resource. 
     Another embodiment of the present invention provides the capability to execute certain requests only when no other requests are pending in the system. A special priority is assigned to these requests that inhibit their execution until no other requests are pending in the system. In accordance with this embodiment, the library controller 121 assigns a priority of 0 to those requests that are to be executed only when no other requests are pending in the system (&#34;priority 0 requests&#34;). The library controller 121 never increments the priority of priority 0 requests. As a result, these priority 0 request will only be executed by the system when no other requests are competing for available resources. 
     Another embodiment of the present invention provides the capability to execute certain requests before all current requests pending in the system. A super priority is assigned to these requests that is greater than any priority that may be assigned by the system. In accordance with this embodiment, the library controller 121 assigns a priority of 99 to those requests that are to be executed before all current requests pending in the system (&#34;priority 99 requests&#34;). The library controller 121 never increments the priority of a request pending in the system to a priority greater than 98. As a result, priority 99 requests are allocated available resources before all non-priority 99 requests pending in the system. 
     Various methods of prioritizing the cartridge requests may be used in accordance with the invention. In one example, the library controller 121 may have numerous hosts. Each host can be set up to send requests at a priority that relates to the response time required for that particular host. For instance, a host connected to customer accounts may have a higher priority attached to its requests than a host which handles cartridge requests for engineers. 
     In another example, priorities could be assigned on a user by user basis. For instance, the system administrator would have a higher priority attached to his requests than say a financial analyst. 
     In a final example, prioritization could be assigned to cartridge requests based on the type of robot motion that will be required. As an example, cartridge entries into the system would have a higher priority than cartridge removals from the system. It should also be noted that a combination of the preceding examples could also be implemented. 
     A computer program listing one embodiment of an implementation of task prioritization for a tape storage system is attached as an appendix hereto. 
     Various embodiments of the invention have been described. The descriptions are intended to be illustrative, not limitative. Thus, it will be apparent to those skilled in the art that certain modifications may be made to the invention as described without departing from the scope of the claims set out below. ##SPC1##