Abstract:
A system, a method, and a computer program product to provide a failover procedure for an automated data storage library are provided. During the operation of a data storage library, a host computer issues control commands to the automated data storage library using one or more control paths. In the event that the host does not receive desirable results, such as when communication is lost between the host computer and the automated data storage library while a command is in process, the host computer may attempt to retry the same command using the same control path or an alternate control path. When the host issues a retry of the same command, the automated data storage library performs an examination of the command and one or more previous commands executed to prevent the automated data storage library from executing an identical command more than once.

Description:
TECHNICAL FIELD  
         [0001]    This invention concerns a system to provide automatic control path failover for an automated data storage library by use of one or more control paths between a host computer and the automated data storage library.  
         BACKGROUND OF THE INVENTION  
         [0002]    Automated data storage libraries are known for providing cost effective storage and retrieval of large quantities of data. Automated data storage libraries provide a means for storing data on data storage media that are not permanently mounted in data storage drives, and that are stored on storage shelves inside the library. One or more robot accessors retrieve selected data storage media from storage shelves and provide them to data storage drives. Host computers communicate with the automated data storage library through control paths. A control path is a communication interface that comprises library control commands or signals. The control path interface may comprise serial interfaces such as RS-232, SCSI (Small Computer Systems Interface), FC-AL (Fibre Channel-Arbitrated Loop), etc. In certain system configurations, a host may include multiple host bus adapters (HBAs) that provide multiple physical paths to a device, such as a dual ported tape device. Each of these physical paths could function as one or more control paths. If a host application, such as a data backup application or device driver, is sending commands to an automated data storage library using a control path and the control path experiences a failure, such as a permanent path error (e.g., a failure in the HBA or cable), then the application would cease operating down the failed path. In such a case, failover procedures may be invoked either by a system administrator or automatically by the host computer to either restart the application, or reissue the command, down an alternate path.  
           [0003]    If the host computer restarts the application, then any operations performed by the library prior to the failure may be lost. This can create large delays because the application may have to repeat work that may have already been completed. If the host computer reissues the command down an alternate path without knowing if the previous command sent to the automated data storage library was successfully completed then errors may occur. For example, a host computer may encounter a loss of library communication while moving media between a storage slot and a drive. The host computer may not be able to determine if the command completed successfully because there was no response from the automated data storage library to the command. If the host computer retries the same command then it may fail because the library has already successfully completed the command. In other situations, the command retry may cause the cartridge to be moved away from its intended destination if the library has successfully completed an exchange command. For these reasons, there is a need in the art for improved techniques for managing failover operations for automated data storage libraries.  
         SUMMARY OF THE INVENTION  
         [0004]    It is an object of the present invention to provide a failover procedure using one or more control paths between a host computer and an automated data storage library.  
           [0005]    Disclosed are a system, a method, and a computer program product to provide a failover procedure for an automated data storage library. During the operation of a data storage library, a host computer issues control commands to the automated data storage library using one or more control paths. In the event that communication is lost between the host computer and the automated data storage library while a command is in process, the host computer may attempt to retry the same command using the same or an alternate control path. In some situations the automated data storage library may have already completed the command but a command completed response was not received by the host computer. When the host issues a retry of the same command, the automated data storage library performs the procedures of the present invention to prevent the automated data storage library from executing an identical command more than once. The procedures of the present invention will also eliminate the need for a host computer to analyze the results of a retry command. For example, if the retry command fails then the host does not have to determine if the error was related to a successful completion of the command on a previous attempt. This invention reduces the occurrence of multiple executions of identical commands by having the automated data storage library examine some or all library control commands before execution.  
           [0006]    In one embodiment, every command sent to the automated data storage library is examined to determine if it is a command that may have already been executed successfully. The automated data storage library examines the command and also performs an examination of one or more commands executed previous to receiving this command to determine if the command was already executed. This situation may occur during a retry of a previously issued command where the host computer did not receive a command completed response from the automated data storage library even though the command was executed successfully. If the automated data storage library determines that the command has already been executed successfully then the automated data storage library does not execute the command a second time and it sends a command completed response to the host computer. By eliminating the duplicate execution of a command, the overall operation of the automated data storage library is improved. In this embodiment, some commands may be excluded from this special handling and they may be executed every time they are received. For example, it may be desired to execute the SCSI Test Unit Ready command every time the host sends this command. The same may be true for some data commands, such as the SCSI Read Element Status command.  
           [0007]    In a second embodiment, every command sent to the automated data storage library is examined to determine if it is a command that moves data storage media. If the command does not move data storage media then the command is immediately executed by the automated data storage library without any further examination of the command. If the command moves data storage media then the automated data storage library does a further examination of the command and an examination of one or more commands executed previous to receiving this command to determine if the command was already executed. This situation may occur during a retry of a previously issued command where the host computer did not receive a command completed response from the automated data storage library even though the command was executed successfully. If the automated data storage library determines that the command has already been executed successfully then the automated data storage library does not execute the command the second time and it sends a command completed response to the host computer. By eliminating the duplicate execution of a command the overall operation of the automated data storage library is improved. In addition, host control path failover is simplified and less prone to errors.  
           [0008]    A third embodiment of the present invention combines additional verification operations with the second embodiment discussed above. The additional verification operations are performed by the automated data storage library to further confirm the successful previous execution of a command. The additional verification operations comprise having the automated data storage library examine the source and/or destination location specified in the command. The automated data storage library then examines the physical source and/or destination location of the data storage media to determine if the data storage media is physically present in either location. Using this additional information the automated data storage library determines if the command has already been executed successfully. If the command was executed successfully then the automated data storage library does not execute the command the second time and it sends a command completed response to the host computer.  
           [0009]    For a more complete understanding of the present invention, reference should be made to the following detailed description taken in conjunction with the accompanying drawings. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0010]    [0010]FIG. 1 is a block diagrammatic representation of the electronics used to operate an automated data storage library of the present invention.  
         [0011]    [0011]FIG. 2 is a block diagrammatic representation of an automated data storage library connected to more than one host computer.  
         [0012]    [0012]FIG. 3 is a flow chart depicting the operation of the automated data storage library for one embodiment of the present invention.  
         [0013]    [0013]FIG. 4 is a flow chart depicting the operation of the automated data storage library for a second embodiment of the present invention.  
         [0014]    [0014]FIG. 5 is a flow chart depicting the operation of the automated data storage library for a third embodiment of the present invention.  
         [0015]    [0015]FIG. 6 is a flow chart depicting the operation of the host computer for the present invention. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0016]    This invention is described in preferred embodiments in the following description. The preferred embodiments are described with reference to the Figures. While this invention is described in conjunction with the preferred embodiments, it will be appreciated by those skilled in the art that it is intended to cover alternatives, modifications, and equivalents as may be included within the spirit and scope of the invention as defined by the appended claims.  
         [0017]    This invention is described in terms of the SCSI command set for a medium changer device. A medium changer is a device that moves data storage media between storage shelves and data storage drives. These devices are also referred to as automated data storage device, storage automation device, library, cartridge loader, etc. References are made to move medium commands and exchange medium commands. These commands are well known to those of skill in the art. For example, the SCSI command set for medium changer devices includes move medium and exchange medium commands. Move medium commands are used to move data storage media from one location to another location within the library. Exchange medium commands are used to improve the performance of a move command by combining more than one move into a single command. For example, a move medium command may move a cartridge from location A to location B. An exchange medium command may move a cartridge from location A to location B, and whatever was present at location B may be moved to location C. Using the SCSI exchange medium command as an example, there are three locations specified in the command. Source address, first destination address and second destination address. Note that address refers to a location identifier and the terms address and location may be used interchangeably. The library location specified by any of the above addresses is a location inside the library that can be accessed by one or more of the robot accessors inside the library. The cartridge at the source address is moved to the first destination address and the cartridge that previously occupied the first destination address is moved to the second destination address. The second destination address may or may not be the same as the source address. In addition, references are made to element status data. Element status data is well known to those of skill in the art. For example, the SCSI command set for medium changer devices includes commands for working with element status data. Element status data comprises a cartridge map of the locations that may contain a cartridge. The cartridge map may comprise information about potential cartridge locations such as whether or not the location contains a cartridge (empty or full), whether or not the cartridge is accessible, the value of the cartridge bar code label, the type of location such as a storage slot or data storage drive, etc. These references are done for descriptive purposes only and one skilled in the art will recognize that other command sets and protocols may be used that provide the same or similar functions without deviating from the spirit and scope of this invention.  
         [0018]    An automated data storage library typically contains one or more library controllers to direct the operation of the automated data storage library. The library controller may take many different forms and may comprise an embedded system, a distributed control system, a personal computer, workstation, etc. FIG. 1 shows a typical library controller  100  with a processor  102 , RAM  103 , nonvolatile memory  104 , device specific circuits  101 , and I/O interface  105 . Alternatively, the RAM  103  and/or nonvolatile memory  104  may be contained in the processor  102  as could the device specific circuits  101  and I/O interface  105 . The processor  102  may comprise an off the shelf microprocessor, custom processor, FPGA (Field Programmable Gate Array), ASIC (Application Specific Integrated Circuit), discrete logic, etc. The RAM  103  is typically used to hold variable data, stack data, executable instructions, etc. The nonvolatile memory  104  may comprise any type of nonvolatile memory such as EEPROM (Electrically Erasable Programmable Read Only Memory), flash PROM (Programmable Read Only Memory), battery backup RAM (Random Access Memory), hard disk drive, etc. The nonvolatile memory  104  is typically used to hold the executable firmware and any nonvolatile data. The I/O interface  105  is a communication interface that allows the processor  102  to communicate with devices external to the controller. Examples may comprise serial interfaces such as RS-232 or USB (Universal Serial Bus), SCSI (Small Computer Systems Interface), FC-AL (Fibre Channel-Arbitrated Loop), etc. The device specific circuits  101  provide additional hardware to enable the library controller  100  to perform unique functions such as motor control of a cartridge gripper, etc. The device specific circuits  101  may comprise electronics that provide Pulse Width Modulation (PWM) control, Analog to Digital Conversion (ADC), Digital to Analog Conversion (DAC), etc. In addition, all or part of the device specific circuits  101  may reside outside the library controller  100 .  
         [0019]    [0019]FIG. 2 shows a block diagram of an automated data storage library  200  and host computers  210 ,  220  and  230 . The host computers  210 ,  220 ,  230  connect to the automated data storage library  200  through host communication lines  211 ,  221 ,  222  and  231 . Host communication lines  211 ,  221 ,  222 ,  231  comprise the control paths into library  200 . The host communication lines  211 ,  221 ,  222 ,  231  may comprise RS-232, USB (Universal Serial Bus), SCSI (Small Computer Systems Interface), FC-AL (Fibre Channel-Arbitrated Loop) or any other communication interface known to those of skill in the art. Host communication lines  221  and  222  are redundant control paths between the host and the automated data storage library. One of ordinary skill in the art will recognize that one or more control paths may exist and be used for the present invention. The automated data storage library  200  comprises a library controller  201  such as the library controller  100  of FIG. 1, control path communication lines  202 , data storage rives  203 , data storage drive communication lines  204 , removable data storage media  208  for he storage of data, a robot accessor  205  for transporting the removable data storage media  208 , storage shelves  206  for holding the removable data storage media  208 , and an optional import/export station  207  for transferring the removable data storage media  208  to/from the data storage library. The library controller  201  may comprise a processor, PC, workstation, embedded system, etc. Alternatively, the library controller  201  may comprise a distributed control system as described in U.S. Pat. No. 6,356,803. The control path communication lines  202  provide control paths into the library  200  and they allow host systems to issue commands to the library  200 . Alternatively, the control paths may go through the data storage drives  203  and through the data storage drive communication lines  204 , as described in U.S. Pat. No. 6,434,090. In this case, control path communication lines  202  would not be required and host communication lines  211 ,  221 ,  222 ,  231  would be connected directly to data storage drives  203 . In any case, the embodiments of this invention do not change, regardless of where the control paths originate.  
         [0020]    The data storage drives  203  provide a means for reading and/or writing data with respect to the removable data storage media  208 . The data storage drives  203  may comprise optical or magnetic tape drives, optical or magnetic disk drives, electronic media drives, etc. The optional data storage drive communication lines  204  provide a means for communication between the data storage drives  203  and the library controller  201 . The data storage drive communication lines  204  may comprise serial connections, such as RS-232 or any other communication protocol as is known to those of skill in the art. The data storage drives may be directly connected to host systems (not shown), such as host computers  210 ,  220 ,  230 , to read and/or write data to/from removable data storage media  208 . In this case, the data storage drive host communication lines (not shown) may comprise RS-232, USB (Universal Serial Bus), SCSI (Small Computer Systems Interface), FC-AL (Fibre Channel-Arbitrated Loop) or any other communication interface known to those of skill in the art. Alternatively, the data storage drives may not be connected to host systems at all. In this case, the library controller  201  would be responsible for the movement of commands and data to/from the data storage drive through data storage drive communication lines  204 .  
         [0021]    [0021]FIG. 3 illustrates the method of the first embodiment of the present invention as performed by the automated data storage library. In step  301 , the automated data storage library receives a host command. At step  302  the automated data storage library decodes the command to determine the type of command and if the command is the same command as a previous good command. A command that was completed successfully would be considered to be a previous good command. The library controller maintains an inventory of previous commands. The inventory may only contain commands that were successfully executed or it may contain the execution status for the commands. This inventory may simply be a copy of the command or it may be some other collection of data that describes the command. The inventory may only comprise the previous command or it may comprise multiple previous commands. In addition, the inventory of previous commands may be organized by the type of command such as a movement command versus a status command. In this example, there may be a command inventory for the previous movement command(s) and there may be another command inventory for the previous status command(s). An inventory of previous commands is used to ultimately determine if a previous command executed successfully. As described above, this may be an inventory of good commands or it may be an inventory of commands that includes enough information about the commands such that the execution success can be determined. Further, an inventory of good commands or an inventory of previous good commands are equivalent and may refer to any of the above descriptions. In this context, a good command is a command that has executed successfully. Herein, when describing the inventory of commands, previous, good, successful, previous good and previous successful all refer to the same thing. The inventory of previous commands may be stored in memory associated with the library controller. RAM  103  and/or nonvolatile memory  104 , memory contained in the processor  102 , or other memory (not shown in FIG. 1) could be used for the memory portion for the storage of command inventory information. Step  302  captures the case where a command that was sent down one control path actually completed successfully, even though a command completed successfully response may not have been received by the host system that issued that command. Step  302  specifically looks for a duplicate command, from the perspective of the automated data storage library. This duplicate command could be the result of the host sending a retry command because the host did not receive a command completed successfully response from the automated data storage library. This failover procedure initiated by the host computer may use the same control path (as used when the command was first sent) or another control path to send the retry command to the automated data storage library. For example, host  210  may send the command using control path  211  of FIG. 2 and then send the retry command on control path  211 . In another example, host  220  may send the command using control path  221  of FIG. 2 and then send the retry command on control path  222  of FIG. 2. Alternatively host  220  may send the retry command on another control path (not shown in FIG. 2) than control path  221  or  222 . The host may send the command more than once using any of the control paths that may exist between the host and the automated data storage library. Referring to FIG. 3, if the command is the same as the previous good command, as indicated in step  302 , then the automated data storage library returns a good command status to the host in step  303 . If however, the command is not the same as the previous good command as indicated in step  302 , then control moves to step  304  where the automated data storage library executes the command. At step  305  the library determines if the command executed successfully this time. If the command executed successfully as indicated in step  305 , then control moves to step  307  where the inventory of previous good commands is updated with the command that just executed successfully. If however, the command did not execute successfully as indicated in step  305 , then control moves to step  306  where the inventory of previous good commands is not updated. Alternatively, the previous good command may be removed from the inventory in step  306 . One skilled in the art can see that variations may be made to the flowchart of FIG. 3 without deviating from the sprit of the invention. For example, there may be more than one previous command inventory and it may be based on the type of command received or some other criteria. In this case, step  302  may be modified to check the corresponding command inventory for a previous command. In another example, steps  305  and  306  may be eliminated if the previous command inventory contains the command execution status as described above. In this case, step  307  would update the inventory of previous commands with the current command and its execution status.  
         [0022]    [0022]FIG. 4 illustrates the method of the second embodiment of the present invention as performed by the automated data storage library. In step  401 , the automated data storage library receives a host command. In step  402 , the automated data storage library decodes the command to determine the type of command and more specifically determines if the command is a move medium command or an exchange medium command. Move medium commands and exchange medium commands are commands of a general class of transport media commands. Both commands transport data storage media in the automated data storage library. Move medium commands are used to move data storage media from one location to another location within the library. Exchange medium commands are used to improve the performance of a move command by combining more than one move into a single command. If the command is not a move command and not an exchange command, as indicated in step  402 , then control moves to step  403  where the automated data storage library executes the host command. Step  402  narrows the problem of correctly handling a failover because any commands other than move or exchange commands will simply be executed with no further checking. If however, the command is a move command or an exchange command, as indicated in step  402 , control moves to step  404  where the automated data storage library determines if the command is the same command as the previous good command. A command that was completed successfully would be considered to be a previous good command. The library controller maintains an inventory of previous commands. The inventory may only contain commands that were successfully executed or it may contain the execution status for the commands. This inventory may simply be a copy of the command or it may be some other collection of data that describes the command. The inventory may only comprise the previous command or it may comprise multiple previous commands. In addition, the inventory of previous commands may be organized by the type of command such as a movement command versus a status command. In this example, there may be a command inventory for the previous movement command(s) and there may be another command inventory for the previous status command(s). Step  404  captures the ease where a command that was sent down one control path actually completed successfully, even though a command completed successfully response may not have been received by the host system that issued that command. Step  404  specifically looks for a duplicate command, from the perspective of the automated data storage library. This duplicate command could be the result of the host sending a retry command because the host did not receive a command completed successfully response from the automated data storage library. This failover procedure initiated by the host computer may use the same control path (as used when the command was first sent) or another control path to send the retry command to the automated data storage library. For example, host  210  may send the command using control path  211  of FIG. 2 and then send the retry command on control path  211 . In another example, host  220  may send the command using control path  221  of FIG. 2 and then send the retry command on control path  222  of FIG. 2. Alternatively host  220  may send the retry command on another control path (not shown in FIG. 2) than control path  221  or  222 . The host may send the command more than once using any of the control paths that may exist between the host and the automated data storage library. Referring to FIG. 4, if the command is the same as the previous good command as indicated in step  404 , then control moves to step  405  where the automated data storage library returns a good command status to the host. If however, the command is not the same as the previous good command, as indicated in step  404  then the automated data storage library executes the command in step  406 . At step  407  the library determines if the command executed successfully this time. If the command executed successfully as indicated in step  407 , then control moves to step  409  where the inventory of previous good commands is updated with the command that just executed successfully. If however, the command did not execute successfully as indicated in step  407 , then control moves to step  408  where the inventory of previous good commands is not updated. Alternatively, the previous good command may be removed from the inventory in step  408 . One skilled in the art can see that variations may be made to the flowchart of FIG. 4 without deviating from the sprit of the invention. For example, step  402  may include commands other than, or in addition to, move commands and exchange commands. In another example, steps  407  and  408  may be eliminated if the previous command inventory contains the command execution status as described above. In this case, step  409  would update the inventory of previous commands with the current command and its execution status.  
         [0023]    [0023]FIG. 5 illustrates the method of the third embodiment of the present invention, as performed by the automated data storage library. In step  501 , the automated data storage library receives a host command. In step  502 , the automated data storage library decodes the command to determine the type of command and more specifically determines if the command is a move medium command or an exchange medium command. Move medium commands and exchange medium commands are commands of a general class of transport media commands. Both commands transport data storage media in the automated data storage library. Move medium commands are used to move data storage media from one location to another location within the library. Exchange medium commands are used to improve the performance of a move command by combining more than one move into a single command. If the command is not a move command and not an exchange command, as indicated in step  502 , then control moves to step  503  where the automated data storage library executes the host command. Step  502  narrows the problem of correctly handling a failover because any commands other than move or exchange commands will simply be executed with no further checking. If however, the command is a move command or an exchange command, as indicated in step  502 , control moves to step  504  where the automated data storage library determines if the command is the same command as the previous good command. A command that was completed successfully would be considered to be a previous good command. The library controller maintains an inventory of previous commands. The inventory may only contain commands that were successfully executed or it may contain the execution status for the commands. This inventory may simply be a copy of the command or it may be some other collection of data that describes the command. The inventory may only comprise the previous command or it may comprise multiple previous commands. In addition, the inventory of previous commands may be organized by the type of command such as a movement command versus a status command. In this example, there may be a command inventory for the previous movement command(s) and there may be another command inventory for the previous status command(s). Step  504  will capture the case where a command that was sent down one control path actually completed successfully, even though a command completed successfully response may not have been received by the host system that issued that command. Step  504  specifically looks for a duplicate command, from the perspective of the automated data storage library. This duplicate command could be the result of the host sending a retry command because the host did not receive a command completed successfully response from the automated data storage library. This failover procedure initiated by the host computer may use the same control path (as used when the command was first sent) or another control path to send the retry command to the automated data storage library. For example, host  210  may send the command using control path  211  of FIG. 2 and then send the retry command on control path  211 . In another example, host  220  may send the command using control path  221  of FIG. 2 and then send the retry command on control path  222  of FIG. 2. Alternatively host  220  may send the retry command on another control path (not shown in FIG. 2) than control path  221  or  222 . The host may send the command more than once using any of the control paths that may exist between the host and the automated data storage library. Referring to FIG. 5, if the command is not the same as the previous successful command as indicated in step  504 , control moves to step  510  where the automated data storage library executes the move or exchange command. If however, the command is the same as the previous successful command, as indicated in step  504 , then control moves to step  505  where the automated data storage library determines if the source location of the move or exchange command is empty or is not accessible. Step  505  further attempts to determine if the command has already been executed. For example, if the last successful command was the same command as the current command then the source location of the move or exchange command should be empty, because this command was already executed successfully. In addition, the accessibility check will further enhance the determination of prior execution as it relates to drives. For example, a cartridge that is loaded into a removable media drive is usually not accessible until the cartridge has been unloaded from the drive. The command may be a move command where the source location and destination location is a single drive. This may happen, for example, as a re-chuck ERP (Error Recovery Procedure) where the media is reloaded because a media load or thread error was detected. The accessibility check would prevent a failover from potentially reloading a cartridge back into a drive. Referring back to FIG. 5, if the source location is empty or is not accessible, as indicated in step  505 , then the automated data storage library returns good command status to the host in step  506 . If however, the source location is not empty and is accessible, as indicated in step  505 , then control moves to step  507  where the automated data storage library determines if the command is an exchange medium command. If the command is not an exchange command, as indicated in step  507 , then control moves to step  510  where the automated data storage library executes the move command. If however, the command is an exchange command, as indicated in step  507 , then control moves to step  508  where the automated data storage library determines if the first destination location is empty or is not accessible. Step  508  further attempts to determine if the exchange command has already been executed, in a similar manner to step  505 . If the first destination location is not empty and is accessible, as indicated in step  508 , then control moves to step  510  where the automated data storage library executes the exchange command. If however, the first destination location is empty or is not accessible, as indicated in step  508 , then the automated data storage library returns good command status to the host in step  509 . As mentioned previously, if the command is not the same as the previous successful command as indicated in step  504 , or if the command is not an exchange medium command as indicated in step  507 , or if the first destination location is not empty and is accessible as indicated in step  508 , then control moves to step  510  where the automated data storage library executes the command. In step  511  the automated data storage library determines if the command executed successfully. If the command executed successfully as indicated in step  511 , then control moves to step  513  where the inventory of the previous good commands is updated with the command that just executed successfully. If however, the command did not execute successfully as indicated in step  511 , then control moves to step  512  where the inventory of the previous good commands is not updated. Alternatively, the previous good command may be removed from the inventory in step  512 . One skilled in the art can see that variations may be made to the flowchart of FIG. 5 without deviating from the method of the current invention. For example, some libraries may not support the exchange medium command and steps  507 ,  508  and  509  may be eliminated. In this case, a no answer in step  505  would lead to step  510 . As another example, steps  502  and  503  may be eliminated or modified to apply the logic of the flowchart to other automated data storage library commands. In this case, the previous good command of steps  504  and  513  may include commands other than, or in addition to, move commands and exchange commands. In addition, step  502  may be eliminated because step  504  would determine if the command has already been executed. In this case, it may be desirable to modify the logic of step  513  to only include certain commands since any automated data storage library command could result in the execution of step  513 . Still further, the checks for empty and/or not accessible in step  505  may be modified or removed. For example, it may be desirable to skip any additional checking for empty and/or not accessible since the automated data storage library contains a record of successfully completing the command as the result of step  504 . In addition, the check to see if the current command is the same as the last good command of step  504  may be removed since the checking for empty and/or not accessible of step  505  may indirectly capture the case where the command has already been executed. As yet another example, the order of step may be changed. For example, steps  502  and  504  may be swapped. Finally, steps  511  and  512  may be eliminated if the previous command inventory contains the command execution status as described above. In this case, step  513  would update the inventory of previous commands with the current command and its execution status.  
         [0024]    [0024]FIG. 6 illustrates the method of all three embodiments described above, as performed by the host computer. In step  601 , the host computer issues a command on a first control path, such as host communication lines  221  of FIG. 2. In step  602 , the host computer determines if the command completed. Command completion may comprise status of successful command completion. Alternatively, command completion may additionally comprise status of unsuccessful command completion. For example, some command errors may be the result of a problem with the control path, while other command errors may be the result of problems that have nothing to do with which control path a command is issued through. Therefore, it may be desirable to treat some errors differently than others. Alternatively, step  602  may only check that the command completed successfully. Referring back to FIG. 6, if the command completed, as indicated in step  602 , control moves to step  603  where the host considers the control path good and handles the command response appropriately. If however, the automated data storage library command did not complete, as indicated in step  602 , then control moves to step  604  where the host issues the same command (retry command) on a second control path, such as host communication lines  222  of FIG. 2. Alternatively, the host may issue the same command (retry command) on the same control path. Steps  602 ,  603  and  604  may be repeated for the same, or additional control paths. This would allow any number of attempted control path failovers.  
         [0025]    In another embodiment of this invention, the host computer sends the same command down two or more control paths at the same time. Since one command will always arrive prior to the other command, from the perspective of the library controller, the logic that the library performs will be the same as described in FIGS. 1 through 5. Instead of managing retries, the host computer would monitor duplicate responses. Herein, any sequence of host commands that results in the library executing the logic described by this invention shall be referred to as host retries.  
         [0026]    While this invention was largely described in terms of a single host computer with redundant control paths, it may be applied to a single control path. For example, a host computer may have more than one host bus adapter connected to a single library control path. A failure of one host bus adapter could cause the host to use another host bus adapter to retry the command on the same control path. The host bus adapter comprises circuits that the host may use to provide communication over a SCSI (Small Computer Systems Interface) bus, FC-AL (Fibre Channel-Arbitrated Loop) bus, USB (Universal Serial Bus), or any other communication method as is known to those of skill in the art.  
         [0027]    While this invention was described in terms of a single host computer with multiple control paths, it may be applied to multiple host computers sharing one or more control paths. In this case, the inventory for previous good commands may be expanded to include information regarding which host the command is associated with. This additional information may be used to compare a current command to a previous command from the same host.  
         [0028]    While the preferred embodiments of the present invention have been illustrated in detail, the skilled artisan will appreciate that modifications and adaptations to those embodiments may be made without departing from the scope of the present invention as set forth in the following claims.