Abstract:
A method of storing data to one of a first or second storage device associated with a data storage system where each storage device provides for the redundant access to and storage of data within the same logical data volumes. The method of storing data consists of defining a storage construct which will direct the performance of a specific storage function. The storage construct is then associated with a logical data volume. The method further consists of mounting the logical data volume residing on one of the two storage devices and executing a storage function in accordance with the storage construct. The storage construct may be defined by a command issued by a host associated with the data storage system. Alternatively, the storage construct may be defined by a user of the data storage system through a user interface. The storage function which is directed by the defined storage construct may consist of selecting which one of the first and second storage devices will execute input/output (I/O) commands received from the data storage system for a particular logical data volume mount. Alternatively, the storage function may consist of determining whether data stored to a logical data volume physically associated with one of the storage devices will be copied to the other storage device. Other storage functions can be directed by a storage construct.

Description:
TECHNICAL FIELD  
       [0001]     The present invention relates to a method, data storage system, and article of manufacture for storing data in a data processing system, in particular controlling whether to copy data from one virtual tape server to another virtual tape server in a peer-to-peer environment, and selecting which peer will initially store data.  
       BACKGROUND ART  
       [0002]     In prior art virtual tape storage systems, hard disk drive storage is used to emulate tape drives and tape cartridges. In this way, host systems performing input/output (I/O) operations with respect to tape are in fact performing I/O operations with respect to a set of hard disk drives emulating the tape storage. In the prior art International Business Machines (IBM) Magstar Virtual Tape Server®, one or more virtual tape servers (“VTS”) are each integrated with a tape library comprising numerous tape cartridges and tape drives, and have a direct access storage device (DASD) comprised of numerous interconnected hard disk drives. The operation of the virtual tape server is transparent to the host. The host makes a request to access a tape volume. The virtual tape server intercepts the tape requests and accesses the volume in the DASD. If the volume is not in the DASD, then the virtual tape server recalls the volume from the tape drive to the DASD. The virtual tape server can respond to host requests for volumes in tape cartridges from DASD substantially faster than responding to requests for data from a tape drive. Thus, the DASD functions as a tape volume cache for volumes in the tape cartridge library.  
         [0003]     Two virtual tape servers can be combined to create a peer-to-peer virtual tape server. In a peer-to-peer virtual tape server, two virtual tape servers, each integrated with a separate tape library, can provide access and storage for the same data volumes (i.e. peer-to-peer environment). By providing two virtual tape server subsystems and two libraries, if an operation to access a logical volume from one virtual tape server subsystem and tape library fails, then the volume may still be accessed from the other virtual tape server subsystem and tape library. This redundant architecture provides greater data and tape availability and improved data shadowing in the event a tape or virtual tape server in one subsystem is damaged. Therefore, when a host system writes to the peer-to-peer virtual tape server, the data will be saved on both virtual tape servers. However, rather than writing to both virtual tape servers simultaneously, which would be a drain on system resources, a virtual tape controller connecting the two virtual tape servers will first write the logical volume to one of the virtual tape servers. An example of a virtual tape controller is the IBM AX0 Virtual Tape Controller (“AX0 VTC”) which acts as an intelligent switch between the two virtual tape servers and transparently connects the host computers with the virtual tape servers. Then, the logical volume is copied by the virtual tape controller from one virtual tape server to the other virtual tape server when the host closes the volume. The copying process between the virtual tape servers can occur immediately upon close of the volume or be deferred based on user preferences. There can also be a case where some elements of the peer-to-peer virtual tape server are unavailable for a period of time and it is necessary to ‘catch’ up on volumes that have not been copied. The automatic copying of volumes between the virtual tape servers that make up a peer-to-peer virtual tape server system is a key part of its data availability characteristics.  
         [0004]     While the automatic copying of a logical volume is a key part of the peer-to-peer virtual tape system, not all logical volume data created by a host have the same level of importance. For some logical volumes, the ability to access it at any time is critical, but for other volumes, it is acceptable to have the volume be temporarily unavailable. For example, a volume that is created as the result of testing a new or modified host application is not critical to the daily operation of the customer&#39;s business and typically has no value after the test is performed. In addition, there are costs associated with the copying of a logical volume in a peer-to-peer virtual tape server. The logical volume takes up space in the disk cache of each of the VTSs as well as space on the physical tapes in the library associated with each VTS. In many peer-to-peer virtual tape server configurations, virtual tape controllers and virtual tape servers are geographically separated and there are costs associated with the transmission of the data between these elements. Thus, there is a need in the art for a method to selectively control the logical volumes that are copied in a peer-to-peer virtual tape server system.  
         [0005]     In the case where the physical elements of the peer-to-peer virtual tape server are geographically separated, it is also desirable to direct the host data creating a logical volume to a specific virtual tape server. The virtual tape server the data is to be directed to may be one virtual tape server for some data and another virtual tape server for other data. For example, a logical volume created by the application development group in a company may need to only go to the virtual tape server that resides at the site that houses that group, typically the customer&#39;s main site. If a virtual tape server remotely located from the customer&#39;s main site is selected, expensive remote connection bandwidth will be used, increasing the cost of the development of a new application. In another example, data that is critical to the continued operations of the company may need to be directed to the remote site first, thus allowing the company&#39;s operations to continue should there be a disaster involving the main site, whether or not the data is copied later to the other virtual tape server at the main site. Thus, there is a need in the art for a method to selectively direct the initial host data to one or the other virtual tape server in a peer-to-peer virtual tape server system where, for example, the data of a logical volume is not to be copied from one virtual tape server to the other or if it is important that the data first be written to a specific virtual tape server before being copied.  
       SUMMARY OF THE INVENTION  
       [0006]     The needs in the art are addressed by a method of storing data to one of a first or second storage device associated with a data storage system where each storage device provides for the redundant access to and storage of data within the same logical data volumes. For example, the data storage devices may be virtual tape servers in a peer-to-peer data storage relationship. The method of storing data consists of defining a storage construct which will direct the performance of a specific storage function. The storage construct is then associated with a logical data volume. The method further consists of mounting the logical data volume residing on one of the two storage devices and executing a storage function in accordance with the storage construct.  
         [0007]     The storage construct may be defined by a command issued by a host associated with the data storage system. Alternatively, the storage construct may be defined by a user of the data storage system through a user interface.  
         [0008]     The storage function which is directed by the defined storage construct may consist of selecting which one of the first and second storage devices will execute input/output (I/O) commands received from the data storage system for a particular logical data volume mount. Alternatively, the storage function may consist of determining whether data stored to a logical data volume physically associated with one of the storage devices will be copied, pursuant to peer-to-peer (PTP) or other protocols, to the logical data volume physically associated with the other storage device. Other storage functions can be directed by a storage construct.  
         [0009]     In one embodiment of the invention where the first storage device and the second storage device are first and second virtual tape servers, the mounting of a logical data volume may consist of sending a mount command from a host to a virtual tape controller. The virtual tape controller may then pass the mount to the first and second virtual tape servers. The mount command will be processed by the first and second virtual tape servers with the processing including the association of the storage construct with the logical storage volume being mounted. The storage construct associated with the logical data volume may also be passed back to the virtual tape controller and retained. A mount operation may be completed by notifying the host.  
         [0010]     Another embodiment of the invention is a data storage system capable of performing the above described steps for storing data.  
         [0011]     A further embodiment of the invention is an article of manufacture comprising a storage medium having logic embedded therein for programming the components of a data storage system to execute the steps described above for storing data. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0012]      FIG. 1  is a block diagram illustrating a computing environment in which a preferred embodiment may be implemented;  
         [0013]      FIG. 2  illustrates a token database record used to access data in accordance with an embodiment of the present invention;  
         [0014]      FIG. 3  illustrates a construct management database and data structures used to associate logical volumes to management actions in accordance with an embodiment of the present invention;  
         [0015]      FIG. 4  is a data flow diagram that illustrates one manner in which one or more constructs for a logical volume are assigned through a control console of a library manager in accordance with an embodiment of the present invention;  
         [0016]      FIG. 5  illustrates logic in the virtual tape controller, virtual tape servers and library managers to receive a mount request and constructs then determine from the management actions associated with the constructs assigned to the logical volume which VTS is to perform the host I/O operations in accordance with an embodiment of the present invention;  
         [0017]      FIG. 6  illustrates logic in the virtual tape controller, virtual tape servers and library managers to receive a mount request and based on user assigned constructs, determine from the management actions associated with the constructs assigned to the logical volume which VTS is to perform the host I/O operations in accordance with an embodiment of the present invention;  
         [0018]      FIG. 7  illustrates logic in the virtual tape controller for the processing of host I/O operations to determine whether a logical volume is to be inhibited or not in accordance with an embodiment of the present invention; and  
         [0019]      FIG. 8  illustrates logic in the virtual tape controller to establish a list of logical volumes that are to be copied taking into account whether copying for a logical volume is to be inhibited or not in accordance with an embodiment of the present invention. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0020]      FIG. 1  illustrates a peer-to-peer (PTP) computing system  1  environment utilizing two virtual tape servers (“VTS”). Although  FIG. 1  shows VTS devices, the invention disclosed herein is applicable to any type of data storage system using two or more storage devices of any nature where each storage device provides for redundant access to an storage of data within the same logical volumes. Additional virtual tape servers can be used by a system, but for purposes of illustration, a single pair arrangement is shown. A plurality of host computers  2   a,    2   b  (two host computers  2   a,    2   b  are shown for illustration purposes) connect to the system  1  through a virtual tape controller  4  (“VTC”). The hosts  2   a,    2   b  may be any computational device known in the art, such as a personal computer, a workstation, a server, a mainframe, a hand held computer, a palm top computer, a telephony device, network appliance, etc. The hosts may include an operating system such as the IBM z/OS operating system, or any other operating system known in the art. Additional virtual tape controllers can be used by the system, but for the purposes of illustration, a single virtual tape controller is shown. The host computers  2   a,    2   b  may connect to the VTC  4  through a channel, such as an Enterprise System Connection (ESCON) channel or other interface mechanism known in the art (e.g., fibre channel, Storage Area Network (SAN) interconnections, etc.). In a peer-to-peer environment, the virtual tape controller  4  is typically transparent to the host computers  2   a,    2   b  (i.e. the host system acts as if the host computers  2   a,    2   b  are writing to a single virtual tape server). The virtual tape controller  4  then routes I/O requests from the hosts  2   a,    2   b  to one of the virtual tape servers  6   a  or  6   b.  The virtual tape servers  6   a,    6   b  control access to direct access storage devices (DASD)  8   a  and  8   b  and tape libraries  12   a  and  12   b,  respectively.  
         [0021]     Each DASD  8   a,    8   b  comprises numerous interconnected hard disk drives. Each tape library  12   a,    12   b  comprises numerous tape cartridges which may be mechanically loaded into tape drives that the virtual tape servers  6   a,    6   b  may access. The virtual tape servers  6   a  or  6   b  may comprise a server system including software to emulate a tape library, such as the IBM TotalStorage Virtual Tape Server. For instance, the virtual tape servers  6   a,    6   b  and the virtual tape controller  4  may be implemented in separate computers comprising an IBM pSeries processor, the IBM AIX operating system, and the IBM ADSTAR Distributed Management (ADSM) software or Tivoli Storage Manager, to perform the data movement operations among the hosts  2   a,    2   b,  DASDs  8   a,    8   b,  and tape libraries  12   a,    12   b.  The tape library may comprise an IBM Magstar Tape Library, such as the Magstar 3494 Tape Library, or any other tape library system known in the art.  
         [0022]     The DASDs  8   a,    8   b  provide a tape volume cache, which extends the performance benefits of disk cache to access the volumes in the tape libraries  12   a,    12   b  and improves performance by allowing host I/O requests to the tape libraries  12   a,    12   b  to be serviced from the DASDs  8   a, b.  The virtual tape servers  6   a,    6   b  appear to the hosts  2   a,    2   b  as tape drives including tape data volumes. The hosts  2   a,    2   b  view the logical tape volumes as actual tape volumes and issue tape management commands, such as mount, and otherwise address the virtual tape servers  6   a,    6   b  as a tape control unit. Further details of the virtual tape server technology in which embodiments may be implemented are described in the IBM publications “IBM TotalStorage Virtual Tape Server: Planning, Implementing, and Monitoring” IBM document no. SG24-2229-06 (Copyright IBM, November 2003) and “IBM TotalStorage Peer-to-Peer Virtual Tape Server: Planning and Implementation Guide” IBM document no. SG24-6115-02 (Copyright IBM, February 2004), which publications are incorporated herein by reference in their entirety.  
         [0023]     Tape data volumes maintained on tape cartridges in the tape library  12   a,    12   b  are logical volumes. A copy of a logical volume can also reside in the DASDs  8   a,    8   b  associated with the virtual tape servers  6   a,    6   b.  A host  2   a,    2   b  accesses the data on a logical volume from the resident copy on the DASD  8   a,    8   b.  After the DASDs  8   a,    8   b  space usage reaches a threshold amount, the virtual tape server  6   a,    6   b  removes logical volumes that have been copied to a tape library  12   a,    12   b  from the DASD  8   a,    8   b  to make room for other logical volumes. Once a logical volume has been removed from the DASDs  8   a,    8   b,  it is no longer accessible by a host. If a host  2   a,    2   b  requests a volume that only resides on a physical tape, then the volume must be recalled and copied from a physical tape in the tape libraries  12   a,    12   b  to the DASDs  8   a,    8   b.  Recall operations can take several minutes and may include mechanical operations concerning the use of a robotic arm to access tape cartridges from the storage cells in tape libraries  12   a,    12   b,  insert it into a tape drive, mount the tape cartridge, rewind the tape, etc. The tape libraries  12   a,    12   b  may not include the same logical volumes since each virtual tape server  6   a,    6   b  typically behaves independently, and each may cache different volumes in DASD. For example, the virtual tape servers  6   a,    6   b  may have different volumes resident in their associated DASDs  8   a,    8   b  as a result of different schedules or algorithms that determine which volumes to remove or which VTS is the target for host data or whether the data is to be copied or not.  
         [0024]     As long as a logical volume is still resident in the DASDs  8   a,    8   b,  it can be accessed again by a host regardless of whether it has been copied to the tape library  12   a,    12   b  or not. By allowing a volume to be mounted and accessed from DASDs  8   a,    8   b,  delay times associated with rewinding the tape, robotic arm movement, and load time for the mounts are avoided because the operations are performed with respect to hard disk drives that do not have the delay times associated with tape access mechanisms. Performing a virtual mount of a logical volume resident in DASD  8   a,    8   b  is often referred to as a cache hit.  
         [0025]     Each virtual tape server  6   a,    6   b  may include a database on DASD  10   a,    10   b  of tokens or records for every logical volume in the tape library  12   a,    12   b  to manage the volumes in the virtual tape servers  6   a,    6   b.  Each tape library  12   a,    12   b  may contain a library manager  14   a,    14   b  which manages the physical tape drives and media and the robotics that moves the media between storage cells and the physical tape drives. The library manager  14   a,    14   b  also may manage a construct management database on DASD  16   a,    16   b  that contains the storage management constructs and management actions associated with each logical volume as disclosed herein. A control console  18   a,    18   b  is typically attached to each library manager. The library manager  12   a,    12   b  and control console  18   a,    18   b  may be any computational device known in the art, such as a personal computer, a workstation, a server, a mainframe, a hand held computer, a palm top computer, a telephony device, network appliance, etc.  
         [0026]      FIG. 2  illustrates representative fields or data maintained in each volume token  50  which is stored in a DASD token database  10   a,    10   b.  The volume ID  52  indicates the identity of the volume. It is common to refer to the identity of a volume by its volume serial number, which is typically 1 to 6 characters in length. A copy flag  54  indicates whether the data needs to be copied to the other virtual tape server in a peer-to-peer environment or other data copying environment. For example, the copy flag  54  is set “on” for a logical volume in one virtual tape server  6   a  if the data needs to be copied to the other virtual tape server  6   b.  If the data is copied, it is placed in the DASD  8   b  just as if the host had originally written it there. After a logical volume has been copied from virtual tape server  6   a,  the copy flag  54  is set “off” again in virtual tape server  6   a&#39; s token database  10   a.  A copy inhibit flag  56  as is fully described below may be used to indicate whether the copying of data for the volume is to be inhibited. The copy inhibit flag  56  is set to “on” for a logical volume if the volume is not to be copied to another virtual tape server, even if the copy flag  54  is set to “on”. Whereas the copy flag  54  is typically only set to “on” in one of the virtual tape servers  6   a,  b and token databases  10   a,    10   b,  the copy inhibit flag  56  can be set to “on” in both virtual tape servers  6   a,    6   b  token databases  10   a, b.  As described below, the copy inhibit flag  56  may be set to “off” again if the volume is again allowed to be copied to another virtual tape server  6   a,    6   b.  Data level  58  indicates the number of times the file has been updated. Every time data is updated to a logical volume, the level field  58  is incremented indicating the number of times a volume in a particular DASD  8   a,    8   b  has been updated. The logical volume in the DASDs  8   a,    8   b  having the highest data level contains the most recent version of the logical volume. For this reason, the virtual tape server  6   a,    6   b  including the most recent version of the data, i.e., having the highest level, will typically be selected when performing I/O operations with respect to the volume. It will be apparent to one skilled in the art that the flags and data level fields of the volume token  50  could be represented by binary, numeric or character values, or a combination thereof.  
         [0027]     Referring to  FIG. 3 , the contents of a construct management database  16   a,    16   b  of a library manager  14   a,    14   b  are depicted. In one embodiment of the invention, the construct management database  16   a,    16   b  contains a volume database  100  that a library manager  14   a,    14   b  uses to store information associated with each of the logical volumes it manages. The volume database  100  contains a plurality of volume IDs  102 , which are typically the volume serial number of the logical volume. Additionally, the volume database  100  contains a plurality of constructs  104 ,  106 ,  108 ,  110  pertaining to each volume ID  102 . The constructs  104 ,  106 ,  108 ,  110  in the depicted embodiment include a data class  104 , a storage class  106 , a management class  108  and a storage group  110 . Furthermore, the volume database  100  may contain additional information  112  about each of the volume IDs  102 , not shown in  FIG. 3 . Although values are shown in  FIG. 3  for data class  104 , storage class  106  and storage group  110 , for the embodiment of the invention, management class  108 , is used to control storage functions such as discussed in detail herein whether the copying of a logical volume is to be inhibited or not or which virtual tape server  6   a  or  6   b  will handle host I/O operations. Although not described in detail herein, one of the other listed storage constructs, or an unlisted storage construct, could be used in the management of logical volumes in a peer-to-peer virtual tape server system in accordance with the invention.  
         [0028]     In one preferred embodiment of the invention, the management class  108  relates to the importance of the data stored on a logical volume and the ability of the system  1  to allow access to the logical volume in the event of a failure of an element of the data storage system  1 . For example, volume ID  102  “ABC123” which contains data that absolutely must be accessible may have a management class  108  of “Critical”, while a management class  108  of “Test” may be assigned to volume ID 102 “ZZZ999” which contains data for which access can be lost for a period of time. It will be understood by those skilled in the art that the example management class  108  constructs depicted in  FIG. 3  are a small subset of the possible names that could be used.  
         [0029]     The construct management database  16   a,    16   b  of the library manager  14   a,    14   b  may also contain a construct actions database  200  that the library manager  14   a,    14   b  uses to store information about the storage management actions associated with each of the storage management constructs to be used. For clarity, only certain management class storage management actions  300  which are particularly associated with the embodiment of the present invention described in detail are shown. The management class storage management actions  300  contains a plurality of names  302 , which are the management class  108  construct names defined for use by the system  1 . Additionally, the management class storage management actions  300  may contain a plurality of copy mode  304  and I/O VTS  306  actions pertaining to each name  302 . For example, a name  302  of “Critical” may have a copy mode  304  of “Immediate” meaning that when the host has finished writing the logical volume and is in the process of closing the logical volume, the copying of the logical volume is completed prior to informing the host that the logical volume has been closed. The name  302  of “Critical” may also have an I/O VTS  306  of “VTC Selected” meaning that the choice of which virtual tape server  6   a,    6   b  is to handle all host I/O operations for the logical volume is to be determined by the virtual tape controller  4  based upon an analysis of system resources, system stability and other factors. Similarly, a name  302  of “Test” may have a copy mode  304  of “Inhibit” meaning that regardless of whether the logical volume should be copied, copying of the logical volume is inhibited. The name  302  of “Test” may also have an I/O VTS  306  of “VTS  6   a”  meaning that the virtual tape server  4  is to use virtual tape server  6   a  to handle all host I/O operations for the logical volume. Furthermore, the management class storage management actions  300  may contain additional actions  308  which are not shown on  FIG. 3  about each of the management class  108  names which are useful for purposes other than the control of the inhibiting of copying a logical volume or controlling which virtual tape server  6   a,    6   b  will handle host I/O operations.  
         [0030]     In one embodiment, the storage management actions  300  are defined by a user through a user interface, for example through a library manager  14   a,    14   b  control console  18   a,    18   b.  In another embodiment, the storage management actions are defined through commands issued by one of the hosts  2   a,    2   b.    
         [0031]     In addition to the above embodiment of the contents of the construct management database  16   a,    16   b,  it will be readily understood to those familiar with the art that the contents of the volume database  100  and the construct actions database  200  may be combined into a single database.  
         [0032]     In one embodiment, the constructs  104 ,  106 ,  108 ,  110  that are to be associated with a logical volume are provided by a host  2   a,    2   b  as part of the volume mount process. In another embodiment, the assignment of constructs  104 ,  106 ,  108 ,  110  is performed independent of a host mount request by a user through a user interface such as a library manager  14   a,    14   b  control console  18   a,    18   b.    FIG. 4  is a data flow diagram that illustrates one manner in which one or more constructs for a logical volume are assigned using a library manager  14   a,    14   b  control console  18   a,    18   b.  Beginning with step  400 , the user determines which volume IDs  102  are to be assigned constructs  104  or  106  or  108  or  110 . For example, the user may decide that all of the logical volumes used by specific host  2   b  are to be inhibited from being copied and so decides that they all would have a management class  108  construct name of “Host  2   b”.  Based on other criteria, the user may select constructs names for data class  104 , storage class  106  and storage group  110 . The user is preferably not restricted to a limited selection of construct  104 ,  106 ,  108 ,  110  names, nor restricted in which volumes IDs  102  are assigned which constructs  104 ,  106 ,  108 ,  110  or if all constructs must be defined for each volume ID  102 . In step  405 , the user enters the data class  104  construct name and either a specific volume ID  102  or a range of volume IDs  102  that are to be assigned data class  104  through the control console  18   a,    18   b.  In step  410 , the library manager  14   a,    14   b,  stores the user provided data class  104  name in the volume database  100  for the user provided volume ID  102  or range of user provided volume IDs  102 . If the user did not provide a data class  104  in step  405 , step  410  is skipped. In step  415 , the user enters the storage class  106  construct name and either a specific volume ID  102  or a range of volume IDs  102  that are to be assigned storage class  106  through the control console  18   a,    18   b.  In step  420 , the library manager  14   a,    14   b,  stores the user provided storage class  106  name in the volume database  100  for the user provided volume ID  102  or range of user provided volume IDs  102 . If the user did not provide a storage class  106  in step  415 , step  420  is skipped. In step  425 , the user enters the management class  108  construct name and either a specific volume ID  102  or a range of volume IDs  102  that are to be assigned management class  108  through the control console  18   a,    18   b.  In step  430 , the library manager  14   a,    14   b,  stores the user provided management class  104  name in the volume database  100  for the user provided volume ID  102  or range of user provided volume IDs  102 . If the user did not provide a management class  108  in step  425 , step  430  is skipped. In step  435 , the user enters the storage group  110  construct name and either a specific volume ID  102  or a range of volume IDs  102  that are to be assigned storage group  110  through the control console  18   a,    18   b.  In step  410 , the library manager  14   a,    14   b,  stores the user provided storage group  110  name in the volume database  100  for the user provided volume ID  102  or range of user provided volume IDs  102 . If the user did not provide a storage group  110  in step  435 , step  440  is skipped. The flow terminates at step  445 .  
         [0033]     The determinations regarding the performance of storage functions such as whether to inhibit the copying of a logical volume and which virtual tape server  6   a,    6   b  is to handle the host I/O operations for a logical volume are preferably made by the virtual tape controller  4  using the management actions  300  established via a library manager  14   a,    14   b  control console  18   a,    18   b.  Hence, the determination of whether the copying of a logical volume is to be inhibited or not or which virtual tape server  6   a,    6   b  will process host I/O for a volume is made in a manner that is transparent to the hosts  2   a,    2   b.  In the one embodiment, a host  2   a,    2   b  provides the constructs  104 ,  106 ,  108 ,  110  associated with each logical volume, but the storage management actions  300  need not be dealt with by the host  2   a,    2   b.  In another embodiment, the constructs  104 ,  106 ,  108 ,  110  associated with each logical volume are assigned by a user through a library manager  14   a,    14   b  control console  18   a,    18   b.  The manner in which a virtual tape server is selected to process host I/O using the storage management actions  300  will be further described in connection with  FIG. 5  and in an alternative embodiment of the invention in connection with  FIG. 6 . The manner in which volume copying is inhibited or not using the storage management actions will be further described in a preferred embodiment of the invention in connection with  FIG. 7 .  
         [0034]     Referring to  FIG. 5 , a data flow diagram illustrates one manner in which the constructs for a logical volume are provided by the host  2   a,    2   b  and retained in the construct management database  100  on DASD  16   a,    16   b.  In step  500 , the host  2   a,    2   b  transmits a mount command to the virtual tape controller  4 . The mount command indicates that the request is for a new logical volume and includes the constructs data class  104 , storage class  106 , management class  108  and storage group  110  to be associated with the logical volume selected for the mount request. As mentioned previously, the constructs  104 ,  106 ,  108 ,  110  in one embodiment do not specify which virtual tape server is to handle the I/O operations for the mount or whether copying of the logical volume is to be inhibited or not. In step  505 , the virtual tape controller  4  passes the mount command and included constructs through one virtual tape server  6   a  or  6   b  to its associated library manager  14   a  or  14   b.  In step  510 , the library manager  14   a  or  14   b  associated with the selected virtual tape server  6   a  or  6   b  processes the mount command, processing which includes indicating in the volume database  100  that the volume is in use, selecting a logical volume from a list of logical volumes available for new data, and updating the volume database  100  record for the selected volume ID  102  with the constructs  104 ,  106 ,  108  and  110 . In step  515 , the library manager  14   a  or  14   b  associated with the selected virtual tape server  6   a  or  6   b  obtains the storage management actions for each of the constructs  104 ,  106 ,  108  and  110  for the selected volume ID  102  and passes them back to virtual tape controller  4  which retains them. In step  520 , the virtual tape controller  4  passes the mount command, the selected volume ID  102  and the constructs  104 ,  106 ,  108 ,  110  through the other virtual tape server  6   a  or  6   b  to its associated library manager  14   a  or  14   b.  In step  525 , the library manager  14   a  or  14   b  associated with the other virtual tape server  6   a  or  6   b  processes the mount command, processing which includes indicating in the volume database  100  that the volume is in use and updates the volume database  100  record for the volume ID  102  passed from the virtual tape controller  4  with the constructs  104 ,  106 ,  108  and  110 . In step  530 , the library manager  14   a  or  14   b  associated with the other virtual tape server  6   a  or  6   b  obtains the storage management actions for each of the constructs  104 ,  106 ,  108  and  110  for the volume ID  102  and passes them back to virtual tape controller  4  which retains them. Referring now to step  535 , the virtual tape controller  4 , using the retained storage management actions passed from the library manager  14   a  or  14   b  associated with the selected virtual tape server  6   a  or  6   b,  and specifically using the storage management action I/O VTS  306  for the associated management class  108 , determines whether a specific virtual tape server is to be used for the duration of the mount. If the I/O VTS  306  action specified one of the virtual tape server IDs, for example “VTS  6   a”,  then the flow proceeds to step  540 . If the I/O VTS  306  action specified “VTC Selected”, then the flow proceeds to step  545 . As shown in step  540 , virtual tape server  6   a  is selected to handle all of the I/O operations for the mount, based on the specified I/O VTS  306  action of “VTS  6   a”  and the flow proceeds to step  550 . Alternatively, as shown in step  545 , the virtual tape controller  4  determines which virtual tape server  6   a,    6   b  is to be selected to handle all of the I/O operations for the mount, based on system resource and other criteria established for the selection and purpose the flow also proceeds to step  550 . In step  550 , the virtual tape controller  4  completes the mount operation by notifying the host  2   a,    2   b  that issued the mount command that the mount request has been successfully satisfied and that I/O operations to the selected logical volume can proceed.  
         [0035]      FIG. 6  illustrates an alternative data flow of an alternative embodiment in which the constructs for a logical volume were provided by a user through a user interface such as the library manager  14   a,    14   b  control console  18   a,    18   b  and retained in the construct management database  100  on DASD  16   a,    16   b.  In step  600 , the host  2   a,    2   b  transmits a mount command to the virtual tape controller  4 . The mount command indicates a specific logical volume is to be used for the mount request. In contrast with the embodiment described in  FIG. 5 , the host  2   a,    2   b  does not provide constructs to associate with the logical volume. In step  605 , the virtual tape controller  4  passes the mount command through a selected virtual tape server  6   a  or  6   b  to its associated library manager  14   a  or  14   b.  In step  610 , the library manager  14   a  or  14   b  associated with the selected virtual tape server  6   a  or  6   b  processes the mount command, processing which includes indicating in the volume database  100  that the specified volume, identified by volume ID  102 , is in use. In step  615 , the library manager  14   a  or  14   b  associated with the selected virtual tape server  6   a  or  6   b  obtains the storage management actions for each of the constructs  104 ,  106 ,  108  and  110  previously assigned to the specified volume ID  102  and passes them back to virtual tape controller  4  which retains them. In step  620 , the virtual tape controller  4  passes the mount command through to the other virtual tape server  6   a  or  6   b,  and thus to its associated library manager  14   a  or  14   b.  In step  625 , the library manager  14   a  or  14   b  associated with the other virtual tape server  6   a  or  6   b  processes the mount command, processing which includes indicating in the volume database  100  that the specified volume, identified by volume ID  102 , is in use. In step  630 , the library manager  14   a  or  14   b  associated with the other virtual tape server  6   a  or  6   b  obtains the storage management actions for each of the constructs  104 ,  106 ,  108  and  110  for the volume ID  102  and passes them back to virtual tape controller  4  which retains them. Referring now to step  635 , the virtual tape controller  4 , using the retained storage management actions passed from the library manager  14   a  or  14   b  associated with the selected virtual tape server  6   a  or  6   b,  specifically using the storage management action I/O VTS  306  for the associated management class  108 , determines whether a specific virtual tape server is to be used or if the virtual tape controller  4  is to select the virtual tape server for all I/O operations for the duration of the mount. If the I/O VTS  306  action specified one of the virtual tape server IDs, for example “VTS  6   a”,  then the flow proceeds to step  640 . If the I/O VTS  306  action specified “VTC Selected”, then the flow proceeds to step  645 . Referring back to step  640 , virtual tape server  6   a  is shown as having been selected to handle all of the I/O operations for the mount, based on the specified I/O VTS  306  action of “VTS  6   a”  and the flow proceeds to step  650 . Alternatively, referring to step  645 , if the virtual tape controller  4  is to determine which virtual tape server  6   a,    6   b  is to be selected to handle all of the I/O operations for the mount, based on system resource and other criteria the selection is made and the flow also proceeds to step  650 . In step  650 , the virtual tape controller  4  completes the mount operation by notifying the host  2   a,    2   b  that issued the mount command that the mount request has been successfully satisfied and that I/O operations to the selected logical volume can proceed.  
         [0036]      FIG. 7  is a flow diagram which illustrates one manner in which the virtual tape controller  4  can process host I/O operations. Control begins at step  700 , when a host computer  2   a  or  2   b  requests a mount of a logical volume and the steps previously described in the embodiment of  FIG. 5  or the alternative embodiment of  FIG. 6  are performed including notifying the requesting host that the mount has been completed. The host issues an I/O operation to the virtual tape controller  4  at step  705 . At step  710 , the virtual tape controller  4  determines whether the I/O operation is a read, write or volume close type operation. If it is a read or write operation, the virtual tape controller  4 , at step  715 , further determines whether the operation is a write operation. If the operation is a read operation, the read occurs and the flow proceeds back to step  705  with the virtual tape controller  4  waiting for the next host I/O operation. If the operation is a write operation, the virtual tape controller  4 , at step  720  determines whether the copy flag  54  is “on” in the volume&#39;s token  50 . If the copy flag  54  is “on” then the write occurs and the flow proceeds back to step  705  with the virtual tape controller  4  waiting for the next host I/O operation. If the copy flag  54  is not “on”, then at step  725  the virtual tape controller  4  turns the flag “on” and also increments the data level  58 . By verifying that a copy flag is turned on after every write operation, the virtual tape controller  4  ensures that a newly written or modified volume is marked for copying. Next, at step  730 , using the retained storage management actions passed from the library manager  14   a  or  14   b  associated with the selected virtual tape server  6   a  or  6   b,  and specifically using the storage management action copy mode  304  for the associated management class  108 , the virtual tape controller  4  determines if copy mode  304  is set to “inhibit”. If the copy mode  304  is set to something other than “inhibit” then the flow proceeds back to step  705  and the virtual tape controller  4  waits for the next host I/O operation. If the copy mode  304  is set to “inhibit” then at step  735  the virtual tape controller  4  turns the inhibit copy flag  56  “on” in the volume&#39;s token  50  and then the flow proceeds back to step  705  and the virtual tape controller  4  waits for the next host I/O operation. If, at step  710 , the I/O operation was a volume close, the virtual tape controller  4 , at step  740 , determines if the copy flag  54  is “on” in the volume&#39;s token  50 . If the copy flag  54  is “on”, then at step  745  the virtual tape controller  4  determines if the inhibit copy flag  56  is “on” in the volume&#39;s token  50 . If the inhibit copy flag  56  is not “on”, at step  750  the virtual tape controller  4  places a copy operation for the logical volume identified by the volume ID  50  in the queue for pending copy operations, and the flow continues to step  755 . If, at step  740 , the virtual tape controller  4  determined that the copy flag  54  was not set to “on” the flow proceeds to step  755 . If, at step  745 , the virtual tape controller  4  determined that the inhibit copy flag  56  was set to “on” the flow proceeds to step  755 . In such a case, at step  755 , the virtual tape controller  4  performs a close operation for the volume without placing a copy operation in the queue for pending copy operations.  
         [0037]     In addition to determining whether a newly created or modified logical volume is to be copied in the steps detailed in  FIG. 7 ,  FIG. 8  illustrates the logic implemented in the virtual tape controller  4  to establish a list of logical volume that are to be copied when a) the peer-to-peer virtual tape server system  1  is powered on, b) when one of the virtual tape servers  6   a  or  6   b  is available to the system after being unavailable due to a failure, maintenance or upgrade, or c) as a periodic background task. As part of determining whether a logical volume is to be copied or not, the state of the copy flag  54  and inhibit copy flag  56  are examined. Beginning with step  800 , the virtual tape controller  4  obtains the contents of the token database  10   a  from virtual tape server  6   a.  In step  805 , the virtual tape controller  4  obtains the contents of the token database  10   b  from virtual tape server  6   b.  The above described order in which the contents of the token databases are obtained from their associated virtual tape server is not important. In step  810 , the virtual tape controller  4  selects a volume token  50  from the contents obtained from each of the token databases  10   a  and  10   b  for a volume ID  52 . In step  815 , the virtual tape controller  4  determines if the copy flag  54  in either volume token  50  is “on”. If “on” the flow proceeds to step  820 . If the copy flag  54  is not “on” in either volume token, the flow proceeds to step  830 . Now referring to step  820 , the virtual tape controller  4  examines the inhibit copy flag  56  to see if it is set to “on” in either volume token  50 . If the inhibit copy flag  56  is “on” for volume ID  52  in either token, the logical volume is not to be copied and the flow proceeds to step  830 . If the inhibit copy flag  56  is not “on” in either volume token  50 , flow proceeds to step  825 . In step  825 , a copy operation for the logical volume identified by the volume ID  50  is placed in the queue for pending copy operations, and the flow continues to step  830 . Now referring to step  830 , the virtual tape controller  4  determines whether volume tokens  50  for all volume IDs  52  have been examined. If not all of the volume tokens  50  have been examined, control passes back to step  810 . If the volume tokens  50  have all been examined, the logic flow is terminated in step  835 .  
         [0038]     The illustrated logic of  FIGS. 4-8  show certain events occurring in a certain order. In alternative embodiments, certain operations may be performed in a different order, modified, or removed. Moreover, steps may be added to the above described logic and still conform to the described embodiments. Further, operations described herein may occur sequentially or certain operations may be processed in parallel. Yet further, operations may be performed by a single processing unit or by distributed processing units.  
         [0039]     The described techniques for selective dual copy control of data storage and copying may be implemented as a method, apparatus or article of manufacture using standard programming and/or engineering techniques to produce software, firmware, hardware, or any combination thereof. The term “article of manufacture” as used herein refers to code or logic implemented in hardware logic (e.g., magnetic storage medium such as hard disk drives, floppy disks, tape), optical storage (e.g., CD-ROMs, optical disks, etc.), volatile and non-volatile memory devices (e.g., EEPROMs, ROMs, PROMs, RAMs, DRAMs, SRAMs, firmware, programmable logic, etc.). Code in the computer readable medium is accessed and executed by a processor. The code in which implementations are made may further be accessible through a transmission media or from a file server over a network. In such cases, the article of manufacture in which the code is implemented may comprise a transmission media such as network transmission line, wireless transmission media, signals propagating through space, radio waves, infrared signals, etc. Of course, those skilled in the art will recognize that many modifications may be made to this configuration without departing from the scope of the implementations and that the article of manufacture may comprise any information bearing medium known in the art.  
         [0040]     The foregoing description of various implementations of the invention has been presented for the purposes of illustration and description. It is not intended to be exhaustive, nor to limit the invention to the precise form disclosed. Many modifications and variations are possible in light of the above teaching. It is intended that the scope of the invention be limited not by this detailed description, but rather by the claims appended hereto. The above specification, examples and data provide a complete description of the manufacture and use of the composition of the invention. Since many embodiments of the invention can be made without departing from the spirit and scope of the invention, the invention resides in the claims hereinafter appended.  
         [0041]     The objects of the invention have been fully realized through the embodiments disclosed herein. Those skilled in the art will appreciate that the various aspects of the invention may be achieved through different embodiments without departing from the essential function of the invention. The particular embodiments are illustrative and not meant to limit the scope of the invention as set forth in the following claims.