Patent Publication Number: US-8996823-B2

Title: Parallel access virtual tape library and drives

Description:
CROSS-REFERENCE TO RELATED APPLICATION(S) 
     This application is a divisional of U.S. patent application Ser. No. 12/202,916, filed Sep. 2, 2008, which claims the benefit of the assignee&#39;s U.S. Provisional Patent Application Nos. 60/969,103 and 60/969,105, both filed Aug. 30, 2007, respectively, each of which is incorporated herein by reference in its entirety. 
    
    
     BACKGROUND 
     Virtual tape libraries (VTL) transform disk drives into virtual tape to improve backup/restore times, as well as to provide other benefits. A VTL operates by providing a layer or interface between a host or client computer and a disk drive system. This VTL layer acts like one or more tape drives with one or more tapes or pieces of media and mimics commands to and from the client computer to make the client believe that it is communicating with tape drives. Instead, however, the VTL actually exchanges data with one or more disk drives. 
     A Jan. 3, 2007 article by B. Pariseau in ComputerWeekly.com, entitled, “Storage Outlook &#39;07: In Search of Better Data Management,” indicates that some consumers prefer tape and VTL because drives capable of up to 4 to 1 compression save storage resources. According to the article,
         The VTL improves upon some of the traditional limitations with physical tape libraries. With VTLs [one] can create hundreds of virtual tape drives. Each server can have its own dedicated drives, reducing the complexity of shared resources. The SCSI tape protocol has some downsides shared by both VTL and physical tape . . . . Neither enables concurrent reads from the same volume by two separate hosts. This means tape replication for [data recovery] and restores cannot happen at the same time.       

     Tape by its nature is sequential and thus does not permit simultaneous read operations from different locations on a given tape. Disk drives, however, do permit simultaneous read and write commands. But, as noted above, a VTL forces the disk drive to operate like a tape drive and thus does not permit concurrent reads, or other capabilities incompatible with tape. Many limitations of VTLs exist because of a lack of disk management solutions or limitations in backup software applications that employ VTLs. For example, when a write command is provided to a drive, the VTL reserves that drive and no other commands can be performed. 
     The need exists for a system that overcomes the above problems, as well as providing additional benefits. Overall, the examples herein describe some prior or related systems, and their associated limitations are intended to be illustrative and not exclusive. Other limitations related to existing or prior systems will become apparent to those of skill in the art upon reading the following Detailed Description. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a block diagram illustrating an example of a VTL or parallel access virtual tape library and drives system. 
         FIG. 2A  is a block diagram illustrating an alternate embodiment of the system shown in  FIG. 1 . 
         FIG. 2B  is a block diagram illustrating components of an API or sequential SCSI-compliant layer shown in  FIG. 2A . 
         FIG. 3  is a flow diagram illustrating an example of a routine for performing parallel access virtualization. 
         FIG. 4  is a virtual tape/disk table for use by the components of  FIG. 2A . 
         FIG. 5  is a block diagram illustrating components of a data stream that may be used in a suitable data storage system. 
         FIG. 6  is a block diagram illustrating an example of a data storage system. 
         FIG. 7  is block diagram illustrating an example of components of a server used in data storage operations. 
     
    
    
     In the drawings, the same reference numbers and acronyms identify elements or acts with the same or similar functionality for ease of understanding and convenience. To easily identify the discussion of any particular element or act, the most significant digit or digits in a reference number generally refers to the Figure number in which that element is first introduced (e.g., element  310  is first introduced and discussed with respect to  FIG. 3 ). 
     DETAILED DESCRIPTION 
     Various examples of the invention will now be described. The following description provides specific details for a thorough understanding and enabling description of these examples. One skilled in the art will understand, however, that the system may be practiced without many of these details. Additionally, some well-known structures or functions may not be shown or described in detail, so as to avoid unnecessarily obscuring the relevant description of the various examples. 
     The terminology used in the description presented below is intended to be interpreted in its broadest reasonable manner, even though it is being used in conjunction with a detailed description of certain specific examples of the system. Certain terms may even be emphasized below; however, any terminology intended to be interpreted in any restricted manner will be overtly and specifically defined as such in this Detailed Description. 
     Described in detail herein is a system that allows a VTL to perform multiple simultaneous or parallel read/write or access sessions with disk drives or other storage media, particularly when subject to a sequential SCSI-compliant layer or the traditional limitations of VTLs. In one embodiment described in detail herein, a virtualizing or transaction layer can establish multiple sessions with one or more clients to concurrently satisfy the read/write requests of those clients for physical storage resources. A table or other data structure then tracks or maps the sessions associated with each client and the location of data on the physical storage devices. The table may also identify virtual drives, virtual media, or other information, such as load data, content data, etc. 
     Referring to  FIG. 1 , an example of a parallel access virtual tape library and drive system  100  is shown. One or more client computers or hosts  102  communicate with a virtual tape or device library  103  that includes one or more disk drives  105 . As explained in more detail below, the VTL  103  looks like one or more tape drives to the client, but in fact the VTL  103  employs disk drives and/or other non-sequential storage media. Indeed, the VTL  103  operates like existing virtual tape libraries, except as explained herein. 
     The VTL  103  causes a tape drive to show up at the operating system level of each client computer. The VTL  103  exposes multiple handles, addresses, or other APIs to the operating systems of each client computer, so that any application on those clients can use the (virtual) tape drive to perform any desired functions, including read or write commands. In this example, the VTL  103  employs one or more disk drives that effectively have a tape command front end that accepts commands through standard protocols, such as standard sequential SCSI protocols. The VTL  103  may employ standard file system-type disk management, such as Dynamic Disk-like functionality under Microsoft® Windows®, to dynamically grow or shrink allocated or available disk space for each session under this example (shown as file system  107 ). 
     Referring to  FIG. 2A , an example of an alternative to the VTL  103  embodiment of  FIG. 1  is shown as having an API or sequential SCSI-compliant layer  104  that communicates with both the clients and a lower layer  106 , which includes a media agent/disk subsystem  108 . The media agent/disk subsystem  108  may communicate with read and write processes  110  that receive and pass to drivers  112  read/write commands initiated by the clients. Data associated with these read/write commands may be cached in an optional intermediate cache  114 , or may flow directly to one or more tape drives  116 , one or more disk drives  118 , one or more optical drives  120  (collectively, drives  116 ,  118 ,  120 ), or other data storage media and devices not shown. To the client computers  102 , the system appears as mounted tape, but instead the media agent/disk subsystem  108  operates routines that receive blocks of data and store and manage them with respect to any form of secondary storage. This permits, for example, client 1 to write commands to a virtual tape drive at the same time that client 2 is reading from it. 
     The lower layer  106  operates, as known by those skilled in the relevant art, to implement communications and functions with data storage devices so as to satisfy read, write, or other data storage commands. Media agents may be employed to execute necessary subroutines for accessing appropriate drives, loading media, performing any necessary data manipulation (e.g., encryption/decryption, compression/decompression, error correction, de-duplication (often involving cryptographic hashing), etc.). The disk subsystem may include an associated file management system (not shown) that can include one or more file access tables (FAT) or other data structures for tracking locations of data stored on particular data storage drives  116 ,  118 ,  120 . Note that the system may dynamically change the allocation of blocks of data on each drive  105 ,  116 ,  118 , and/or  120 , to permit greater flexibility and improve performance of the system. Note also that when multiple commands for the same tape, i.e., tape drive  116 , are received, they may be stored temporarily in cache  114 . The lower layer  106  can, for example, provide multiple concurrent lines of data flow to or from one or more disk drives  118 . Note that the lower layer  106  provides access to multiple data storage media types, e.g., drives  116 ,  118 ,  120 , even though the system receives tape drive commands from a client or host computer. 
     The media agent/disk subsystem  108  may communicate with other systems  113  so that data or commands from the clients receive further processing to provide further benefits. Such other operations can include snapshots, single instancing/de-duplication, data classification, load balancing, or other data management and data storage techniques, some of which are described in detail herein. 
     Referring to  FIG. 2B , layer  104  includes a client interface  202  that receives read/write commands from the clients and passes them on to other components. For example, a session mapper  204  receives the read/write commands and responds by associating a session handle with each read/write request. The client interface  202  then passes the session handle to the appropriate client. The session mapper  204  also builds one or more data object mapping tables  206  to track read/write commands with session handles and physical locations of data written to and read from one or more drives or storage devices  105 ,  116 ,  118 , and/or  120  (as described above with reference to  FIGS. 1-2A ). A data store interface  208  interfaces with these drives to implement the read/write commands. 
     The client interface  202  exposes a sequential SCSI interface (e.g., tape interface or API), or similar sequential media interface to any application running on the client computers. The client interface  202  thus accepts read/write commands like a typical tape drive. The data store interface  208  then passes standard VTL commands to any secondary storage device (via lower layer  106 ). Layer  104  of  FIG. 2B  acts as a standard VTL, but receive commands are based on sessions created and associated with received client commands via session mapper  204 . In other words, an SCSI interface is provided on top of a file input/output handler. 
     Under an alternative implementation, components of  FIG. 2A  may be combined to form a storage appliance, similar to Network Attached Storage (NAS). Thus, within a single enclosure, components  104 ,  106 ,  113 ,  114 , and one or more storage devices  116 ,  118 ,  120  can be combined to provide an appliance to client computers  102 . Layer  104  can include a file system interface to provide uniform access to the appliance for the client&#39;s  102 , while avoiding specific APIs, or sequential SCSI compliant commands. (The APIs of layer  104  can be specifically tailored for individual VTLs in the example above.) Thus, the file system of layer  104  will include uniform naming conventions and APIs through the file system, and provide client with a common or uniform interface (e.g., UNIX, Windows, etc.). The media agent/disc subsystem  106  is an independent component to help handle various processes that may be performed by the appliance, including snapshots, single instancing, data compression/decompression, data encryption/decryption, and other processes described herein. 
       FIG. 3  shows an example of a flow diagram illustrating VTL or virtualization routine  300 . Beginning in block  302 , the system receives read/write commands from a client, along with associated parameters. The parameters can indicate or specify a particular tape drive, media (typically identified by a barcode number), or possibly an offset, data size, etc. While barcodes are shown in this example, any identifier may be employed to identify, e.g., a desired disk. In block  304 , the system returns a session label to the client, which the client then uses for subsequent association with that request. If the same media is requested by two different clients, then a different session handle is provided to each client. 
     While a session-based example is described herein, alternatively or additionally the system may employ a threaded architecture. Under a threaded architecture, the system could employ one or more threads to handle different requests from the same client. A multi-threaded architecture may be implemented at the operating system level to cater to multiple client requests simultaneously, where, e.g., Windows provides such access through APIs. The system described herein, in this example, creates threads via the specific Windows APIs. Each client may still be given a client session, where a thread is a parallel request. So multiple sessions can run in parallel, as assisted by the use of multiple threads under Windows. 
     In block  306 , the system maps the received request and parameters to storage devices, as described below with respect to  FIG. 4 . In block  307 , the system performs the read/write commands with respect to one or more drives and performs any additional operations, such as snapshotting, single instancing, data classification, load balancing, or other processes, described herein. 
     In decision block  308 , the system determines whether a new request has been received. If so, the routine returns to block  302 . If not, then in decision block  310  the system determines whether the read/write operation has been complete. If not, the routine moves back to  308 ; else, the session is released in block  312 . 
     Referring to  FIG. 4 , an example of a virtual tape/disk table is shown. While a table is shown, any data structure may be employed. Here, the system provides a mapping for received read/write commands to associated physical data storage resources. For example, a “Client 1” provides a read/write command to the system and receives in return a session handle “Handle 1.” Parameters received from Client 1 include identification of a tape drive and media, “Drive 1” and “Barcode 1.” The system then maps that command to a particular storage device and block ID, which in this example is “Drive 1” and “Block ID 1,” and which can refer to a particular magnetic disk drive and one or more blocks on that drive. 
     Similar examples are evidenced from the table of  FIG. 4 . Note that Client 1 provides two additional requests, which are associated with “Handle 3” and “Handle 5,” where Handle 3 requests (tape) Drive 1 and media or Barcode 3, but the system maps that request to Drive 1 and Block ID 3. The system can provide simultaneous read/write commands to multiple clients and for different identified drives and media, or even for the same drives and media, despite the limitations of prior VTL technology. Of course, many more entries may exist in the table, and the table grows and shrinks dynamically as sessions are created and released. 
     As noted above, the system can provide other processing of the data under block  307 . This can include, for example, performing characteristic analysis, such as data classification for data passing through the system. Further details regarding data classification may be found in the assignee&#39;s U.S. Patent Application No. 60/752,203, entitled “Systems and Methods for Classifying and Transferring Information in a Storage Network”. The system can also perform content indexing, which is described in the assignee&#39;s U.S. patent application Ser. No. 11/694,869, filed 30 Mar. 2007, entitled “Method and System for Offline Indexing of Content and Classifying Stored Data”. Further, the system can perform single instancing or deduplication, such as is described in the assignee&#39;s U.S. Patent Application No. 60/871,737, filed 22 Dec. 2006, entitled “System and Method for Storing Redundant Information”. Further, the system can provide load balancing operations among the one or more secondary storage drives  116 ,  118 ,  120 , and dynamically allocate streams of data to and from such drives, as is described in further detail in the assignee&#39;s copending U.S. patent application Ser. No. 11/615,800, filed 22 Dec. 2006, entitled “Systems and Methods of Data Storage Management, Such as Dynamic Data Stream Allocation”. Note that the present system allows multiple data processing techniques to be employed in a virtual tape library. The system can perform many of various data processing and data storage management techniques before or during the writing (or reading) of data to the secondary storage drives  116 ,  118 ,  120 . 
     One example of the type of further processing that may be utilized is the implementation of snapshots. Snapshots are a type of data backup known to those skilled in the relevant art. A snapshot driver, in this example, creates a snapshot of a disk drive, such as “Drive 1” referred to in  FIG. 4 , which has data from Clients 1 and 2 written to it. Multiple snapshots may be created over a period of time, with each snapshot associated with a particular time they were created. A user may then access a list or table of snapshots and select a point in time to which the user may wish to roll back and view an image of the disk at that specific time (based on when the selected snapshot was created). Each of the snapshots would store not only a bitmap image of the disk, but also the table that indicates block identifications or locations where data for each client is stored. Therefore, the system can identify those blocks associated with Client 1 and re-create an image of the data or disk based on the user-selected snapshot. Note that this snapshot represents (virtual tape. Thus, snapshots may be applied to a VTL under this example. 
     As long as snapshots are taken sequentially over a period of time and associated with the time at which those snapshots were taken, a user may be able to roll back and view the state of a virtual tape drive for a particular client at any time that a snapshot was created. The system can provide a user interface that allows a user to browse through the available snapshots. Alternatively or additionally, the system can store copies of each data object mapping table, such as the table of  FIG. 4 . If the time associated with each snapshot is added as another column to the table, the user can simply use such stored tables to identify a desired snapshot. Furthermore, if the table tracks individual files and times associated with snapshots, then a user may be able to obtain earlier versions of particular files by obtaining the appropriate bitmap associated with a client and file from the snapshot and pulling up the desired file copy. In other words, the system has granularity down to the file level assuming that the locations of individual files for each client on the secondary storage (e.g., disk  118 ) are tracked in the table. 
     Other operations, such as additional reads or writes, may be performed simultaneously on Drive 1. Thus, one operation may be performed while another is simultaneously performed. A read operation, for example, may be performed simultaneously with a replication of that same data, or a write operation for a particular data object being done simultaneously with replication of that data (albeit to a different portion of a particular drive or other drive of the system). 
     Another alternative is to employ an effectively unmodified VTL, and instead allow two drives to mount the same media. Traditionally, a VTL allows only one client to read or access a given tape drive and a given piece of media at one point of time. In this alternative, one or more clients may specify two or more drives. However, those clients would also specify the same media to be accessed. The VTL would then permit two or more simultaneous reads of that data back to the clients. In other words, under this alternative, an existing VTL would expose the same drive twice, such as to two different clients. 
     Under this alternative, the VTL itself will be able to do parallel or non-sequential SCSI commands, where two clients may request, e.g., reads at the same time for the same media. Again, sessions would be employed, but within the VTL. So, in one example each client/user session has a read request in each session, which may be mapped to non-sequential access on the same disk. Each client gains access to a tape drive and issues sequential reads in each session, which then become multiple random accesses on the disk. A middle layer converts these multiple sessions sequentially into random access on the disk and returns the clients responses as sequential reads. (This middle layer is in the VTL in this alternative, as opposed to that described in the system above, but similar operations occur.) Overall, each client believes it has requested and received a sequential read, but internally it is random access to the disk. 
     Such an alternative would require only minor or no modifications to existing VTLs to permit such functionality. For example, changes may be required to client-side applications that use the VTL. The client-side application using the VTL will have to manage its own requests to the VTL, e.g., requests to mount media in a different drive and to know that the drive is actually using that media, regardless of requests by other clients. With an unmodified VTL, the application must be modified e.g., to know that it is reading media already mounted in another drive. 
     In this alternative, a drawback would be that the client would have to do some management on its own to track the media. For example, the application&#39;s regular operation or logic requires that if one tape is already being used by a drive, then it cannot be used by another drive. Here, the application would need to modify that logic so that even if a certain tape is being used by one client, it could mount the same tape in a different drive. 
     Alternatively or additionally, minor modifications to the VTL may be implemented. Normally, if a given piece of media is already mounted in a drive, the VTL does not allow that media to be mounted to a different drive. So, here, a minor modification to the VTL would be to simply allow it to mount the same media to a different drive. 
     Yet another alternative is to expose the file system employed by the VTL. By providing access to the VTL file system (via appropriate APIs), simultaneous reads of data identified in the VTL file system may be performed. VTLs typically do not expose their underlying file system, i.e., how they store data on the disk, etc., but instead just provide a sequential access interface (e.g., sequential SCSI interface), which does not describe exactly how files are laid on disk by the VTL file system. In other words, the VTL completely manages, and completely hides, how files are managed and distributed by the VTLs internal disk drive(s). 
     If the VTL file system instead were opened up (via appropriate APIs, like in Microsoft Windows), simultaneous reads, writes, and other access may be provided, as described herein. However, such an alternative would require a VTL manufacturer or VTL subsystem to open up the underlying file system, which may require the manufacturer to manage multiple, concurrent write sessions, etc., thus potentially expanding the required scope of modification. 
     Suitable System 
     The above parallel access VTL system may be incorporated within a data storage system and may be subjected to a data stream during a data copy operation. Referring to  FIG. 5 , a block diagram illustrating components of a data stream  510  utilized by a suitable data storage and recovery system is shown. The data stream  510  may include client  102 , a media agent  512 , and a secondary storage device, such as VTL  103 . For example, in storage operations, the system may store, receive, and/or prepare data to be stored, copied, or backed up at a server or client. The system may then transfer the data to be stored to media agent  512 , which may then refer to storage policies, schedule policies, and/retention policies (and other policies) to choose a secondary storage device. The media agent  512  may include or be associated with an intermediate component, to be discussed herein. 
     The secondary storage device receives the data from the media agent  512  and stores the data as a secondary copy, such as a backup copy. Secondary storage devices may be magnetic tapes, optical disks, USB and other similar media, disk, and tape drives, and so on. Of course, the system may employ other configurations of data stream components not shown in  FIG. 6 . 
     Referring to  FIG. 6 , a block diagram illustrating an example of a data storage and recovery system  600  is shown. Data storage systems may contain some or all of the following components, depending on the needs of the system.  FIG. 6  and the discussion herein provide a brief, general description of a suitable computing environment in which the system can be implemented. 
     For example, the data storage system  600  contains a storage manager  610 , one or more clients  102 , one or more media agents  512 , and one or more storage devices  103 ,  613 . Storage manager  610  controls media agents  512 , which may be responsible for transferring data to storage devices. Storage manager  610  includes a jobs agent  611 , a management agent  612 , a database  613 , and/or an interface module  614 . Storage manager  610  communicates with client(s)  102 . One or more clients may access data that is stored by the system from database  622  via a data agent  621 . The system uses media agents  512 , which contain databases  631 , to transfer and store data into storage devices. Client databases  622  may contain data files and other information, while media agent databases  631  may contain indices and other data structures that assist and implement the storage of data in secondary storage devices, for example. 
     The data storage and recovery system may include software and/or hardware components and modules used in data storage operations. The components may be storage resources that function to copy data during storage operations. The components may perform other storage operations (or storage management operations) other that operations used in data stores. For example, some resources may create, store, retrieve, and/or migrate primary or secondary data copies. Additionally, some resources may create indices and other tables relied upon by the data storage system and other data recovery systems. The secondary copies may include snapshot copies and associated indices, but may also include other backup copies such as HSM copies, archive copies, and so on. The resources may also perform storage management functions that may communicate information to higher level components, such as global management resources. 
     In some examples, the system performs storage operations based on storage policies, as mentioned above. For example, a storage policy includes a set of preferences or other criteria to be considered during storage operations. The storage policy may determine or define a storage location and/or set of preferences about how the system transfers data to the location and what processes the system performs on the data before, during, or after the data transfer. In some cases, a storage policy may define a logical bucket in which to transfer, store, or copy data from a source to a data store, such as storage media. Storage policies may be stored in storage manager  610 , or may be stored in other resources, such as a global manager, a media agent, and so on. Further details regarding storage management and resources for storage management will now be discussed. 
     Referring to  FIG. 7 , a block diagram illustrating an example of components of a server or system  700  used in data storage operations is shown. A server, such as storage manager  610 , may communicate with clients  102  to determine data to be copied to storage media. As described above, the storage manager  610  may contain a jobs agent  611 , a management agent  612 , a database  613 , an interface module  614 , and/or other agents  720 . Jobs agent  611  may manage and control the scheduling of jobs (such as copying data files) from clients  102  to media agents  512  (not shown). Management agent  612  may control the overall functionality and processes of the data storage system or may communicate with global managers. Database  613  or another data structure may store storage policies, schedule policies, retention policies, or other information, such as historical storage statistics, storage trend statistics, and so on. Interface module  615  may interact with a user interface, enabling the system to present information to administrators and receive feedback or other input from the administrators or with other components of the system (such as via APIs). 
     CONCLUSION 
     Systems and modules described herein may comprise software, firmware, hardware, or any combination(s) of software, firmware, or hardware suitable for the purposes described herein. Software and other modules may reside on servers, workstations, personal computers, computerized tablets, PDAs, and other devices suitable for the purposes described herein. Modules described herein may be executed by a general-purpose computer, e.g., a server computer, wireless device, or personal computer. Those skilled in the relevant art will appreciate that aspects of the invention can be practiced with other communications, data processing, or computer system configurations, including: Internet appliances, hand-held devices (including personal digital assistants (PDAs)), wearable computers, all manner of cellular or mobile phones, multi-processor systems, microprocessor-based or programmable consumer electronics, set-top boxes, network PCs, mini-computers, mainframe computers, and the like. Indeed, the terms “computer,” “server,” “host,” “host system,” and the like, are generally used interchangeably herein and refer to any of the above devices and systems, as well as any data processor. Furthermore, aspects of the invention can be embodied in a special purpose computer or data processor that is specifically programmed, configured, or constructed to perform one or more of the computer-executable instructions explained in detail herein. 
     Software and other modules may be accessible via local memory, a network, a browser, or other application in an ASP context, or via another means suitable for the purposes described herein. Examples of the technology can also be practiced in distributed computing environments where tasks or modules are performed by remote processing devices, which are linked through a communications network, such as a Local Area Network (LAN), Wide Area Network (WAN), or the Internet. In a distributed computing environment, program modules may be located in both local and remote memory storage devices. Data structures described herein may comprise computer files, variables, programming arrays, programming structures, or any electronic information storage schemes or methods, or any combinations thereof, suitable for the purposes described herein. User interface elements described herein may comprise elements from graphical user interfaces, command line interfaces, and other interfaces suitable for the purposes described herein. 
     Examples of the technology may be stored or distributed on computer-readable media, including magnetically or optically readable computer disks, hard-wired or preprogrammed chips (e.g., EEPROM semiconductor chips), nanotechnology memory, biological memory, or other data storage media. Indeed, computer-implemented instructions, data structures, screen displays, and other data under aspects of the invention may be distributed over the Internet or over other networks (including wireless networks), on a propagated signal on a propagation medium (e.g., an electromagnetic wave(s), a sound wave, etc.) over a period of time, or they may be provided on any analog or digital network (packet switched, circuit switched, or other scheme). 
     Unless the context clearly requires otherwise, throughout the description and the claims, the words “comprise,” “comprising,” and the like are to be construed in an inclusive sense, as opposed to an exclusive or exhaustive sense; that is to say, in the sense of “including, but not limited to.” As used herein, the terms “connected,” “coupled,” or any variant thereof, means any connection or coupling, either direct or indirect, between two or more elements; the coupling or connection between the elements can be physical, logical, or a combination thereof. Additionally, the words “herein,” “above,” “below,” and words of similar import, when used in this application, refer to this application as a whole and not to any particular portions of this application. Where the context permits, words in the above Detailed Description using the singular or plural number may also include the plural or singular number respectively. The word “or,” in reference to a list of two or more items, covers all of the following interpretations of the word: any of the items in the list, all of the items in the list, and any combination of the items in the list. 
     The above Detailed Description is not intended to be exhaustive or to limit the invention to the precise form disclosed above. While specific examples for the invention are described above for illustrative purposes, various equivalent modifications are possible within the scope of the invention, as those skilled in the relevant art will recognize. For example, while processes or blocks are presented in a given order, alternative implementations may perform routines having steps, or employ systems having blocks, in a different order, and some processes or blocks may be deleted, moved, added, subdivided, combined, and/or modified to provide alternative or subcombinations. Each of these processes or blocks may be implemented in a variety of different ways. Also, while processes or blocks are at times shown as being performed in series, these processes or blocks may instead be performed or implemented in parallel, or may be performed at different times. Further any specific numbers noted herein are only examples: alternative implementations may employ differing values or ranges. 
     The teachings of the invention provided herein can be applied to other systems, not necessarily the system described above. The elements and acts of the various examples described above can be combined to provide further implementations of the invention. 
     Any patents and applications and other references noted above, including any that may be listed in accompanying filing papers, are incorporated herein by reference. Aspects of the invention can be modified, if necessary, to employ the systems, functions, and concepts of the various references described above to provide yet further implementations of the invention. 
     These and other changes can be made to the invention in light of the above Detailed Description. While the above description describes certain examples of the invention and describes the best mode contemplated, no matter how detailed the above appears in text, the invention can be practiced in many ways. Details of the system may vary considerably in its specific implementation, while still being encompassed by the invention disclosed herein. As noted above, particular terminology used when describing certain features or aspects of the invention should not be taken to imply that the terminology is being redefined herein to be restricted to any specific characteristics, features, or aspects of the invention with which that terminology is associated. In general, the terms used in the following claims should not be construed to limit the invention to the specific examples disclosed in the specification, unless the above Detailed Description section explicitly defines such terms. Accordingly, the actual scope of the invention encompasses not only the disclosed examples, but also all equivalent ways of practicing or implementing the invention under the claims. 
     While certain aspects of the invention are presented below in certain claim forms, the applicant contemplates the various aspects of the invention in any number of claim forms. For example, while only one aspect of the invention is recited as a means-plus-function claim under 35 U.S.C. §112, ¶ 6, other aspects may likewise be embodied as a means-plus-function claim, or in other forms, such as being embodied in a computer-readable medium. (Any claims intended to be treated under 35 U.S.C. §112, ¶ 6, will begin with the words “means for”.) Accordingly, the applicant reserves the right to add additional claims after filing the application to pursue such additional claim forms for other aspects of the invention.