Abstract:
A data storage library efficiently utilizes I/O slots while maintaining software compatibility by using functional addresses to conduct virtual cartridge moves from storage slots to I/O slots. More particularly, a location-centric library host manages cartridge movement according to functional storage addresses and functional I/O addresses. In reality, the library has multiple cartridge receiving slots, which include physical I/O slots and physical storage slots. In contrast with the physical I/O slots and physical storage slots, functional I/O addresses and functional storage addresses are virtual locations used by the host in managing cartridge locations. Thus, host knowledge of cartridge locations is limited to their functional addresses. The library includes a library map that correlates functional addresses with physical addresses. Initially, an eject command is received from the host. The eject command requests transfer of a cartridge from a source functional storage address to a target functional I/O address. In response to the eject command, irrespective of any physical movement of the cartridge, the library reports successful completion of the requested eject command to the host. The library promptly gives a functional I/O address to the physical storage slot containing the cartridge. When a physical I/O slot becomes available, the library physically moves the cartridge there and either correlates the cartridge&#39;s functional I/O address with this physical I/O slot, or registers the cartridge&#39;s functional I/O address as empty.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to data storage libraries housing multiple tapes or other data storage cartridges in various slots. More particularly, the invention concerns a data storage library that efficiently utilizes physical input/output (“I/O”) slots by using functional addresses to conduct virtual cartridge ejects to the I/O slots. 
     2. Description of the Related Art 
     Many data processing systems require a large amount of data storage, for use in efficiently accessing, modifying, and re-storing data. Data storage is typically separated into several different levels, each level exhibiting a different data access time or data storage cost. A first, or highest level of data storage involves electronic memory, usually dynamic or static random access memory (“DRAM” or “SRAM”). Electronic memories take the form of semiconductor integrated circuits where millions of bytes of data can be stored on each circuit, with access to such bytes of data measured in nanoseconds. The electronic memory provides the fastest access to data since access is entirely electronic. 
     A second level of data storage usually involves direct access storage devices (“DASD”). DASD storage, for example, includes magnetic and/or optical disks. Data bits are stored as micrometer-sized magnetically or optically altered spots on a disk surface, representing the “ones” and “zeros” that comprise the binary value of the data bits. Magnetic DASD includes one or more disks that are coated with remnant magnetic material. The disks are rotatably mounted within a protected environment. Each disk is divided into many concentric tracks, or closely spaced circles. The data is stored serially, bit by bit, along each track. An access mechanism, known as a head disk assembly (“HDA”) typically includes one or more read/write heads, and is provided in each DASD for moving across the tracks to transfer the data to and from the surface of the disks as the disks are rotated past the read/write heads. DASDs can store gigabytes of data, and the access to such data is typically measured in milliseconds (orders of magnitudes slower than electronic memory). Access to data stored on DASD is slower than electronic memory due to the need to physically position the disk and HDA to the desired data storage location. 
     A third or lower level of data storage includes tapes, tape libraries, and optical disk libraries. Access to library data is much slower than electronic or DASD storage because a robot or human is necessary to select and load the needed data storage medium. An advantage of these storage systems is the reduced cost for very large data storage capabilities, on the order of Terabytes of data. Furthermore; tape storage is especially useful for backup purposes. That is, data stored at the higher levels of data storage hierarchy is reproduced for safe keeping on magnetic tape. Access to data stored on tape and/or in a library is presently on the order of seconds. 
     There are a number of different data storage libraries on the market today, including models made by International Business Machines (“IBM”). A number of today&#39;s data storage libraries utilize the small computer system interface (“SCSI”) medium changer standard. This standard is “location-centric” because it requires the host to manage cartridge movement by specifying source and destination locations in the system. Each location is a site capable of holding a cartridge, and is referred to as an “element.” Each element is given a fixed element address, either at the time of manufacture or at the time of system installation or configuration. The SCSI medium changer protocol defines four types of elements: medium transport element, storage element, import/export element, and data transfer element. In physical terms, the medium transport element is an accessor gripper, a storage element is a storage slot, an import/export element is a library I/O slot or pass-through slot, and a data transfer element is a removable media drive. 
     Moves from one element to another are requested on the SCSI interface. Typically, moves from one element to another element are the responsibility of SCSI initiator software, also called independent software vendor programming. This includes moves between the I/O slots and the storage slots. 
     Even though some data storage libraries enjoy considerable commercial success today, IBM engineers are continually seeking to improve the performance and efficiency of these systems. One area of possible focus concerns the manner in which the library ejects cartridges and receives inserted cartridges. When an operator wishes to load a number of cartridges into a library without disrupting the accessor motion, the operator inserts the cartridges into the I/O slots. However, data storage libraries only have a finite number of I/O slots for use in transferring cartridges to and from the library. Consequently, eject/insert operations are blocked if the I/O slots fill up, until the independent software vendor programming moves the inserted cartridges to storage slots using the SCSI interface. 
     In addition, many libraries are slow to transfer cartridges into the library from I/O slots because they rely on human operators to issue commands to the host using a library control panel. This is because the host is needed to supervise cartridge insertion operations by issuing appropriate commands to library robotics. This situation may be exacerbated if the host is located remotely from the library, since the operator (and library control panel) are located at the host, but the operator must physically insert or remove cartridges from I/O slots at the library. Accordingly, the process of adding a large number of cartridges may involve many trips between the library&#39;s I/O station and the control panel. 
     Furthermore, when the independent software vendor programming needs to eject some cartridges by operator request or automatically, the operator must ensure there is an empty I/O slot for each cartridge. Otherwise, the attempt may be blocked, causing error, failure, or other delay. For these and other reasons, known data storage libraries are amenable to improvement. 
     SUMMARY OF THE INVENTION 
     Broadly, the present invention concerns a data storage library that efficiently utilizes I/O slots while maintaining software compatibility by using functional addresses to conduct virtual cartridge moves from storage slots to I/O slots. More particularly, a location-centric library host manages cartridge movement according to functional storage addresses and functional I/O addresses. In reality, the library has multiple cartridge receiving slots, which include physical I/O slots and physical storage slots. In contrast with the physical I/O slots and physical storage slots, functional I/O addresses and functional storage addresses are virtual locations used by the host in managing cartridge locations. Thus, host knowledge of cartridge locations is limited to their functional addresses. The library includes a library map that correlates functional addresses with physical addresses. 
     Initially, an eject command is received from the host. The eject command requests transfer of a cartridge from a source functional storage address to a target functional I/O address. In response to the eject command, irrespective of any physical movement of the cartridge, the library reports successful completion of the requested eject command to the host. The library first gives a functional I/O address to the physical storage slot containing the cartridge. When a physical I/O slot becomes available, the library physically moves the cartridge there and either correlates the cartridge&#39;s functional I/O address with this physical I/O slot, or registers the cartridge&#39;s functional I/O address as empty. 
     Accordingly, in one embodiment, the invention may be implemented to provide a method to manage eject operations in a data storage library. In another embodiment, the invention may be implemented to provide an apparatus, such as a data storage library, configured to manage eject operations as explained herein. In still another embodiment, the invention may be implemented to provide a signal-bearing medium tangibly embodying a program of machine-readable instructions executable by a digital data processing apparatus to perform operations for managing eject operations in a data storage library. Another embodiment concerns logic circuitry having multiple interconnected electrically conductive elements configured to perform operations in a data storage library as discussed herein. 
     The invention affords its users with a number of distinct advantages. For example, host workload is reduced because the host can direct ejection of a cartridge without waiting for physical ejection to complete, and regardless of whether a physical I/O slot is available at that time. As another advantage, the invention maintains broad software compatibility between the host and library controller. The invention also provides a number of other advantages and benefits, which should be apparent from the following description of the invention. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a block diagram of the hardware components and interconnections of a data storage system according to the invention. 
     FIG. 2 is a block diagram of a digital data processing machine according to the invention. 
     FIG. 3 shows an exemplary signal-bearing medium according to the invention. 
     FIG. 4 depicts flowcharts of cartridge intake sequences according to the invention. 
     FIG. 5 is a flowchart of an operational sequence for processing host eject commands according to the invention. 
    
    
     DETAILED DESCRIPTION 
     The nature, objectives, and advantages of the invention will become more apparent to those skilled in the art after considering the following detailed description in connection with the accompanying drawings. 
     Introduction 
     One aspect of the invention concerns a data storage system, which may be embodied by various hardware components and interconnections, with one example being described in FIG.  1 . FIG. 1 shows a data storage library  100  coupled to a hierarchically superior host  102 . The library  100  may include one or different types of portable data storage media, such as magnetic tape cartridges, optical cartridges, writeable CDs, etc. For ease of reference, the portable data storage media of the library  100  are referred to as “cartridges.” 
     Host 
     Among other possible functions, the host  102  supplies data to the library  100  for storage on the cartridges, and sends requests to the library  100  to retrieve data from the cartridges. The host role may be satisfied by various types of hardware, such as a digital data processing computer, logic circuit, construction of discrete circuit components, interface to a human operator, etc. As an example, the host  102  may comprise an IBM RS/6000 machine employing an operating system such as AIX. The host  102  is also coupled to an interface  104  and a host catalog  120 . The interface  104  enables the host  102  to exchange information with a human operator, and may comprise a control panel, video monitor, computer keyboard/mouse, or another appropriate human/machine interface. 
     The host  102  manages data in the library  100  using “location-centric” commands, and may utilize the SCSI medium changer protocol as one example. The host manages cartridge movement by specifying source and destination locations in the system. According to the present invention, the source and destination locations are “functional addresses,” rather than physical storage sites. The functional addresses may also be considered “imaginary” or “virtual” storage addresses, since they do not necessarily correspond to physical storage sites in the library  100  (although they might on an incidental basis). Nonetheless, to satisfy the host&#39;s location-centric nature, the host  102  associates each functional address with various physical attributes, such as a medium transport element, storage element, import/export element, or data transfer element. In the illustrated example, the host&#39;s functional addresses include “functional I/O addresses” (which the host perceives to be I/O slots) and “functional storage addresses” (which the host perceives to be storage slots). As an example, the host&#39;s functional addresses may be established upon configuration of the library  100  with the host  102 , and would not normally change. There is a different, underlying layer of mapping that correlates the host&#39;s functional addresses with the actual storage sites, called “physical storage addresses.” This configuration introduces several benefits for the library  100 , as discussed in greater detail below. 
     To support its management of the data storage library  100  according to functional addresses, the host  102  maintains the host catalog  120 . The host catalog  120  cross-references each functional address with any data storage cartridge that is stored therein, according to the host&#39;s view. TABLE 1 shows an example of the catalog  120 . Each row depicts one functional address, and cross-references this address against: 
     1. The type of imaginary location in the data storage library  100  represented by the functional address. This information, which includes whether the functional address is a “functional I/O address” or a “functional storage address,” is fixed during operation of the host  102 . 
     2. Whether the functional address contains a cartridge or not; this information varies during normal operation of the library  100 . 
     3. The identity of the cartridge (if any) stored at the functional address; one type of identification is by volume serial number (“VOLSER”), as illustrated. This information varies during normal operation of the library  100 . 
     
       
         
               
             
               
               
               
               
             
           
               
                 TABLE 1 
               
             
             
               
                   
               
               
                 HOST CATALOG 
               
             
          
           
               
                    FUNCTIONAL 
                   
                   
                   
               
               
                 ADDRESS 
                 TYPE 
                 FULL OR EMPTY? 
                 VOLSER 
               
               
                 (FIXED) 
                 (FIXED) 
                 (CHANGEABLE) 
                 (CHANGEABLE) 
               
               
                   
               
               
                 001 
                 FUNCTIONAL STORAGE 
                 FULL 
                 929475 
               
               
                   
                 ADDRESS 
               
               
                 002 
                 FUNCTIONAL STORAGE 
                 FULL 
                 988928 
               
               
                   
                 ADDRESS 
               
               
                 003 
                 FUNCTIONAL STORAGE 
                 FULL 
                 329820 
               
               
                   
                 ADDRESS 
               
               
                 004 
                 FUNCTIONAL STORAGE 
                 EMPTY 
                 NONE 
               
               
                   
                 ADDRESS 
               
               
                 . . . 
                 . . . 
                 . . . 
                 . . . 
               
               
                 098 
                 FUNCTIONAL I/O ADDRESS 
                 FULL 
                 333820 
               
               
                 099 
                 FUNCTIONAL I/O ADDRESS 
                 EMPTY 
                 NONE 
               
               
                 100 
                 FUNCTIONAL I/O ADDRESS 
                 EMPTY 
                 NONE 
               
               
                   
               
             
          
         
       
     
     Drive 
     The data storage library  100  includes a drive  106  to conduct read/write operations with cartridges in the library  100 . The library  100  may utilize multiple drives  106  if desired. Each drive  106  comprises suitable hardware to access the format of data storage cartridge in the library  100 . For example, in the case of magnetic tape cartridges, the drive  106  may comprise an IBM model 3590 tape drive. Cartridges are directed to/from the drive  106  by robotics  110 , described below. 
     Physical Cartridge Storage &amp; Management 
     The library  100  also includes equipment to physically move and store the cartridges. For instance, physical storage slots  114  house cartridges when they are not being used. The physical storage slots  114  comprise shelves or other data storage library compartments. 
     Physical I/O slots  112  are provided to transfer cartridges to/from the library  100 . The physical I/O slots  112  include any slots that are marked, known, set aside, positioned, or otherwise designated for operator to insert cartridges into the library and remove cartridges therefrom. Using the I/O slots  112 , an operator can introduce cartridges into the library  100  (“insert” operation), or the library  100  can expel cartridges (“eject” operation). The physical I/O slots  112  may be accessible by the operator without disrupting operation of the robotics  112  or drive  106  (such as through an external door), although this is not necessary. Some examples of physical I/O slots  112  include “pass-through” slots, a carriage, conveyor, normal storage-type slots designated as I/O slots, etc. 
     To move cartridges between the drive  106 , I/O slots  112 , and storage slots  114 , the library  100  includes robotics  110 . The robotics  110  accesses these components by respective paths  110   a ,  110   b , and  110   c . The robotics  110  may be implemented by any suitable cartridge movement machinery, such as robotic arms, integrated cartridge loading equipment, conveyors, grippers movable on an x-y coordinate system, etc. 
     Controller 
     The library  100  operates under supervision of a controller  108 , which receives commands from the host  102  requesting the controller  108  to move cartridges from one functional address to another. The controller  108  communicates with the host  102  by interfaces such as wires/cables, one or more busses, fiber optic lines, wireless transmission, intelligent communications channel, etc. In addition to this host-controller interface, which constitutes a “control” path, the library  100  also includes a “data” path that carries data between the host  102  and the drive  106 . 
     The controller  108  comprises a digital data processing machine, logic circuit, construction of discrete circuit components, or other automated mechanism, and operates according to suitable programming, physical configuration, etc. To provide a specific example, the controller  108  may comprise an IBM PowerPC processor. 
     After receiving location-centric commands from the host  102  referencing imaginary “functional addresses,” the controller  108  translates these commands into physical locations present in the library  100  and implements the requested operations by directing the robotics  110 . To map between the host&#39;s functional addresses and the library&#39;s physical storage locations, the controller  108  maintains a library database including a library map  116  and library status table  118 . 
     For each functional address, the library map  116  identifies a corresponding physical storage address, if one has been associated with that functional address. TABLE 2 shows an example of the library map  116 . Each row depicts one functional address and the associated physical storage address. In this example, the physical storage addresses comprise horizontal-vertical coordinates for a robotic gripper. 
     
       
         
               
             
               
               
               
             
           
               
                 TABLE 2 
               
             
             
               
                   
               
               
                 LIBRARY MAP 
               
             
          
           
               
                   
                   
                 ASSOCIATED 
               
               
                 FUNCTIONAL 
                   
                 PHYSICAL 
               
               
                 ADDRESS 
                 TYPE OF FUNCTIONAL 
                 ADDRESS 
               
               
                 (FIXED) 
                 ADDRESS (FIXED) 
                 (CHANGEABLE) 
               
               
                   
               
               
                 001 
                 STORAGE 
                 (1,1) LEFT 
               
               
                 002 
                 STORAGE 
                 (1,1) RIGHT 
               
               
                 003 
                 STORAGE 
                 (1,2) LEFT 
               
               
                 004 
                 STORAGE 
                 UNASSOCIATED 
               
               
                 . . . 
                 . . . 
                 . . . 
               
               
                 098 
                 I/O 
                 (4.5) LEFT 
               
               
                 099 
                 I/O 
                 UNASSOCIATED 
               
               
                 100 
                 I/O 
                 UNASSOCIATED 
               
               
                   
               
             
          
         
       
     
     The library status table  118  (TABLE 3) lists all physical storage addresses in the library  100 . This listing depends upon the physical configuration of the library  100 , which is established upon manufacture, initial configuration, etc. Also, for each physical storage address, the library status table  118  tells: 
     1. Whether the physical storage address contains a cartridge or not, which may change from time to time. 
     2. What physical configuration embodies that physical storage address (e.g., read/write drive, storage slot, I/O slot, etc.). This is fixed at an appropriate time, such as the initial configuration of the library. 
     3. The VOLSER or other identity of cartridge stored in the physical storage address. This changes from time to time, as cartridges are moved about in the library. 
     
       
         
               
             
               
               
               
               
             
           
               
                   
               
               
                 LIBRARY STATUS TABLE 
               
             
          
           
               
                   
                   
                 CONFIGURATION OF 
                   
               
               
                 PHYSICAL ADDRESS 
                 FULL OR EMPTY? 
                 PHYSICAL ADDRESS 
                 VOLSER 
               
               
                 (FIXED) 
                 (CHANGEABLE) 
                 (FIXED) 
                 (CHANGEABLE) 
               
               
                   
               
               
                     (1,1) LEFT 
                 FULL 
                 STORAGE SLOT 
                 929475 
               
               
                 (1,1) RIGHT 
                 FULL 
                 STORAGE SLOT 
                 988928 
               
               
                 (1,2) LEFT 
                 FULL 
                 STORAGE SLOT 
                 329820 
               
               
                 . . . 
                 . . . 
                 . . . 
                 . . . 
               
               
                 (4,5) LEFT 
                 FULL 
                 I/O SLOT 
                 333820 
               
               
                 (5,8) RIGHT 
                 EMPTY 
                 I/O SLOT 
                 NONE 
               
               
                 . . . 
                 . . . 
                 . . . 
                 . . . 
               
               
                 (10,10) RIGHT 
                 EMPTY 
                 DRIVE 
                 NONE 
               
               
                   
               
             
          
         
       
     
     Exemplary Digital Data Processing Apparatus 
     The controller  108  may be implemented in various forms, including a digital data processing apparatus as one example. This apparatus may be embodied by various hardware components and interconnections; one example is the digital data processing apparatus  200  of FIG.  2 . The apparatus  200  includes a processor  202 , such as a microprocessor or other processing machine, coupled to a storage  204 , In the present example, the storage  204  includes a fast-access storage  206 , as well as nonvolatile storage  208 . The fast-access storage  206 may comprise random access memory (“RAM”), and may be used to store the programming instructions executed by the processor  202 . The nonvolatile storage  208  may comprise, for example, one or more magnetic data storage disks such as a “hard drive,” a tape drive, or any other suitable storage device. The apparatus  200  also includes an input/output  210 , such as a line, bus, cable, electromagnetic link, or other means for the processor  202  to exchange data with other hardware external to the apparatus  200 . 
     Despite the specific foregoing description, ordinarily skilled artisans (having the benefit of this disclosure) will recognize that the apparatus discussed above may be implemented in a machine of different construction, without departing from the scope of the invention. As a specific example, one of the components  206 ,  208  may be eliminated; furthermore, the storage  204  may be provided on-board the processor  202 , or even provided externally to the apparatus  200 . 
     Logic Circuitry 
     In contrast to the digital data storage apparatus discussed previously, a different embodiment of the invention uses logic circuitry instead of computer-executed instructions to implement the controller  108 . Depending upon the particular requirements of the application in the areas of speed, expense, tooling costs, and the like, this logic may be implemented by constructing an application specific integrated circuit (“ASIC”) having thousands of tiny integrated transistors. Such an ASIC may be implemented with CMOS, TTL, VLSI, or another suitable construction. Other alternatives include a digital signal processing chip (“DSP”), discrete circuitry (such as resistors, capacitors, diodes, inductors, and transistors), field programmable gate array (“FPGA”), programmable logic array (“PLA”), and the like. 
     In addition to the various hardware embodiments described above, a different aspect of the invention concerns a method, discussed below. 
     Signal-Bearing Media 
     In the context of FIGS. 1-2, such a method may be implemented, for example, by operating the controller  108 , as embodied by a digital data processing apparatus  200 , to execute various sequences of machine-readable instructions. These instructions may reside in various types of signal-bearing media. In this respect, one aspect of the present invention concerns a programmed product, comprising signal-bearing media tangibly embodying a program of machine-readable instructions executable by a digital data processor to perform a method of managing eject operations in the data storage library  100 . 
     This signal-bearing media may comprise, for example, RAM (not shown) contained within the controller  108 , as represented by the fast-access storage  206 . Alternatively, the instructions may be contained in another signal-bearing media, such as a magnetic data storage diskette  300  (FIG.  3 ), directly or indirectly accessible by the processor  200 . Whether contained in the storage  206 , diskette  300 , or elsewhere, the instructions may be stored on a variety of machine-readable data storage media, such as direct access storage (e.g., a conventional “hard drive,” redundant array of inexpensive disks (“RAID”), or another DASD), magnetic tape, electronic read-only memory (e.g., ROM, EPROM, or EEPROM), optical storage (e.g., CD-ROM, WORM, DVD, digital optical tape), paper “punch” cards, or other suitable signal-bearing media including transmission media such as digital and analog and communication links and wireless. In an illustrative embodiment of the invention, the machine-readable instructions may comprise software object code, compiled from a language such as “C,” etc. 
     Logic Circuitry 
     In contrast to the signal-bearing medium discussed above, the method aspect of the invention may be implemented using logic circuitry, without using a processor to execute instructions. In this embodiment, the logic circuitry is implemented in the controller  108 , and is configured to perform operations to implement the method of the invention. The logic circuitry may be implemented using many different types of circuitry, as discussed above. 
     Cartridge Insertion 
     FIG. 4 shows cartridge intake sequences 400/450, to provide one example of the invention. For ease of explanation, but without any intended limitation, the example of FIG. 4 is described in the context of the hardware components and interconnections shown in FIG. 1, described above. 
     In the routine  400 , the controller  108  manages the library  100  to automatically empty new cartridges from the physical I/O slots  112 . This helps the operator by ensuring that the I/O slots do not fill up, which would prevent the operator from inserting more cartridges. This also facilitates eject operations, since the controller  108  is more likely to find an available physical I/O slot. Another benefit of the routine  400  is that the controller  108  automatically recognizes external placement of a cartridge into the physical I/O slots  112 . The sequence  400  automatically empties cartridges from the physical I/O slots  112  regardless of any host involvement. 
     The sequence  400  is initiated in step  402 . In step  404 , the controller  108  determines whether any new cartridge(s) have been placed into the I/O slots  112 . This step may be performed by physically checking the physical I/O slots  112  (“polling”) according to a desired repeating schedule, polling the physical I/O slots  112  whenever a door to the physical I/O slots is opened, etc. As an alternative, some or all of the physical I/O slots  112  may include sensors that are activated when a cartridge is received. When step  404  finds a newly inserted cartridge, the controller  108  accesses the library status table  118  to identify an empty physical storage slot  114  (step  406 ), and then moves the inserted cartridge there (step  408 ). Step  408  also updates the library status table  118  to show the cartridge&#39;s presence in the empty storage slot. Step  408  has the effect of quickly clearing the physical I/O slot  112  where the cartridge was inserted, making it available for other insert or eject operations. Moreover, this step is invisible to the host  102 . 
     After step  408 , the controller  108  makes the cartridge known to the host  102 . First, the controller  108  selects an available functional I/O address from the library map  116 , and updates the library map  116  to associate this functional I/O address with the physical storage slot that now contains the cartridge (step  410 ). After step  410 , the routine  400  returns to step  404  to process any other newly inserted cartridges. 
     Apart from the sequence  400 , the sequence  450  is performed by the controller  108  to assist the host  102  in completing the cartridge insertion operation. The sequence  450  begins in step  412 . In step  414 , the controller  108  determines whether it has received any host commands. In response to a host status command, the controller  108  reports the newly received cartridge to the host  102  (step  413 ). Particularly, the controller  108  reports the cartridge&#39;s functional I/O address (from step  410 ) and VOLSER to the host  102 . This is how the host  102  learns of the cartridge&#39;s presence in the library, namely, by querying the controller  108 . After the host  102  becomes aware of the new cartridge&#39;s presence in the functional I/O address (via step  413 ), the host  102  responds (not shown) by updating its host catalog  120  to show the functional I/O address as “full,” and entering the cartridge&#39;s VOLSER or other identity. Then, at some later time depending upon host programming, the host  102  elects to move the cartridge from its functional I/O address into a functional storage address. When this occurs, the host  102  sends an appropriate “insert” command, which is received by the controller  108  in step  414 . The controller  108  reflexively responds to the insert command of step  414  with an immediate report that the requested insertion has been completed (step  416 ). To actually carry out insert operation, the controller  108  performs certain additional steps as part of step  416 , these additional steps being invisible to the host  102 . Namely, as the cartridge already resides in a physical storage slot, no physical movement is needed. Instead, the controller  108  chooses an available functional storage address from the library map  116  and associates it with the cartridge&#39;s physical storage address by updating the library map  116  (step  416 ). The controller  108  also de-associates the cartridge&#39;s previous functional I/O address by listing this functional I/O address as “unassociated” in the library map  116 . After step  416 , the controller  108  returns to step  414  to await another host command. 
     With the benefit of this disclosure, ordinarily skilled artisans should recognize that the order of operations within the sequences  400 ,  450  may be changed in certain respects without departing from this invention. Moreover, although the foregoing sequences  400 ,  450  have been described in a rigid, serial form for ease of illustration, some of the operations  400 ,  450  may employ hardware interrupts or multi tasking to perform concurrent operations for different cartridges, etc. 
     Cartridge Ejection 
     FIG. 5 shows one example of a cartridge ejection sequence  500 . For ease of explanation, but without any intended limitation, the example of FIG. 5 is described in the context of the hardware components and interconnections shown in FIG. 1, described above. 
     Advantageously, the controller  108  manages the library  100  to perform a near immediate virtual eject, so that the host  102  is never blocked by the perception of full physical I/O slots. As shown below, this is done by immediately associating a functional I/O address with the cartridge&#39;s current physical storage slot, whether any physical I/O slots are available or not. 
     The steps  500  are initiated in step  502 . In step  504 , the controller  108  receives an eject request from the host  102 . The eject request, which is location-centric in accordance with host programming, tells the controller  108  to move the cartridge from a specified functional storage address to a specified, available functional I/O address. In response, the controller  108  reflexively reports that the requested eject operation is complete (step  506 ) thereby satisfying the host request promptly. At this time, the host  102  may delete the cartridge from the host catalog  120  (step not shown); alternatively, the host  102  may wait until the cartridge is physically removed from the library or another appropriate event, determined by querying the controller  108 . To actually carry out the eject request, the controller  108  performs certain other steps, which are invisible to the host  102 . Namely, the controller  108  updates the library map  116  to free the functional storage address currently associated with the physical storage slot, and replace the functional storage address with the specified functional I/O address step (step  506 ). This achieves a near immediate virtual eject, since the host  102  now perceives the cartridge to be located in an I/O slot. 
     As an alternative to steps  504 - 506  as illustrated above, the host&#39;s eject request may omit the functional I/O address, leaving the controller  108  identify, select, and report (when queried) an available functional I/O address. 
     After step  506 , the controller  108  checks to see whether a physical I/O slot is available (step  510 ) to truly eject the cartridge. If not, the controller  108  waits in step  512 , and then repeats step  510 . One implementation of step  512 , for example, involves entering the cartridge-to-be-ejected into queue that advances each time a physical I/O slot becomes available. 
     When a physical I/O slot becomes available for the a waiting cartridge, the controller  108  moves the subject cartridge into the available physical I/O slot (step  514 ). Then, the controller  108  updates the library map  116  (step  516 ) so that the cartridge&#39;s current functional I/O address is associated with the cartridge&#39;s physical I/O address, instead of the physical storage address of its previously occupied storage slot. Additionally, the controller  108  updates the library status table  118  to show the cartridge&#39;s presence in the physical I/O slot. 
     After step  516 , the controller  108  waits until the cartridge is removed from its physical I/O slot by a human operator, another machine, etc. At this time, the controller  108  updates the library map  116  and library status table  118  to show removal of the cartridge from the library (step  517 ). As an alternative, the controller  108  may omit step  516 , in which case the cartridge&#39;s move to its physical I/O slot is not recorded. After step  517 , the eject routine  500  ends in step  518 . 
     Other Embodiments 
     While the foregoing disclosure shows a number of illustrative embodiments of the invention, it will be apparent to those skilled in the art that various changes and modifications can be made herein without departing from the scope of the invention as defined by the appended claims. Furthermore, although elements of the invention may be described or claimed in the singular, the plural is contemplated unless limitation to the singular is explicitly stated. Additionally, ordinarily skilled artisans will recognize that operational sequences must be set forth in some specific order for the purpose of explanation and claiming, but the present invention contemplates various changes beyond such specific order.