Patent Publication Number: US-9418696-B2

Title: Fast-access self-repairing tape library using mobile robots

Description:
RELATED APPLICATIONS 
     This application is a continuation of copending U.S. patent application Ser. No. 13/888,287 filed May 6, 2013, which is a continuation of U.S. patent application Ser. No. 13/558,232 filed Jul. 25, 2012, which is a continuation of U.S. patent application Ser. No. 13/217,181, filed Aug. 24, 2011, from all of which priority is claimed and which are all herein incorporated by reference. 
    
    
     BACKGROUND 
     The present invention relates to data storage systems, and more particularly, to mobile robots for use in tape libraries. 
     In magnetic storage systems, data are read from, and written onto, a magnetic recording medium utilizing magnetic transducers. Data are written on the magnetic recording medium by moving a magnetic recording transducer to a position over the medium where the data are to be stored. The magnetic recording transducer then generates a magnetic field, which encodes the data into the magnetic medium. Data are read from the medium by similarly positioning the magnetic read transducer and then sensing the magnetic field of the magnetic medium. Read and write operations may be independently synchronized with the movement of the medium to ensure that the data can be read from, and written to, the desired location on the medium. 
     In a tape drive system, magnetic tape is moved over the surface of the tape head at high speed. Usually the tape head is designed to minimize the spacing between the head and the tape. The spacing between the magnetic head and the magnetic tape is crucial so that the recording gaps of the transducers, which are the source of the magnetic recording flux, are in near contact with the tape to effect writing sharp transitions. Also when the read element is in near contact with the tape, effective coupling of the magnetic field from the tape to the read element is possible. 
     In the near future, with the adoption of improved media, the cost of storing information (on a per byte basis) on tape is expected to decline by a factor of five or more with respect to magnetic disk. Also, short-term and long-term reliability will continue to favor tape-based storage. Furthermore, as more mass storage is allocated to cloud networks, most storage will be in large libraries, rather than on individual drives, which is a consideration favoring tape-based storage. One disadvantage of tape-based storage with respect to disk-based storage is the relatively poor access time associated with tape-based storage, with the time required to bring the tape to the tape drive and then spool the tape to the file location typically averaging about 40 seconds. If the access time could be reduced, tape-based storage would be better positioned versus disk-based storage for storing files around 10 MB and greater, such as photographs, videos, collections, etc., which are accessed only rarely, but which users want to be able to retrieve almost immediately, e.g., within about a second. 
     BRIEF SUMMARY 
     A method according to one embodiment includes causing, using a controller, a mobile robot having a volume of less than about 1000 cubic inches to selectively retrieve one or more of a plurality of tape reels, and causing the mobile robot to transport the one or more retrieved tape reels to a tape drive that is configured for reading data from tape stored on at least one of the plurality of tape reels. The mobile robot threads a tape on the one or more reels onto the tape drive. 
     A method according to another embodiment includes causing, using a controller, a mobile robot to selectively retrieve at least one of a plurality of tape reels, causing the mobile robot to transport the at least one retrieved tape reel to a reel chuck, and threading a tape on the at least one retrieved tape reel across a tape drive by moving at least one of the tape reel and a take up reel coupled to the tape. 
     A method according to yet another embodiment includes causing, using a controller, a mobile robot to selectively retrieve a pair of tape reels coupled to a tape, causing the mobile robot to transport the at least tape reels to a pair of reel chucks, and threading the tape across a tape drive using at least one of rollers and cylinders. 
     Other aspects and embodiments of the present invention will become apparent from the following detailed description, which, when taken in conjunction with the drawings, illustrate by way of example the principles of the invention. 
    
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
         FIG. 1  is a schematic diagram of a simplified tape drive system according to one embodiment. 
         FIG. 2  illustrates a system using mobile robots for faster access to tape, according to one embodiment. 
         FIG. 3  shows a more detailed view of a mobile robot, according to one embodiment. 
         FIG. 4A  illustrates an optical pattern on a surface, according to one embodiment. 
         FIG. 4B  illustrates a surface design, according to one embodiment. 
         FIG. 4C  illustrates a surface design, according to one embodiment. 
         FIG. 4D  illustrates a surface design, according to one embodiment. 
         FIGS. 5A-5B  show detailed views of a mobile robot, according to one embodiment. 
         FIGS. 6A-6B  show detailed views of a tape library using mobile robots, according to one embodiment. 
         FIGS. 7A-7D  illustrate tape threading using a mobile robot, according to one embodiment. 
         FIGS. 8A-8D  show steps of self tape threading using a tape drive, according to one embodiment. 
         FIGS. 9A-9C  show steps of using flange extenders, according to one embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     The following description is made for the purpose of illustrating the general principles of the present invention and is not meant to limit the inventive concepts claimed herein. Further, particular features described herein can be used in combination with other described features in each of the various possible combinations and permutations. 
     Unless otherwise specifically defined herein, all terms are to be given their broadest possible interpretation including meanings implied from the specification as well as meanings understood by those skilled in the art and/or as defined in dictionaries, treatises, etc. 
     It must also be noted that, as used in the specification and the appended claims, the singular forms “a,” “an” and “the” include plural referents unless otherwise specified. 
     The following description discloses several preferred embodiments of magnetic storage systems, as well as operation and/or component parts thereof. 
     In one general embodiment, a system includes a plurality of tape reels, a tape drive configured for reading data from tape stored on at least one of the plurality of tape reels, a mobile robot having a volume of less than about 1000 cubic inches, the robot configured for selectively retrieving one or more of the plurality of tape reels and transporting the one or more retrieved tape reels to the tape drive, wherein the robot moves unconstrained along a first surface, and a controller for directing movement of the robot. 
     In another general embodiment, a system includes a plurality of tape reels positioned on a lower surface, a tape drive for reading data from a tape on one of the plurality of tape reels, at least one mobile robot configured for selectively retrieving at least one of the plurality of tape reels and transporting the at least one retrieved tape reel to the tape drive, wherein the at least one mobile robot moves unconstrained along an upper surface that overlies the plurality of tape reels, and a controller for directing movement of the at least one mobile robot. 
       FIG. 1  illustrates a simplified tape drive  100  of a tape-based data storage system, which may be employed in the context of the present invention. While one specific implementation of a tape drive is shown in  FIG. 1 , it should be noted that the embodiments described herein may be implemented in the context of a variety of tape drive systems. 
     As shown, a tape supply cartridge  120  and a take-up reel  121  are provided to support a tape  122 . One or more of the reels may form part of a removable cartridge and are not necessarily part of the system  100 . The tape drive, such as that illustrated in  FIG. 1 , may further include drive motor(s) to drive the tape supply cartridge  120  and the take-up reel  121  to move the tape  122  over a tape head  126  of any type. Such head may include an array of readers, writers, or both. 
     Guides  125  guide the tape  122  across the tape head  126 . Such tape head  126  is in turn coupled to a controller assembly  128  via a cable  130 . The controller  128  typically controls head functions such as servo following, writing, reading, etc. The controller may operate under logic known in the art, as well as any logic disclosed herein. The cable  130  may include read/write circuits to transmit data to the head  126  to be recorded on the tape  122  and to receive data read by the head  126  from the tape  122 . An actuator  132  controls the position of the head  126  relative to the tape  122 . 
     An interface  134  may also be provided for communication between the tape drive and a host (integral or external) to send and receive the data and for controlling the operation of the tape drive and communicating the status of the tape drive to the host, all as will be understood by those of skill in the art. 
     As shown by the partial top down view of  FIG. 2  and the cross-sectional view of  FIG. 3 , a system  200 , such as a tape library, may include one or more mobile robots  210  for transporting at least one tape reel  202 , which may or may not be part of a tape cartridge, to and from tape drives  204  for reading data from the tape. The tape on the selected tape reel or pair of tape reels may be accessed by the mobile robots  210 , which may be miniature remote-controlled vehicles that move on a surface (such as an upper surface  302 , used interchangeably with the term “ceiling,” as shown in  FIG. 3 ). Thus, the tape reels may lie on a lower surface, such as a “floor.” while a mobile robot maneuvers on the upper surface unconstrained, e.g., without the use of rails, tracks, pathways, etc., such as through magnetic attraction between the robot and the upper surface or portions of the upper surface (see  FIG. 3 ). In one approach, the tape library may comprise more than one level. On each level, tape reels may be arranged on a lower surface (used interchangeably with the term “floor”) in a dense pattern. This arrangement is shown in partial top down view of  FIG. 2 , according to one embodiment. 
     A contiguous tape may be stored on a single reel, and may include a pin or other end piece that enables threading of the tape in the tape drive. In other approaches, the tape may be coupled to a pair of reels, e.g., in a tape cartridge. 
     According to one embodiment, magnetic tape may be included in miniature tape reels  202  which have only a fraction of the tape length of a standard tape cartridge, thereby decreasing seek time. Such shorter length may be, e.g., less than about ⅕th the tape length of a standard Linear Tape Open (LTO) tape cartridge, less than about 1/25 th  the tape length of a standard tape cartridge, less than about 1/50th the tape length of a standard tape cartridge, etc. In some approaches, a length of tape on the plurality of tape reels  202  may be less than about 50 meters for each tape reel  202 , and may be preferably less than about 25 meters, more preferably 20 meters or less, etc. 
     The seek time of tape-based systems using such tape reels may also or alternatively be decreased by increasing the locate speed, e.g., to about 20 meters per second (m/s) or faster. “Locate time” refers to the time required for the tape to wind to the beginning of a data set (or file) after the tape is loaded in the tape drive. If the tape length is reduced to about 20 m or less and the locate speed is about 20 m/s, the average locate time is (20 m/20 m/s)/2=0.5 s. In addition to decreasing the seek time in the drive, the system seek time may be reduced by structuring the tape library to achieve a mean load time of about 0.5 s. “Load time” refers to the time between the first time when a request reaches the tape library and a second time when the relevant tape reel(s) are loaded in the tape drive. 
     According to one embodiment, the tape system is configured such that the average seek time is less than about 2 seconds, preferably about 1 second or less. The seek time is the delay between a first time when a request to access data is received by the tape library, and a second time when the tape library begins to provide the data to the requester. 
     With continued reference to  FIG. 2 , the tape reels  202  may be closely packed, for example in a hexagonal array, ordered array, circular arrangement, etc., to maximize the number of tape reels that may be stored in a limited amount of space. 
     According to one embodiment, a diameter of each of the plurality of tape reels  202  may be less than about 100 mm, such as less than about 50 mm, less than about 40 mm, less than about 30 mm. 
     In some approaches, a single tape drive  204  is available for reading and/or writing tape on the tape reels  202 . Depending on the frequency with which files are read, a plurality of tape drives  204  may be available for reading and/or writing tape on the tape reels  202 , as shown in  FIG. 2  by the leftmost tape drive  204  reading a tape. The tape drives  204  may be located about the surface in any arrangement as would be known to one of skill in the art, preferably such that quick access to the tape drives  204  is possible to reduce loading and data access times. 
     In one approach, the system  200  may be configured to permit the mobile robot  210  to locate, retrieve, and transport any one selected tape reel  202  to the tape drive  204 , and to initialize reading of the tape by the tape drive  204  within about five seconds of receiving a request to read data from the tape of the selected tape reel  202 , more preferably within about 2 seconds, and ideally less than about 1 second. 
     In several embodiments, several mobile robots  210  move unconstrained along the surface, e.g., two, five, or more. In other embodiments, only a single robot may be present on a given level. It may be advantageous for the robots  210  to be unconnected by cables to any other part of the system  200 . This design favors fast robot motion, and facilitates the robots  210  being capable of moving between levels, rooms, enclosures, etc. Also, when multiple robots  210  are used, any problem with cables getting tangled together from different robots  210  is eliminated when the robots  210  are controlled wirelessly. 
     The mobile robots  210  may have predetermined “resting” locations, as shown by mobile robots  210 , according to one embodiment. As shown in  FIG. 2 , mobile robot  211  has moved from a resting location, acquired tape reels, and is moving toward a tape drive to load the tape reels in the tape drive for reading tape therefrom. Also, mobile robot  208  has retrieved tape reels from a tape drive which has completed a reading operation, and is returning the tape reels to their storage location. Of course, this movement is exemplary only, and not meant to be limiting on the invention in any way. 
     The mobile robots  210  may move in straight line increments, or may move more naturally in arcing patterns between positions on the surface, according to various embodiments. Also, the mobile robots  210  may avoid objects in their path according to any method as would be understood by one of skill in the art upon reading the present descriptions. 
     In some previous tape library geometries, a library gripper accesses an array of tape reels and/or tape cartridges, through motion of a first carriage along a rail or pair of rails. The first carriage in turn holds another set of rails or guides which enables motion of a second carriage holding the gripper. The scheme enables the gripper to access anywhere in two dimensions, where the tape reels or tape cartridges are located. Alternately, in some previous designs, the second carriage has been replaced by a rotary motion which operates about the axes of the first carriage. This scheme limits the flexibility of the library, because grippers (or robots) may interfere with one another. For example, if the grippers use the same set of rails, then they cannot move past each other. Also, there is generally no available space in tape libraries for the installation of an additional set of rails, so this scheme is severely limited in its functionality. In addition, these previous designs do not facilitate easy recovery of access to the tape reels and/or tape cartridges when a gripper fails. 
     On the other hand, the systems herein use “unconstrained” robots, which facilitate access to the tape reels and/or tape cartridges. Here, the term “unconstrained” indicates that movement is not constrained to rails, tracks, guideways, pathways, etc., but instead movement is free in at least two dimensions, e.g., along a surface. Thus, unconstrained mobile robots are easily added or removed from the surface, and they can easily maneuver around each other along the surface, since they are not fixed to a rail or track. 
     As shown in  FIG. 3 , according to one embodiment, the surface across which the mobile robots  210  travel unconstrained may be an upper surface  302  (such as a ceiling in one approach) and may be unpatterned so that the mobile robots  210  are unconstrained, e.g., not limited to motion on particular tracks, paths, rails, etc. Thus each mobile robot  210  is capable of movement independent of movement of any other mobile robot  210 , e.g., each mobile robot  210  may cross the path that any other mobile robot  210  has taken or will take. Similarly, by attaching the mobile robot  210  to a surface  302  separate from a surface  304  supporting the tape reels  202 , the mobile robots  210  are not constrained to follow aisles, paths, corridors, etc., between groups, columns, rows, etc., of tape reels  202 . In one example, this movement may be analogous to the movement of a shopping cart through a supermarket. However, instead of being constrained to moving between the shelves of the supermarket, the mobile robots  210  are capable of moving unconstrained along the ceiling of the supermarket, capable of selecting any desired item from below in the shelves, as an example. The ability to use multiple mobile robots  210  to access a group of tape reels  202  provides faster access of short, popular files. This geometry is much more flexible than previous configurations, in which the tape cartridge grippers were supported on x and y positioners which could not cross paths. The mobile robots  210  may be coupled to the upper surface  302  using magnets  310  or some other coupling or attraction device that biases the mobile robots  210  towards the upper surface  302 . By allowing the mobile robots  210  to maneuver on the upper surface  302 , the tape reels  202  may simply rest on a lower surface  304 , such as a floor. The magnets  310  may bias the mobile robot  210  toward the upper surface  302  with much more force than gravity biases the mobile robot  210  toward the lower surface  304 , allowing a much higher frictional force of the mobile robot wheels  306 ,  308 , thereby enabling faster robot acceleration and thus faster seek times. 
     In one approach, the mobile robot  210  may have three wheels  306 ,  308 : two rear wheels  308  and one front (maneuvering) wheel or ball  306 . In another three-wheel configuration, the mobile robot  210  may have two front wheels  308 , and a rear (maneuvering) wheel or ball  306 . For sake of clarity, a maneuvering wheel in this discussion indicates a wheel whose direction of positioning or rotation with respect to the robot body is not fixed. Of course, the mobile robot  210  may have any number of wheels  306 ,  308  or other apparatus for causing movement of the mobile robot  210  as would be known to one of skill in the art. Steering of the robot may be accomplished in any known manner, such as by independently driving two of the wheels with a caster maneuvering wheel, steering using the maneuvering wheel, and driving and steering with the maneuvering wheel, etc. 
     The mobile robot  210  may have a reel gripper  312 , which when the mobile robot  210  is positioned above a desired tape reel  202  or set of tape reels  202 , may grab, attract (for example, magnetically), secure, or otherwise take hold of the tape reel  202  such that it may be loaded into the mobile robot  210  and transported to a tape drive or back from a tape drive to the tape reel&#39;s storage location. 
     With continued reference to  FIG. 3 , in some embodiments, the mobile robot  210  may have a height such that it may be able to maneuver above the tape reels  202  in a space H of about 1.75″, which measures about 1 U in a standard rack configuration. The smallest units used for rack-mounted computer components are “1 U” units, which are 1.75″ high. Since a reel for standard 0.5″ wide magnetic tape may be as thin as 0.58″, this 1 U unit may be thick enough to hold a single layer of storage having a layer of tape reels  202  plus mobile robots  210 . Thus a single layer design having mobile robots  210 , tapes  202 , and drives may be used for various tape library configurations, ranging from a single layer to multiple stacked layers, which may fill an entire storage room, or any size desired. 
     In one approach, the plurality of tape reels  202  may be spaced from and located within a distance of about 15 cm from the upper surface, e.g., 15±1.5 cm, less than about 20 cm, less than about 10 cm, less than about 5 cm, or any value in the foregoing ranges. 
     The system  200  may also comprise a controller  212  for directing movement of the robot  210 . The controller  212  may be on board the robot  210 , or away from the robot  210  (as shown in  FIG. 2 ) and in communication therewith via any type of communication channel (such as wireless, wired, infrared, etc.). 
     According to one illustrative embodiment, a system  200  comprises at least one tape drive  204  configured for reading data from tape stored on one of a plurality of tape reels  202 , at least one mobile robot  210  having a volume of less than about 1000 cubic inches (and in some approaches less than about 900 in 3 , less than about 750 in 3 , less than about 500 in 3 , less than about 250 in 3 , less than about 100 in 3 , less than about 50 in 3 ) configured for selectively retrieving one or more of the plurality of tape reels  202  and transporting the one or more retrieved tape reels  202  to the tape drive  204 . The mobile robot  210  moves along a surface, and is preferably not mechanically constrained to move along a pre-determined trackway or path (i.e., it is unconstrained). In some approaches, the robot  210  may be mechanically unconstrained and may be able to move autonomously across the surface via any desired path. 
     To assist in navigation of the mobile robot  210 , at least one of the lower and upper surfaces may include an optical pattern usable for navigation of the mobile robot  210 , and the mobile robot  210  may be configured for recognizing the optical pattern. 
     As shown in  FIG. 4A , the lower surface or floor  304 , for example, may include an optical pattern  402  designed for the mobile robot to locate its position. This optical pattern  402  may be a rectangular grid extending over the entire floor  304 , with each grid square  404  labeled with readable code identifying the row and column of the particular grid square  404 . This readable code may take any form as would be understood by one of skill in the art. 
     The floor  304  may be planar, as shown in  FIG. 4B , covered with small indentations as shown in  FIG. 4C , or have a lattice defining receptacle areas in which the tape reels  202  are positioned as shown in  FIG. 4D , according to various embodiments. By extending the lattice up past the tape reels  202 , it may form a surface on which the mobile robot  210  may be supported, and the mobile robots  210  would not maneuver along the upper surface but instead would maneuver on the lower surface formed by the top surface of the lattice, according to one embodiment. 
     The design of the mobile robot  210  may include additional features, abilities, etc., as would be understood by one of skill in the art upon reading the present descriptions. In one embodiment, a mobile robot  210  is shown in  FIGS. 5A-5B , according to one approach. In this example, one or more motors  508  power symmetric drives wheels  308  that allow forward and backward motion, and in a further approach, the drive wheels  308  may provide steering if the drive wheels  308  are operated independently using two symmetric drive motors  508 . These motors  508  may be attached to each wheel  308  by a single step-down gear. The mobile robot  210  may additionally be supported by one or more other omni-directional passive wheels  306 , which may move in any direction, and may be maneuverable (e.g., steerable, positionable, etc.), in some approaches. The omni-directional wheel(s)  306  may be caster wheels, and more preferably may be spherical balls which are also referred to as ball transfers. 
     To pick up the tape reel(s)  202 , a simple platform  312  may be lowered and raised by a solenoid or motor (not shown). If the tape reels  202  are topped by a magnetic plate, the tape reels  202  may be gripped to the platform  312  by energizing electromagnets  502 . One or more cameras  504  may allow for navigation of the mobile robot  210 . A camera  504  may be placed above each reel holding position  506 , enabling the mobile robot  210  to determine its position and to deliver a tape reel  202  directly into a tape drive. The mobile robot  210  need not be made precisely, because the cameras  504  may simultaneously image the tape reels  202  and location grid (or the tape reel chucks on the drive) to precisely position the tape reels  202 , in some approaches. 
     To enable multiple mobile robots  210  to work in the same work space, such as the same floor, there are preferably no cables attached to the mobile robots  210 . The mobile robot  210  uses little power, and a peak speed of about 2 m/s in some embodiments is sufficient to pick up the tape reel  202  and bring it to the drive within a half second. For a 150 gram mobile robot  210 , the corresponding kinetic energy would be about 0.3 Joule. This amount of energy may be supplied by a rechargeable battery, through inductive coupling, etc., but a capacitor, with its extremely long lifetime, may be preferable. In one embodiment, an 80 volt, 1000 μF electrolytic capacitor which includes 3.2 Joules, yet is only 16 mm diameter by 40 mm long may be used. The mobile robot  210  may be recharged at its parking position, or at the tape drive when it loads the tape reel  202 . 
     As shown in  FIG. 5B , according to a preferred embodiment, the tape reels  202  are positioned on a floor  304 , the surface  302  is opposite the floor  304 , and the mobile robot  210  is biased toward the surface  302 , such as through magnetic biasing (by using magnets  310 ), thereby suspending the robot  210  above the tape reels  202 . For example, the mobile robot  210  may be magnetically biased toward the upper surface  302 . 
     As shown in  FIGS. 6A-6B , a library controller may communicate with the mobile robots  210  by light, such as infrared (IR); radio frequency (RF); etc., and may be differentiated on each level of the library  200  to avoid cross-talk. The library controller computes a path the mobile robot  210  is to take to pick up and drop off the tape reels and load the tape reels in the tape drives  204 . The mobile robot  210  may servo along the path by using its cameras, in one approach, or it may use encoders on its motors or wheels to servo, using the cameras only for fine adjustments, in another embodiment. 
     In one embodiment, the tape library  200  may comprise a plurality of tape drives  204 , where each tape drive  204  is positioned on the floor supporting the tape reels, the floor being below the surface on which the mobile robots  210  maneuver by a distance sufficient to allow movement of the mobile robots  210  therebetween (between the tape reels and the surface). 
     In large multilayer libraries  200 , mobile robots  210  may move between floors using ramps  604 . This enables the mobile robots  210  themselves to load different floors with tape reels, and to rebalance the work load by optimally locating the mobile robots  210  and organizing the tape reels. The relative number of mobile robots  210 , tape reels, and tape drives  204  may be determined by the access frequency of the files, data, etc. 
     According to one approach, multiple surfaces having a spaced and stacked configuration may be used, and the mobile robot  210  is configured to travel between the multiple surfaces. In this approach, at least one ramp  604  may be provided, connecting at least two of the multiple surfaces, thereby permitting the mobile robot  210  to travel between the multiple surfaces. In these or any other approaches, there may be no cable coupled between the mobile robot  210  and any other component of the system. 
     As illustrated in  FIG. 6B , according to one embodiment, one or more tape drives  204  may be secured directly to the floors  606  of a tape library  200  having one or more levels. In addition, the mobile robots  210  may be attracted to the ceilings  602 . In this design, the drive(s)  204  may be easily positioned at any location in the tape library  200  by simply leaving space in the array of reels (not shown). This arrangement facilitates reconfiguration of the tape library  200  after it has been manufactured, along with replacement of failed tape drives should they occur. For example, the mobile robots  210  may be adapted for moving and/or relocating a tape drive  204 . At least one tape drive  204  may be positioned on each level of the tape library  200 , according to one embodiment. 
     The library can be configured to have “spare” tape drives  204  and mobile robots  210  ready to be put into use upon failure of other tape drives and mobile robots, according to one embodiment. In another embodiment, when extra tape drives  204  or mobile robots  210  are to be used, due to some factor, such as an increased work load, failed mobile robots and/or tape drives, etc., the tape library  200  may allow adding or removing tape drives, tape reels, and/or mobile robots by the user after manufacture of the tape library  200 . 
     Any type of tape threading system known in the art may be adapted for use with the systems described herein, according to various embodiments. For example, if tape reels are used individually (not as pairs), a standard threader mechanism may be used to thread the tape reel onto the tape drive, according to one embodiment. Storing the tape on pairs of reels has an advantage in that after loading, the tape is ready to be used and does not need to be wound onto another inboard reel. Also, when the tape is finished being used, both reels may be removed with the tape, and the tape does not need to be unwound from the inboard reel in order to be removed. 
     In another embodiment, as shown in  FIG. 7A , the tape drive  204  itself may have no threader. Rather, the mobile robot  210  may be configured to thread tape  706  of the retrieved tape reel  702  onto the tape drive  204 . In one approach, a mobile robot  210  loads the tape  706  provided on a pair of tape reels  704 ,  704 , by first dropping off one reel  702  on the drive  204 , and then the other reel  704  on a reel chuck  708 , for which a reel motor may wind the tape  706 . Alternately, if the tape  706  is stored on one reel only (such as reel  702 ), the mobile robot  210  may move an end of the tape not on the reel  702  to secure it to the inboard drive wheel, as described in relation to  FIGS. 7B-7D . The precision of the mobile robot  210  motion may be much greater than that of a leader pin loader mechanism currently used in tape drives, in some approaches. 
     In another embodiment, as shown in  FIGS. 7B-7D , a single reel  702  of tape  706  may be loaded onto a reel chuck and motor  714  which are fixed to a carriage  716  that is adapted to follow a guide  718  that allows for motion of the chuck  714  around the tape drive  204  to facilitate threading of the tape  706 . Before loading the reel  702  on the chuck  714 , the chuck  714  is brought into proximity of the inboard (take-up) reel  710 , as shown in  FIG. 7B . After the mobile robot (not shown for clarity) places the reel  702  on the chuck  714 , a mechanism  712  attaches the end of the tape  706  to the take-up reel  710 . See  FIG. 7C . At this point, the tape  706  may be transferred between reels  702  and  710  as part of a preliminary locate operation, before the tape  706  contacts the recording head  720  or any guide surfaces, such as rollers, thus enabling faster tape locate with reduced tape damage and drive wear. Then, the chuck and reel motor  714  and tape reel  702  are moved along the guide  718  to a final position, as shown in  FIG. 7D , where the tape  706  may be read from or written to. 
     In another embodiment, rather than moving the library reel  702  and carriage  716 , the take-up reel  710  and its motor may be moved to thread the drive  204 . In yet another embodiment, if the tape  706  is stored on a pair of reels, the mobile robot may position the two reels directly on the tape drive  204 , where the tape  706  may be located before threading. 
     Alternatively, as shown in  FIGS. 8A-8D , the tape  706  may be held on two tape reels  202 , and a method may be used which avoids the unreliability of a leader pin. In this approach, the reel drive chucks  802  may be spaced with the same close spacing of the tape reels  202  in their storage position. The mobile robot places the tape reels  202  on the chucks  802 , as shown in  FIG. 8B , and two moveable rollers  804 ,  806  sequentially thread the tape  706  in position. First, roller  804  threads the tape  706 , as shown in  FIG. 8C , and then roller  806  threads the tape  706 , as shown in  FIG. 8D . The tape  706  may locate before the rollers  804 ,  806  move into their final position, thus avoiding the extra wear and control difficulties associated with contact of the tape  706  with the roller  806  or other guide surfaces and the head. Alternately, two smooth cylinders which form air bearings may be used to thread the tape drive, in another embodiment. 
     In one embodiment, each reel may be large enough (e.g., the flanges are big enough) to hold the entire length of tape, if all the tape is to be accessed. If the tape is stored in pairs of these large reels, the capacity of the library may be reduced by a factor of two from what it may be by using single reels. 
     In another approach, the tape may be stored on a pair of reels, where at least one reel is not large enough to accept the entire length of tape. As shown in  FIGS. 9A-9C , a reel-size reduction may be effected by incorporating temporary flange extenders  902 ,  904  in the tape drive. In the tape drive, a temporary flange extender  902  is fixed to the motor/reel chuck  908  so that the reel flange  906  is extended when it is on the drive. Once the reels  202  are in the drive, another flange extender  904  is placed on the top of each of the reels  202 , so that tape can be spooled onto one of the reels. Before the tape reels  202  are unloaded from the tape drive, the tape is spooled back evenly between the tape reels  202 , so that it lies only between the permanent flanges  906  on the tape reels  202 . 
     In another approach, a packing roller may be used to keep the tape on the reel after its diameter becomes bigger than the flange. 
     In some approaches, the tape library may be fault tolerant and/or self repairing. For example, each tape cartridge may be read by a number of different drives, if a robot fails then other robots may maneuver around the failed robot, a failed robot may be towed away by another robot and replaced, and/or a failed tape drive may be towed away and replaced. 
     According to one embodiment, if a mobile robot fails, it may be towed away and replaced by another mobile robot. The tape drives may be configured so that a mobile robot may easily tow away and replace a tape drive, after temporarily clearing away the tape reels that may be in the path taken to remove the tape drive. The power and data connections may be made to use very little force, for example, using simple sliding spring contacts for the power and optical link for the tape drive data transfer. 
     According to one embodiment, the system may be configured to allow removal of a failed mobile robot using another mobile robot, in which case the failed mobile robot may then be replaced with the mobile robot performing the removing or another mobile robot, in some approaches. 
     In another embodiment, the mobile robot may be configured to remove the tape drive, e.g., if the tape drive is broken. Likewise, the robot may install a replacement or repaired tape drive. 
     It will be clear that the various features of the methodologies and embodiments described herein may be combined in any way, creating a plurality of combinations from the descriptions presented herein. 
     Communications components such as input/output or I/O devices (including but not limited to keyboards, displays, pointing devices, etc.) may be coupled to the system either directly or through intervening I/O controllers. 
     Communications components such as buses, interfaces, network adapters, etc., may also be coupled to the system to enable the data processing system, e.g., host, to become coupled to other data processing systems, remote printers, storage devices, etc., through intervening private or public networks. Modems, cable modems, and Ethernet cards are just a few of the currently available types of network adapters that maybe used, in some approaches. 
     It will be further appreciated that embodiments described herein may be provided in the form of a service deployed on behalf of a customer to offer service on demand. 
     While various embodiments have been described above, it should be understood that they have been presented by way of example only, and not limitation. Thus, the breadth and scope of an embodiment of the present invention should not be limited by any of the above-described exemplary embodiments, but should be defined only in accordance with the following claims and their equivalents.