BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to techniques for digitally storing data. More particularly, the invention concerns a particularly efficient storage system for making redundant copies of data on tape by waiting until a rewind/unload command is received and then copying stored data objects en masse, thus benefitting from any data compression used to store the objects along with the lower overhead for the bulk copy operation. 
     2. Description of the Related Art 
     Many data processing systems require a large amount of data storage space, 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 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. Tape storage is often used 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. 
     Having a backup data copy is mandatory for many businesses for which data loss would be catastrophic. The time required to recover lost data is also an important recovery consideration. In this respect, tape or library backup has been found useful for periodically backing up primary data by making a copy of the primary data. In contrast to the serial access of tape storage, DASD backup storage is random access and therefore facilitates more advanced backup techniques. One example is “dual copy,” which operates by mirroring contents of a primary DASD device with a nearly identical secondary DASD device. An example of dual copy involves providing additional DASDs so that data is written to the additional DASDs substantially in real time along with the primary DASDs. Then, if the primary DASDs fail, the secondary DASDs can be used to provide otherwise lost data. A drawback to this approach is that the number of required DASDs is doubled. 
     A different data backup alternative that avoids the need to provide double the storage devices involves writing data to a redundant array of inexpensive devices (RAID). In this configuration, the data is apportioned among many DASDs. If a single DASD fails, then the lost data can be recovered by applying error correction procedures to the remaining data. Several different RAID configurations are available. 
     A number of other solutions have been developed for DASD backups, including synchronous remote copy, asynchronous remote dual copy, peer-to-peer remote copy, and other solutions, many having been developed by International Business Machines Corp. (IBM). 
     DASD backup solutions have been more thoroughly developed than tape backup solutions. Due to ability to randomly access DASD media, DASD offers more reliable storage, faster commit times, and more flexibility in crafting backup strategies. Still, tape backup remains popular because it is cost effective. According to one approach, when a system receives new data for storage, it concurrently copies the data to separate tapes using tape drives operating in parallel. Under still another approach, when the system receives new data for storage, it first copies the data using tape drive, and then begins to copy the data from the first tape drive to another tape at a second tape drive. 
     Although known tape backup strategies enjoy widespread use today, engineers at IBM are continually seeking to improve the performance and efficiency of tape backup systems. One area of possible focus is reducing the time required to complete tape backups. 
     SUMMARY OF THE INVENTION 
     Broadly, the present invention concerns a particularly efficient storage system for making redundant copies of data on tape media. The invention is implemented in a tape storage system that includes a tape director coupled to one or more tape drives, the entire tape storage system being coupled to a host. In response to a write command, the invention automatically makes redundant copies of the data, this process being invisible to the host. 
     Initially, the host sends the tape director one or more data objects, such as logical “volumes” of data. The host also sends corresponding instructions to write the data objects to tape media. In response, the tape director stores the data objects using a first tape drive, and then transmits a message to the host signaling successful completion of the storage. The tape director waits until receiving a rewind/unload command from the host to create a backup copy of the data objects. At this time, the tape director creates the backup copy by copying all data from the first tape drive to a second tape drive, regardless of the data objects&#39; boundaries. This copy operation, called “bulk copy,” is more efficient than the data objects&#39; original storage, because there is less overhead in copying the data objects en masse, and there is actually less data to copy if the data objects were stored with any data compression. After copying the data objects, the tape director completes the host request by instructing the tape drive to rewind and unload the tape containing the original data objects. 
     The invention also contemplates a “dual tape” modification to this technique, where the tape directory stores some data objects using one tape drive, while storing other data objects using another tape drive. When the host sends the tape director its rewind/unload command, the tape director responds by directing the tape drives to exchange their respective data objects, thereby creating the backup copy. Namely, the tape director operates the tape drives to all data objects from the backup set of tapes to the primary set of tapes, and to copy all data objects from the primary set of tapes to the backup set of tapes. After copying these data objects, the tape director commands the tape drives to rewind the primary and backup tapes and unload the tapes from their respective tape drives. 
     In one embodiment, the invention may be implemented to provide a method to create redundant data copies on tape media. In another embodiment, the invention may be implemented to provide an apparatus, such as a tape storage system, programmed or otherwise configured to create redundant copies of data on tape by waiting until a rewind/unload command is received and then copying stored data objects en masse, thus taking advantage of any data compression used to store the objects and also the lower overhead for the copy operation. 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 direct components of a tape storage system to create redundant data copies on tape media as mentioned above. 
     The invention affords its users with a number of distinct advantages. First, in contrast to previous techniques, the tape backup process of this invention finishes more quickly. This is because the tape director delays creation of the backup copy until the host issues a “rewind-and-unload” command. This delayed backup completes more quickly because it takes advantage of any data compression used to store the objects, and also benefits from the bulk copy operation&#39;s low overhead. Overhead is low because the bulk copy operation involves transferring a large block of information, rather than many smaller data objects as such. 
     Furthermore, the present invention does not incur any additional delay that is detectable by the host. This is because most conventional backup systems do not commit a backup copy until the rewind-and-unload operation succeeds, even though the conventional backup has been performed previously. The invention may be implemented to gain further advantage by dividing the initial storage of data objects among paired tape drives, and then cooperatively operating the tape drives to exchange data upon the rewind/unload command. The tape drives, operating in parallel, operate twice as fast. As another advantage, the invention permits earlier recovery than prior tape backup techniques; the data is available upon completion of the first copy, even if the backup copy has not been made. 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 tape backup 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 in accordance with the invention. 
     FIG. 4 is a flowchart of an expedited tape backup sequence according to the invention. 
     FIGS. 5A-5B are block diagrams showing a conventional tape storage system in various stages of performing a conventional data backup. 
     FIGS. 6A-6C are block diagrams showing a tape storage system in various stages of backing up data 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. As mentioned above, the invention concerns a tape backup system configured to perform a particularly expedited tape backup. 
     Hardware Components &amp; Interconnections 
     Introduction 
     One aspect of the invention concerns a tape storage system, which may be embodied by various hardware components and interconnections as shown by the arrangement  100  of FIG.  1 . Generally, the tape storage system  104  stores and retrieves data to/from tape media in accordance with commands received from a host  102 . 
     Host 
     The host  102  manages the system  104  by transmitting commands to the system  104  to store and retrieve data therefrom. The host  102  may be implemented in a variety of different ways, such as one or more mainframe computers, computer workstations, personal computers, or even a human operator that interacts with the tape storage system  104  by an interface such as a keyboard and display monitor. To provide some particular examples, the host  102  may be implemented by an IBM System/390 computer utilizing the MVS operating system, an IBM model RS 6000 machine, IBM model AS 400 computer, IBM 3090 mainframe computer, or another suitable digital data processing machine. 
     Tape Storage System 
     The tape storage system  104  includes a tape director  106 , one or more tape drives (exemplified by the two illustrated tape drives  108 ,  110 ), and corresponding tape media (exemplified by the two illustrated tapes  109 ,  111 ). The system  104  may be implemented by interconnecting components as shown, or by designating part of a larger tape library for use as discussed herein. The tape director  106  comprises a digital computer that manages the tape drives  108 ,  110  according to commands from the host  102 . As an example, the tape director  106  may be implemented by a tape system controller from an IBM model 3590 tape storage system. 
     The tape drives  108 ,  110  comprise mechanisms for exchanging machine-readable data with the tapes  109 ,  111 . The tape drives  108 ,  110  may be implemented by tape drives of the IBM model 3590 tape storage system, as an example. Although magnetic tapes are shown by example, the tapes  109 ,  110  may be implemented by any serially accessible, removable data storage media, whether implemented in “tape” format or not. One example of suitable tapes includes IBM 3590 tape cartridges. The tapes  109 ,  110  are loaded into the tape drives  108 ,  110  for access by the tape drives, and may be unloaded or “ejected” from the tape drives for the purpose of storing the tapes and freeing the tape drives to access other tapes. 
     Digital Data Processing Apparatus 
     As mentioned above, the tape director  106  is implemented by a digital computer. In this respect, another aspect of the invention concerns such a digital data processing apparatus programmed or otherwise configured to implement the tape director  106 . 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 implements the tape director  106  with logic circuitry instead of computer-executed instructions. 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 using 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. 
     Operation 
     In addition to the various hardware embodiments described above, a different aspect of the invention concerns a particularly efficient storage system for making redundant copies of data on tape media. This is achieved by waiting until a rewind/unload command is received and then copying stored data objects en masse, thus taking advantage of any data compression used to store the objects and also the lower overhead for the bulk copy operation. 
     Signal-Bearing Media 
     In the context of FIGS. 1-2, such a method may be implemented, for example, by operating the tape director  106 , as embodied by a digital data processing apparatus  200 , to execute a sequence 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 make redundant copies of data on tape by waiting until a rewind/unload command is received and then copying stored data objects en masse, thus taking advantage of any data compression used to store the objects and also the lower overhead for the bulk copy operation. 
     This signal-bearing media may comprise, for example, RAM (not shown) contained within the storage  204 , as represented by the fast-access storage  206  for example. 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  204 , 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 direct access storage device (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. 
     Overall Sequence of Operation 
     Introduction 
     FIG. 4 shows a sequence  400  to illustrate one example of the method aspect of the present invention. For ease of explanation, but without any intended limitation, the example of FIG. 4 is described in the context of the tape storage system  104  described above. 
     Receiving Data Objects &amp; Write Instruction 
     After the steps  400  are initiated in step  402 , the tape director  106  receives data objects and one or more corresponding write instructions from the host  102  in step  404 . The host&#39;s write instructions comprise commands to store the data objects on a single tape. A “data object” may comprise any convenient unit of data, such as a logical volume, file, record, page, byte, block, etc. 
     Writing Data Objects 
     In step  406  the tape director  106  instructs the tape drive  108  to store the data object(s) received in step  404 . Although the tape drive  108  is used as one example, the tape drive  110  may be used instead. Optionally, the tape director  106  may apply a data compression technique when storing the data objects to occupy less space on the tape  109 . One exemplary data compression technique is “LZ1.” When the write operation completes, the tape director  106  transmits a suitable message, such as “write successful” to the host  102 . 
     Dual Tape 
     As explained below, one alternative to step  406  is to perform “dual tape” storage. This operation pairs data objects provided by the host for storage  102 , and saves time by concurrently storing the data objects using separate tape drives. The tape director  106  may pair two data objects according to order of receipt, logical relationship of the underlying data, characteristics of the data, etc. 
     With dual tape storage, step  406  involves the tape director  106  instructing the tape drives  108 ,  110  to concurrently store different data objects to the tapes  109 ,  110 . Each of the two tape drives  108 ,  110  stores one data object. When both tape drives&#39; write operations succeed, the tape director  106  transmits a suitable message, such as “write successful” to the host  102 . Since dual tape storage is performed in spite of the host&#39;s command to store the data objects together on the same tape medium, it is therefore conducted so as to be invisible to the host  102 . 
     Rewind/Unload 
     After writing the data object(s), the tape director  106  determines whether it has received a “rewind-and-unload” command from the host  102  (step  408 ). This command, which may also be referred to by other names, is a host command for the system  104  to rewind the presently loaded tape medium. In the absence of a rewind-and-unload command, the system  104  proceeds to receive more data (step  419 ) and write the data objects (step  406 ) to the currently loaded tape(s)  109  and/or  110 . 
     Although not illustrated, the system  104  may receive data objects (step  419 ) asynchronously relative to the writing of data objects step (step  406 ). For example, data objects may be received at a faster rate than they are written out to tape. In this event, data objects are received and accumulated in a queue. 
     If the optional dual tape storage feature is implemented, the tape director  106  limits the amount of data written to each tape to half of the tape&#39;s capacity. As the tapes  109 ,  111  fill with data in parallel, room must be reserved to copy the other tape&#39;s contents and thereby form a backup copy, as discussed in step  410  below. When the tapes  109 ,  111  approach half-full, the tape director  106  may send an appropriate “tape full” message to the host  102 , from which the host  102  may eventually respond by issuing a rewind-and-unload command. Alternatively, the host  102  may issue status queries to the tape director  106  to monitor the progress of filling the tapes, and preemptively issue the rewind-and-unload command at the appropriate time. 
     High Efficiency Data Exchange—Single Tape 
     Step  408  determines whether the host has issued a rewind-and-unload command. When the host  102  finally does send the rewind-and-unload command, the tape director  106  does not immediately respond by rewinding and unloading the tape because the backup copy has not been created yet. Instead, the tape director in step  410  initiates a “bulk copy” operation to produce a high efficiency data exchange. The bulk copy operation, which may involve a single command issued to the tape drive  108 , results in copying of the data objects en masse. Generally, the bulk copy operation treats the stored data objects as a single, larger data object. Therefore, there is less overhead incurred by copying the data objects together than the process of copying them individually, as was performed in step  406 . 
     More particularly, the bulk copy operation (step  410 ) involves the following sub steps. First, the tape director  106  copies all of the data on the current tape  109  into volatile memory accessible to the tape director  106 . This step effectively groups many smaller data objects on the tape  109  to form a single, large data object. The tape director  106  ignores “records” or other data boundaries utilized by the host  102 . Instead, the tape director  106  copies all of the tape&#39;s individual data units (such as “logical blocks” or other units), regardless of whether they contain metadata utilized by the tape director  106 , metadata compatible with the tape drive  108 , or customer data. The copying of data in this manner also takes advantage of any compression that has been applied to stored customer data, since this data occupies less space and is therefore faster to copy. After the contents of the tape  109  are copied into the tape directors volatile memory, the tape director  106  directs the drive  110  to write the data back out “en masse” to the tape  111 . If desired, the tape director  106  may start writing to the tape  111  before reading all data from the tape  109 , in effect buffering and spooling the data transfer rather than waiting until all data has arrived at the tape director  106 . Furthermore, the data may be written out instead to a different tape (not shown) coupled to the drive  108 . The operation of writing this data enjoys particular efficiency because underlying data is written free from any constraints of records or other host-compatible data boundaries. 
     High Efficiency Data Exchange—Dual Tape 
     In the dual tape storage embodiment, step  410  involves each tape drive  108 ,  110  cooperatively copying its data objects to the other tape drive&#39;s tape medium. Since the tape drives cannot concurrently read and write, this is a sequential process. One tape drive reads while the other writes, and then vice versa. Respective reading and writing operations among the tape drives may be interleaved if desired. 
     This process is highly efficient for various reasons. Unlike data received from the host  102  for storage in the system  104 , the stored data objects are already configured in tape format, and therefore expeditiously copied from one tape drive to the other. Additionally, the data objects may be stored with some compression, further boosting the data transfer efficiency. Thus, the copying of stored data objects from one tape drive to another is conducted with the highest efficiency of which the system  104  is capable. 
     The performance of step  410  is invisible to the host  102 . From the host perspective, the host sends the rewind-and-unload command and then waits for a message from the tape director  106  signaling successful completion of the operation. 
     Rewind &amp; Unload 
     Following step  410 , the tape director  106  directs the tape drives  108 ,  110  to satisfy the host&#39;s rewind/unload command (step  408 ) by performing a rewind-and-unload operation (step  412 ). In response, the tape drives rewind and then eject their respective tapes. The tape drives may also place the ejected tapes in a designated place, such as a transient tape bin or slot, input/output slot, pass-through slot, etc. As an example, the tape director  106  may wait until both tape drives&#39; rewind-and-unload operations are complete, and then send the host  102  a message to that effect. Having finished step  412 , the tape storage system  104  then proceeds to mount new any tapes (step  414 ), and the process begins anew (step  404 ). 
     Comparison by Example 
     To more thoroughly explain the operation of this invention, a specific comparative example is provided below in the context of “dual tape” storage. This example is given to compare a conventional tape backup with the backup of this invention. In this example, the tape system receives two data objects of 2400 Mb size, and an accompanying write instruction. While storing data, the tape drives inherently apply 3:1 compression. Thus, when each 2400 Mb data object is stored on tape, it occupies 800 Mb. Each tape drive stores data at the rate of 10 Mb/second. 
     Conventional Backup 
     The conventional backup sequence is diagrammatically shown by FIGS. 5A-5B. In FIG. 5A, the tape director  502  directs the tape drive  504  to store the first data object to a tape  508 , and the tape drive  506  to store the second data object to its tape  510 . The tape drives can operate in parallel, so this operation takes 240 seconds (i.e., 2400 Mb data object written at 10 Mb/second). 
     Next, in FIG. 5B, the tape director  502  directs the tape drive  504  to store the second data object on the tape  508 , while the tape drive  506  stores the first data object on the tape  510 . As with the operation of FIG. 5A, this takes another 240 seconds. The overall time of completing the operations of FIGS. 5A-5B is 480 seconds. 
     Efficient Backup According to This Invention 
     The backup sequence of the present invention is shown by FIGS. 6A-6C. In FIG. 6A, the tape director  602  directs the tape drive  604  to store the first data object on its tape  608 , and the tape drive  606  to store the second data object on its tape  610 . Since the tape drives can operate in parallel, so this operation takes 240 seconds (i.e., 2400 Mb data object written at 10 Mb/second). its tape  606 , and the tape drive  606  to store the second data object on its tape  610 . Since the tape drives can operate in parallel, so this operation takes 240 seconds (i.e., 2400 Mb data object written at 10 Mb/second). 
     Next, in FIG. 6B, the tape director  602  directs the tape drives  604 ,  606  to cooperatively copy the first data object from the tape  608  to the tape  610 . Since the first data object was compressed to 800 Mb when initially stored by the tape drive  604 , the operation of FIG. 6B takes 80 seconds (i.e., 800 Mb data object copied at 10 Mb/second). 
     Next, in FIG. 6C, the tape director  602  directs the tape drives  604 ,  606  to cooperatively copy the second data object from the tape  610  to the tape  608 . This operation, like that of FIG. 6B, requires 80 seconds. The overall time of completing the operations of FIGS. 6A-6C is  400  seconds, an improvement of twenty percent over the 480 seconds required by the conventional technique of FIGS. 5A-5B. 
     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.
A particularly efficient storage system makes redundant copies of data on tape by waiting until a rewind/unload command is received and then copying stored data objects en masse, thereby benefitting from any data compression used to store the objects along with the lower overhead for the copy operation. This copy operation is more efficient than the data objects' original storage, because there is less overhead in copying the data objects en masse; also, there is less data to copy if the data objects were stored with any data compression. Efficiency may be further increased by dividing the task of originally writing the data objects between paired tape drives, and then exchanging data objects between the tape drives upon issuance of the rewind/unload command.