Patent Publication Number: US-6910098-B2

Title: Method and apparatus for maintaining data coherency

Description:
This application is continuation of 09/981,058 filed on Oct. 16, 2001, now U.S. Pat. No. 6,543,001 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention generally relates to the storage of data for use in data processing systems. More particularly, this invention relates to maintaining data integrity and consistency in redundant storage systems. 
     2. Description of Related Art 
     Nearly all data processing system users are concerned with maintaining back-up data in order to insure continued data processing operations should their data become lost, damaged or otherwise unusable. Such back-up operations can be achieved through a variety of procedures. In one approach, copies of data on a primary storage device are made on the same or other media such as magnetic tape to provide an historical backup. Typically, however, these systems require all other operations in the data processing system to terminate while the backup is underway. 
     More recently disk redundancy has evolved as an alternative or complement to historical tape backups. Generally a redundant system uses two or more disk storage devices to store data in a form that enables the data to be recovered if one disk storage device becomes disabled. For example, a first disk storage device stores the data and a second disk storage device mirrors that data. Whenever a transfer is made to the first disk storage device, the data also transfers to the second disk storage device. Typically separate controllers and paths interconnect the two disk storage devices to the remainder of the computer system. One advantage of this type of system is that the redundant copy is made without interrupting normal operations. 
     Several systems have been proposed for providing concurrent backups to provide the advantage of a tape backup without interrupting normal operations. For example, U.S. Pat. No. 5,212,784 to Sparks discloses an automated concurrent data backup system in which a central processing unit (CPU) transfers data to and from storage devices through a primary controller. The primary controller connects through first and second independent buses to first and second mirrored storage devices respectively (i.e., a primary, or mirrored, storage device and a secondary, or mirroring, storage device). A backup controller and device connect to one or more secondary storage devices through its bus. Normally the primary controller writes data to the primary and secondary data storage devices. The CPU initiates a backup through the primary controller. In response the backup controller takes control of the second bus and transfers data from one secondary data storage device to the backup media. Applications continue to update the primary and any additional secondary storage devices. After a backup operation is completed, the primary controller resynchronizes the storage devices by updating the secondary storage device that acted as a source for the backup with any changes that occurred to the primary data storage device while the backup operation was underway. 
     U.S. Pat. Nos. 5,241,668 and 5,241,670 to Eastridge et al. disclose different aspects of concurrent backup procedures. In accordance with these references a request for a backup copy designates a portion of the stored data called a “dataset”. For example, if the data storage devices contain a plurality of discrete data bases, a dataset could include files associated with one such data base. In a normal operation, the application is suspended to allow the generation of an address concordance for the designated datasets. Execution of the application then resumes. A resource manager manages all input and output functions between the storage sub-systems and associated memory and temporary memory. The backup copy forms on a scheduled and opportunistic basis by copying the designated datasets from the storage subsystems and updating the address concordance in response to the copying. Application updates are processed during formation of the backup copy by buffering the updates, copying the effected uncopied designated datasets to a storage sub-system memory, updating the address concordance in response to the copying, and processing the updates. The designated datasets can also be copied to the temporary storage memory if the number of designated datasets exceeds some threshold. The designated datasets are also copied to an alternate memory from the storage sub-system, storage sub-system memory and temporary host memory utilizing the resource manager and the altered address concordance to create a specified order backup copy of the designated datasets from the copied portions of the designated datasets without user intervention. 
     Still referring to the Eastridge et al. patents, if an abnormal event occurs requiring termination of the backup, a status indication is entered into activity tables associated with the plurality of storage sub-systems and devices in response to the initiation of the backup session. If an external condition exists that requires the backup to be interrupted, the backup copy session terminates and indications within the activity tables are reviewed to determine the status of the backup if a reset notification is raised by a storage sub-system. This enables the determination of track extents which are active for a volume associated with a particular session. A comparison is then made between the track events which are active and volume and track extents information associated with a physical session identification. If a match exists between the track extents which are active and the volume of and track extent information associated with a physical session identification, the backup session resumes. If the match does not exist, the backup terminates. 
     U.S. Pat. No. 5,473,776 to Nosaki et al. discloses a concurrent backup operation in a computer system having a central processing unit and a multiple memory constituted by a plurality of memory devices for on-line storage of data processed by tasks of the central processing unit. A data backup memory is provided for saving data of the multiple memory. The central processing unit performs parallel processing of user tasks and a maintenance task. The user tasks include those that write currently processed data into the multiple memory. The maintenance task stops any updating of memory devices as a part of the multiple memory and saves the data to a data backup memory. 
     More recently the concept of redundancy has come to include geographically remote data facilities. As described in U.S. Pat. No. 5,544,347 to Yanai et al. for Remote Data Mirroring and U.S. Pat. No. 5,742,792 to Yanai et al. for Remote Data Mirroring (both assigned to the assignee of this invention), a computer system includes one or more local and one or more remote data facilities. Each local and remote data facility typically includes a data processing system with disk storage. A communications path, that may comprise one or more individual communications links, interconnects a local storage facility with a remote storage facility that is a mirror for the local storage facility. The physical separation can be measured in any range between meters and hundreds or even thousands of kilometers. In whatever form, the remote data facility provides data integrity with respect to any system errors produced by power failures, equipment failures and the like. 
     In prior art systems one dataset normally is stored in a single storage facility, so data consistency has been achieved whenever the remote storage facility exactly mirrors the local storage facility; i.e, the two facilities are in synchronism. Generally if a communications path comprising one or more communications links, fails (i.e., no data can be transferred over any of the communications links), the dataset remains in the remote storage facility, but no longer is updated. This becomes particularly important when data must be recovered because without consistency or synchronism data in a dataset that has not yet reached the remote or backup facility may be lost. 
     U.S. Pat. No. 5,720,029 to Kern et al. discloses one approach for providing a disaster recover, system that utilizes a synchronous remote data shadowing to obtain a backup copy of data. A host processor at the primary, or local, site transfers a sequentially consistent order of copies of record updates to the secondary site for backup purposes. The copied record updates are stored on the secondary storage devices at the remote site that form remote copy pairs with the primary data storage devices. One track array, as an active track array, is used to set elements according to which tracks on the primary storage device receive record updates from the host processor at the primary site. The other track array, as a recovery track array, designates which record updates comprise the copy record updates currently transferred from the primary site to the secondary site for data shadowing and is used for recovery should an error interrupt the transfer. The track arrays are toggled once the consistency group transfer completes and a recovery track array becomes the active track array and the active track array becomes the recovery track array. 
     U.S. Pat. No. 5,649,152 to Ohran et al. discloses another method and system for providing a static snapshot of data stored on a mass storage system. In accordance with this approach a preservation memory is provided and a virtual device is created in that preservation memory. Whenever a write operation is to be performed on the mass storage system, a check is made of the preservation memory to determine if it contains a block associated with the mass storage write device. If no block is present, a copy of the block in the mass storage system at the block write address is placed in the preservation memory. Whenever a read is to be performed on the virtual device, a check is made of the preservation memory to determine if it contains a block associated with the virtual device read address. If a block exists, that block is returned in response to the read operation. Otherwise, a block at the virtual device block read address is returned from the mass storage device. 
     U.S. Pat. No. 5,680,580 to Beardsely et al. discloses a remote copy system that incorporates dynamically modifiable ports on storage controllers such that those ports can operate either as a control unit link-level facility or as a channel link-level facility. When configured as a channel link-level facility, a primary storage controller can appear as a host processor to a secondary storage controller. The primary storage controller can thereafter initiate multiple request connects concurrently for servicing a single I/O request. In this manner, a first available path can be selected and system throughput is improved. In this system host write commands at the primary storage controller are intercepted for a remote dual copy process. As a result of the intercept, the system determines whether a unit check write I/O flag is set. If it is not set, data is written to the primary cache or MVS and thereafter to the primary device. Once the data is stored at the primary storage controller, a connection is established to the secondary storage controller to allow a remote copy to proceed to transmit the data to the secondary storage controller. 
     Each of the foregoing references describes a different method of obtaining a backup and particularly addresses data consistency as between a specific storage controller and its backup facility whether that facility comprises a magnetic disk or tape device. The broad or basic object of these patents, particularly the Ohran et al. and Kern et al. patents, is to provide a method of tracking any changes that are in transit so that a disaster recovery will identify those items that need to be recovered. 
     Now storage facilities using redundancy including remote data facilities have become repositories for large databases. Recently, these databases and other types of datasets have grown to such a size that they are distributed across multiple independent storage controllers or facilities. This has a led to a new definition of data consistency. In the following description we use “synchronism” in a conventional context and “consistency” in a modified context to account for such distributed datasets. As between a single storage controller and a single backup facility, such as disclosed in the foregoing Yanai et al. patents, the storage devices are in synchronism when the data at the local site corresponds exactly to the data on a secondary storage facility coupled by a single communications path. When multiple independent communications paths are involved with the transfer of data in different portions of a dataset, such as the journal log file and the data base, and the transfer of data over one path is interrupted, the remote storage facility associated with that communications path loses synchronism. In addition, even though other remote sites may remain in synchronism, the data across the remote storage facilities storing the dataset will no longer be consistent. If this occurs, the remotely stored dataset becomes corrupted. Conversely, if data transfers can occur over all the communications paths associated with a dataset and all the corresponding remote storage facilities are in synchronism with their local storage facility counterparts, the dataset is consistent. Consequently, what is needed is a method and apparatus for enabling a user to be assured that the data at the remote data facilities in such multiple communications path configurations is consistent, even when data can not be transferred across one or more communications paths. 
     SUMMARY 
     Therefore it is an object of this invention to provide a method and apparatus for assuring consistency of data at one or more remote sites coupled to one or more local sites by multiple communications paths. 
     Another object of this invention is to provide such data consistency at a remote site transparently to any user application. 
     Still another object of this invention is to provide such data consistency to a remote site with minimal impact on other data processing operations. 
     In accordance with this invention, a host interacts with a first dataset copy. Transfers to a second dataset copy occur over multiple independent communications paths. If a transfer over one of the independent communications paths, is not efficacious, all transfers from the first to the second dataset copy over all the independent paths are terminated. However, operations between the host and the first dataset copy continue. When the cause of the transfer interruption is corrected, transfers to the second dataset copy over all the independent communications paths resume. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The appended claims particularly point out and distinctly claim the subject matter of this invention. The various objects, advantages and novel features of this invention will be more fully apparent from a reading of the following detailed description in conjunction with the accompanying drawings in which like reference numerals refer to like parts, and in which: 
         FIG. 1  is a block diagram of a data processing system adapted for benefitting from this invention; 
         FIG. 2  depicts an alternative embodiment of a data processing system that can benefit from this invention; 
         FIG. 3  depicts another alternative embodiment of a data processing system that can benefit from this invention; 
         FIG. 4  depicts the organization of a local host as shown in any of  FIGS. 1 through 3 ; 
         FIG. 5  depicts a particular data structure that is useful in implementing this invention; 
         FIG. 6  depicts an embodiment of an initialize module useful in implementing this invention; 
         FIG. 7  depicts an embodiment of control blocks useful in accordance with the embodiment of  FIG. 5 ; 
         FIG. 8  depicts a status table useful in one embodiment of this invention; 
         FIG. 9  is a flow diagram of a monitor module shown in  FIG. 1 ; 
         FIG. 10  is a chart depicting the general operation of this invention in the context of a specific digital computer operating system; 
         FIG. 11  is a flow diagram of a unit check module shown in  FIGS. 1 and 10 ; 
         FIG. 12  is a flow diagram of a module that responds to the receipt of a unit check sense from the module of  FIG. 10 ; and 
         FIG. 13  depicts the operation of a module that suspends operations to storage devices. 
     
    
    
     DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS 
     There are many possible examples of data processing system configurations adapted for providing data redundancy within the context of this invention.  FIG. 1  depicts one such configuration in which local hosts  20  and  21  connect to a local storage facility with local storage controllers  22  and  23 . All transactions required by the local hosts  20  and  21  are then carried out with the local storage controllers  22  and  23 . In this particular embodiment a remote storage controller  24  connects to the local storage controller  22  over a communications path  25  that, as previously described, may contain multiple communications links. The structure of the local storage controller  22 , remote storage controller  24  and communications path  25  are as depicted in the U.S. Pat. No. 5,742,792. In essence the remote storage controller  24  maintains a copy of some or all of the data stored in the local storage controller  22 . The controllers  22  and  24  normally operate to maintain the remote storage controller  24  in synchronism with the local storage controller  22 . 
     A communications path  26 , like the communications path  25 , connects the local storage controller  23  to a remote storage controller  27 . In accordance with the prior discussion, the remote storage controller  27  acts as a mirror for some or all of the data in the local storage controller  23 . 
     With this configuration remote hosts  28  and  29  are connected to communicate with both remote storage controllers  24  and  27 . As the communications paths  25  and  26  can extend for many miles a disaster at the local facility will not interrupt operations at the remote facility whereupon the remote hosts  28  and  29  interact with the data in the remote storage controllers  24  and  27 . 
     As previously indicated, it now is possible for all the data in a single dataset, such as a dataset in the form of a database, to be so large as to be stored in a plurality of local storage controllers. Alternatively it is possible for such a dataset to be stored in a single local storage controller and mirrored in multiple remote storage controllers. In whatever form, in such systems redundancy is achieved with one or more remote storage controllers are connected to one or more local storage controllers through plural independent communications paths. These paths are subject to failure or interruption for any number of reasons including failures of third-party communications systems, failure of some portion of a remote storage controller or related electronics or even removal of an individual storage device from a remote storage controller. 
     With prior art embodiments, if communications over one path are interrupted, such as the path  26 , communications will continue with the remote storage controller  24  over the communications path  25 . Consequently, the remote storage controller  24  will remain in synchronism with the local storage controller  22 , but the remote storage controller  27  will lose synchronism with the local storage controller  23 . Consequently the data for the entire dataset will be inconsistent. 
       FIG. 2  depicts in block form another configuration wherein two remote storage controllers mirror a dataset contained in a single local storage controller. More specifically, local hosts  30  and  31  connect to a local storage controller  32 . A remote storage controller  34  mirrors a portion of the dataset in the local storage controller  32 , such as a journal log file in a database application, over a communications path  35 ; a second communications path  36  enables a remote storage controller  37  to mirror the other portion of the dataset in the local storage controller  32 , such as the database. In this configuration and with an interruption in the communications path  35 , the remote storage controller  37  continues to operate and mirror the corresponding dataset portion while the data in the remote storage controller  34  remains frozen at a point earlier in time. The database distributed over the remote storage controllers  34  and  37  site therefore no longer is consistent after a first write to the remote storage controller  34  fails to be completed. 
       FIG. 3  depicts another embodiment in which a single remote storage controller mirrors a dataset distributed over two local storage controllers. More specifically, a local host  40  has access to the dataset distributed over a local storage controller  42  and local storage controller  43 . A remote storage controller  44  has two independent communications paths  45  and  46  for allowing the remote storage controller  44  to mirror the dataset portions in the local storage controllers  42  and  43 , respectively. If transfers over the communications path  46  are interrupted, only those changes made to portions of the dataset in the local storage controller  42  will be reflected in the remote storage controller  44 . Again, the copy of the dataset in the remote storage controller  44  will not be consistent. 
     In general terms, this invention prevents such inconsistency automatically and transparently to any user. Each local storage controller monitors the ability of a communications path and remote storage controller to update data. If the monitoring indicates such updates are not possible, a special error signal is generated. The operating system in the local host, modified in accordance with this invention, processes that special error signal by suspending all further transfers over all the independent communications paths related to the dataset until the problem is corrected. When this occurs, all synchronism is lost between individual pairs of local and remote storage controllers. However, as the dataset copy at the remote site remains in a fixed consistent state so that the dataset remains usable. 
     For purposes of a further and more detailed understanding of this invention,  FIG. 1  depicts a particular embodiment of a data facility incorporating magnetic disk storage controllers of a type provided by the assignee of this invention and described in the foregoing Yanai et al. patents. The communications path  25  includes electronic transmission media that may include any known high-speed data communications link such as a link provided by fiber optics, T 1  and T 3  telecommunications links and the like. A remote adapter (RA)  50  resides in the local storage controller  22  and a corresponding RA  51  resides in the remote storage controller  24  to control all the links in a specific communications path. 
     Referring to the remote storage controller  24 , the RA  51  connects to a bus  52  that communicates with host adapters  53  and  54  connected to remote hosts  28  and  29  respectively. A system memory  55 , commonly called a cache memory, also connects to the bus  52 . Data storage is provided by a device controller (DC)  56  that connects to one or more physical storage devices  57 . Another device controller  58  attaches to a plurality of physical storage devices  59 . During normal operation the remote storage controller  24  mirrors the local storage controller  22  as known in the art and described in the above-identified Yanai et al. patents. A further understanding of the operation details can be attained by referring to these references. 
     As also shown in  FIG. 1 , the local storage controller  23  includes a remote adapter  60  that communicates over the communications path  26  with a remote adapter  61  in the remote storage controller  27 . These controllers are similar to the storage controllers  22  and  24  so no additional discussion of their structures is necessary. 
     The hosts shown in  FIG. 1  operate utilizing some commercially-available operating system, such as the IBM MVS operating system. The local host  20  in such an MVS environment includes a plurality of CPs.  FIG. 4  depicts two control processors CP( 1 ) and CP(n) identified by reference numerals  62  and  63 , respectively, by way of example. They communicate with a main storage unit  64  that, as known, is divided into private, common, and extended private storage areas. A console device  65  permits an operator to communicate with the system for performing a number of configuration, diagnostic and other procedures independently of operations that occur in response to any application programs. 
     When any application program is loaded into the system, the MVS operating system provides an address space for that program.  FIG. 4  depicts an address space  66  for a user application (APPL) program and an address space  67  assigned for a consistency group program that incorporates this invention. As shown in  FIG. 5  the CGROUP address space  67  in storage unit  64  includes an INITIALIZE module  70  that establishes various data structures and loads the remaining program into the system. The INITIALIZE module also modifies the MVS system to route responses to particular events to software included in the consistency group program  67 . Control blocks  71  contain information about the physical storage devices (hereinafter devices) that are organized into a consistency group and related status information. The functional modules of this system include an ENABLE module  72  that enables the local controller to monitor each writing operation. A DISABLE module  73  terminates the monitoring operation. When transfers over a communications path are interrupted for any reason, a SUSPEND module  74  suspends further transfers to all the devices in the consistency group. After corrections are made, a RESUME module  75  enables the remote storage controllers to be brought into synchronism and for the testing for consistency to resume. 
       FIG. 6  depicts the INITIALIZE module  70  in more detail. This module is processed after the consistency group program is loaded in the address space. As a first operation, step  77  stores a control block, such as control block  71 , for each such consistency group in the allocated common storage area. As shown in  FIG. 7 , control blocks  71  are stored in a table having an initial location designated by a specific subsystem control block (SSCT). With respect to  FIG. 7 , the SSCT for the consistency group contains a pointer (SSCT ‘CGRP’) to the control blocks  71 . The CGLB contents provides a header function. The next location includes the name of one consistency group. In this particular embodiment it is identified as a CGRP 1  consistency group. The CGLB location contains a pointer to the CGRP 1  location as first location. The CGRP 1  location, in turn, contains a first pointer to the next consistency group name; in this particular embodiment that is the location for the CGRP 2  consistency-group. The CGRP 1  location also references a second pointer to a CGC entry that represents one local controller within the CGROUP. Thus, if one consistency group includes a dataset distributed over the local storage controllers  22  and  23 , the CGC entries identify the local controllers  22  and  23 . The CGC entry also contains a pointer to the next CGC entry with the last entry being a pointer to a null location that acts as an end-of-list code. 
     Control blocks  71  also includes a CGD area that is an index of all devices in all consistency groups sorted in device (CUU) order. Each location has a pointer to its corresponding consistency group. 
     Once this control block data  71  has been produced, step  78  attaches a subtask for each consistency group with each subtask waiting on an event control block (ECB). The process of attaching subtasks is a standard procedure implemented in the MVS operating system. 
     Normally the MVS operating system includes an EOS exit routine for responding to particular events. In accordance with this invention, step  79  loads an EMC-EOS exit routine into a page fixed storage area. Step  80  then creates a copy of the standard device descriptor table (DDT) provided in the MVS operating system. Step  81  modifies that copy by replacing a pointer to the standard EOS exit routine or procedure by a, pointer to the EMC-EOS exit routine. For each device in the consistency group, step  82  loads a pointer in a corresponding UCB to the modified data descriptor table. 
     Next the INITIALIZE module uses step  83  to construct a “suspend” channel program for each controller within each consistency group. That is, if a consistency group spans n controllers, step  83  will generate n suspend channel programs. When a suspend channel program runs, it will identify each device of the corresponding controller within the corresponding consistency group. 
     When this action is completed, step  84  issues a “CGROUP ENABLE” call to enable all the listed consistency groups in step  84 . 
     Referring again to  FIG. 1 , each local storage controller includes a status table, such as a status table  85  in the local storage controller  22 .  FIG. 8  depicts elements of such a table that are important to an understanding of this invention. One or more RAE flags indicate whether any storage device attached to a corresponding remote adapter is in a consistency group. For example, the RAE( 1 ) flag  86  indicates whether the remote adapter  50  in  FIG. 1  was attached to a device in any consistency group. As shown in  FIG. 8 , there may be more than one RAE flag in a local storage controller. The “GROUP ENABLE” call identifies each remote adapter in a consistency group and uses the ENABLE module  72  in  FIG. 5  to produce a conventional I/O request procedure modified to set the RAE( 1 ) flag  86  for each such remote adapter. The DISABLE module uses an analogous procedure for clearing each RAE flag whenever none of the devices attached to a remote adapter is in any consistency group. This means the RAE flag associated with each remote adapter, such as the RAE flag  86  for remote adapter  50 , when set, indicates that at least one device in communication with that remote adapter is in a consistency group that is enabled. 
     Still referring to  FIG. 8 , the status table  85  also contains information for each storage device in a remote storage controller with which the local storage controller communicates through any remote adapter. For example, a register is associated with each such device. These are designated as registers  87 A through  87 D for devices  0 ,  1 , n−1 and n, respectively. As each has the same structure, only the register  87 A associated with Device  0  is discussed in detail. Register  87 A includes a remote status (RS) flag  88  and a series of path group notification (PGN) flags  89 . The functions of these flags is described later. 
     Step  84  completes the operation of the INITIALIZE module  70 . There is no further activity within the consistency group program until a communications path interruption is detected. 
     When a consistency group is enabled in step  84  of  FIG. 6 , unit check modules, such as unit check modules  90  and  91  in  FIG. 1 , are also enabled to monitor communications path status during each input/output request. Specifically the unit check module  90  will generate a unique code at any time it is determined that a one of the storage devices, such as any one of storage devices  57  and  59 , will not be able to transfer data to the corresponding remote storage controller, such as remote storage controller  24 , in response to a write request. 
     For a better understanding of this invention, it will be helpful to discuss other features of this system particularly the method by which a monitor in each remote adapter, such as a monitor module  92  in remote adapter  50 , maintains a current status in a remote status table, such as the remote status table  85  associated with the remote adapter  50 . The module for maintaining the current status of the remote status table  85  is depicted in FIG.  9 . This is a generally adopted approach used in storage controllers of the assigned of this invention. 
     Specifically, the operation of the monitor  92  as depicted in  FIG. 9  assumes that, at step  94 , the initial state of remote status (RS) flag for each communications path and each storage device connected to that communications path is valid. RS flags, such as the RS flag  88  in  FIG. 8 , reflect this state. Thus, the remote status table  85  will identify the validity of the combination of the path and a storage device, such as a storage device in the remote storage controller  24  attached to the remote adapter  50 . The storage device may be represented as a physical storage device or a logical storage device (i.e., a logical volume) depending upon the particular configuration of the remote storage controller. In storage controllers provided by the assignee of this invention, logical volumes constitute logical storage devices. The phrase “storage device” is meant to cover all such definitions as may be pertinent to a particular implementation of the invention on a specific storage controller. 
     In  FIG. 9  a monitor, such as monitor  92 , operates iteratively using step  95  to select a communications path, such as communications paths  25  or  26  in FIG.  1 . In step  96  the monitor  92  selects a storage device, such as a logical volume associated with one of the physical storage devices  57  or  59  attached to the selected communications path. In step  97  the monitor  92  performs various conventional tests that determine, among other things, whether the path to that particular storage device is valid and therefore whether it is likely that a request will be made successfully to that device. 
     As previously stated, there are a number of reasons why such a test would not produce a valid response. A particular storage device may have been removed for maintenance or be inoperative. The system operator may have turned off a series of storage devices for maintenance. All the communications links that constitute the path  25  have been interrupted. An external cause may have rendered the remote storage controller  24  to be inoperable while the remote storage controller  28  at perhaps a separate physical location continues to operate. 
     Whatever the reason, the monitor  92  makes that determination in step  97 . If the status is satisfactory, step  98  branches to step  99  and no subsequent action is taken. If additional storage devices connected to the selected communications path must be tested, step  99  passes control to step  96 ; otherwise control passes to step  95 . 
     If the step  97  determines that a particular communications path and storage device combination is not efficacious, step  98  branches to step  100 . In this step the monitor changes the remote status flag for that specific communications path-storage device combination to an invalid state. Normally the system will then use step  101  to generate an error report to the system operator. 
     In essence, the procedure set forth in  FIG. 9  constantly monitors the efficacy of the paths from a remote adapter, such as the remote adapter  50 , to each of the storage devices (physical or logical) in the corresponding remote storage controller and maintains the status of each path and each communications path-storage device combination in the remote status table, such as the remote status table  85 . 
     As previously stated,  FIG. 9  depicts a continuing, repetitive, asynchronous process. It will be apparent that this process can be initiated in other ways. For example, the program may be operated asynchronously, but periodically in response to a tinier signal. Alternatively the process might be initiated by the occurrence of an event, such as a system operator making a change in the local storage controller. In still other systems it might be appropriate to use some combination of the foregoing approaches or still some other approach that will cause a periodic test of the communications path and storage device to identify the inability of a remote storage device to receive data in an expeditious and timely fashion. 
       FIG. 10  depicts the manner in which the embodiment of  FIGS. 1 and 4  through  7  operates in response to a communications path interruption after the consistency group program is enabled. A particular sequence begins when an application program, such as the APPL program  66  in  FIG. 4 , issues a write request shown at  110  in FIG.  9 . The MVS operating system responds to that write request by establishing an I/O request at  111  and dispatching the I/O request at  112  according to conventional MVS operating procedures. The I/O request is directed to one of the local storage systems containing distributed dataset. At  113  in  FIG. 10  the local storage controller receives the I/O request. For write I/O requests, the local storage controller completes a local write at step  114 . 
     At  115  the local storage controller processes the write request for transfer to a designated remote storage controller.  FIG. 11  depicts a module for performing this process. This module is replicated in each local storage controller. First, the module uses step  116  to retrieve the status (RS) flag for the device identified in the write request; e.g. the RS flag  88  if Device  0  is the addressed device. If the RS flag has a valid state, step  117  transfers control to step  118 . In the context of  FIG. 10 , this represented as the step of enabling the transfer and the processing of that transfer at  119 . The process at,  119  ends with the generation of a response indicating the success or failure of the operation at the remote storage controller. 
     Referring again to  FIG. 11 , if a transfer is completed successfully, step  120  transfers control to step  121  thereby to transmit a response in the form of an acknowledgment signal back to indicate the successful completion of the writing operation. 
     If the RS flag for the addressed device is in an invalid state indicating that for some reason the write operation can not be completed over the communication path to the identified storage device, step  117  transfers control to step  122 . 
     Likewise, if the response from the process  119  in  FIG. 10  indicates the operation was not successful, step  120  transfers control to step  123  that tests the RAE flag in the status flags  85  for the remote adapter, such as the remote adapter  50 , identified for transferring the data. As previously indicated, this indicates whether the remote adapter is even associated with any consistency group. If it is not, control transfers to step  121  to send an appropriate response in the form of an error signal indicating a failure of the write operation. If the RAE flag is in a valid state, however, control transfers to step  122 . 
     Step  122  tests the state of the corresponding PGN status bit for the path group being utilized for the write request. In an MVS environment a path group represents multiple paths or links that interconnect a local host, such as the local host  20  in  FIG. 1 , to a local storage controller. As known and also shown in  FIG. 1 , multiple local hosts, such as local hosts  20  and  21 , can connect to a single local storage controller, such as local storage controller  22 . In the specific example of  FIG. 1 , the local storage controller has two path groups  124  and  125  from the local hosts  20  and  21  respectively. 
     Step  122  retrieves the corresponding PGN flag for the addressed device. For example, if the write operation is to Device  0  from local host  20 , the PGN-A flag in the register  87 A is the corresponding PGN flag. A valid state indicates that no interruptions have occurred in the transfers to the remote storage controllers. 
     If step  122  determines that the corresponding PGN status is at an invalid state, one independent path between the local and remote storage controllers has failed previously. There is no need to issue another unit check code, so control passes to step  121 . If the corresponding PGN flag is at a valid state, this is a first failure in the path group. Step  126  thereupon transfers control to step  127  that generates a unique unit check code for transfer to the MVS operating system with the response of step  121 . Then step  128  changes the corresponding PGN flag to an invalid state. If a subsequent write operation from the same local host is received, step  126  diverts control to step  121  immediately so redundant generation of the unique unit check code is avoided. 
     As will be apparent, although the transfer of a response in step  121  has been shown in a single step, the exact nature of the response will depend upon the method of entry to step  121 ; that is, whether the entry to step  121  is from step  120 ,  123 ,  126  or  128 . 
     Still referring to  FIG. 10 , at  129  the local storage controller transfers the response to the MVS operating system and the MVS operating system begins to process the response in a conventional manner at  130 . 
     However, as will be recalled, the INITIALIZE module of  FIG. 6  has modified the MVS operating system to intercept these responses and transfer control to  131  in  FIG. 10  to process the response in the consistency group module.  FIG. 12  depicts the receipt of a response at  130  and transfer to the consistency group module  131  to monitor the acknowledgment signal to determine the presence of a special unit check sense at step  132 . If no such special unit check sense is included, control transfers to step  133  to process the acknowledgement signal and transfer control back to the MVS standard end of sense exit. 
     If the special unit check sense is received, control passes to step  134  that begins the suspend process. In step  134  the process retrieves the device identification-from the UCB. If this device is not in a consistency group, step  135  transfers control to step  133  to allow conventional processing of the end of sense exit by the operating system. Otherwise control passes to step  136 . The process uses this CUU to gain access to the appropriate consistency group through the control blocks  71  in  FIG. 7  in step  136 . 
     Next step  137  attempts to obtain a lock to serialize operations. If that process is successful, step  137  transfers control to step  140  whereupon the EMC EOS exit routine  130  raises the IOS level for each device in the consistency group. Raising the IOS level for each device in the consistency group assures normal I/O requests directed to a device can not be processed so long as the IOS level for that device is raised. 
     The subtask posted in step  141  is designed to cause a suspension of any further write operations to devices in the consistency group in the remote storage controllers. Thus, the subtask as posted will contain a list of all the devices obtained from the control blocks  71 . When the subtask is posted, the transfer of step  133  is made to the MVS end of sense exit. 
       FIG. 13  depicts the operation of the subtask posted at step  141  represented in  FIG. 10  at  142 . As will be apparent from  FIG. 12 , when this process begins it operates at the raised IOS level. Step  143  selects a local storage controller. Step  144  then selects a device in the consistency group to provide a basis for issuing the I/O request that will suspend transfers. This I/O request, depicted in step  145  sets the remote status (RS) flag for each storage device in the selected local storage controller and consistency group to an invalid state. That is, for the selected local storage controller, the RS flag, such as RS flag  88  associated with Device  0  is set to an invalid state. 
     Step  146  then sets to an inactive state all the PGN flags, such as the PGN flags in register  87 A for Device  0 , for all the storage devices in the selected local storage controller and in the consistency group. Step  147  then transfers control back to step  143  if other local storage controllers exist. 
     When the RS flags and PGN flags associated with all the storage controllers in the consistency group have been set to an invalid state, step  147  transfers control to step  148  to reset the IOS level for each device in the consistency group. Step  147  then unlocks the consistency group, and step  150  represents a procedure by which a message may be generated for transfer to the application, the system operator or both. 
     The foregoing procedures cooperate to maintain consistency under a wide variety of conditions. For example, if a single write operation causes suspension, a later write operation from the same MVS system will be prevented from reaching a remote storage controller because the RS flag and corresponding PGN flags will be set to an inactive state. 
     As another example, assume that multiple MVS systems can access a single consistency group over multiple path groups. Further assume that a write operation from a first MVS system has produced the specific unit check code. As previously indicated, step  146  in  FIG. 13  has set all the PGN flags to an inactive state for all the devices. Now assume a second MVS system issues a write operation to another device. When the module of  FIG. 11  processes that module, the tests at steps  117  and  126  will fail. Consequently, the module will not send a redundant unit check code. 
     Another possibility occurs if a second MVS systems issues a write request to another device over another path group with an invalid RS flag that is processed after the unit check code is generated in step  127  of FIG.  11  and the suspension produced by steps  145  and  146  in FIG.  13 . In that situation control passes from step  117  in  FIG. 11  to step  122  and from step  126  to step  127  because the PGN flag for that path will still be valid. Although this produces a redundant operation under normal conditions, it assures that the suspension occurs even if, for some reason, the response to the first write operation fails to suspend transfers to all the devices in the consistency group. 
     Conversely, the PGN flags prevent repeated unit check code processing in the operating system in other situations. If two paths should fail, only the first to fail will interrupt the operating system. The second will merely produce a response that will free the local write operation for completion if it occurs after steps  144  and  146  in  FIG. 3  are processed in response to the first failure. 
     If two write requests issue unit check codes essentially simultaneously, the locking process of  FIG. 12  serializes the operations. As previously indicated, the successful lock produces the suspension. As unsuccessful lock merely waits in step  151  of  FIG. 12  until the IOS level is raised. It then merely transfers an acknowledgment to the operating system so the I/O request can be completed at the local storage controller. 
     After the transfers to all the remote storage controllers in a group have been suspended, the local hosts continue to operate with the local storage controllers without interruption. However, no additional transfers occur with respect to the remote storage controller for the devices in the consistency group. Thus, even though synchronism is lost between a remote and local storage controller, the data in the remote storage controller remains unchanged and fixed in time, so it is consistent. 
     When the cause of the condition that interrupted the transfers is overcome, the system operator utilizes the console device  65  in  FIG. 4  to run the resume module  75 . The resume module performs two functions. First, it eliminates the suspension mode utilizing a similar process to that performed by the suspension module by resetting all the relevant RS and PGN flags to a valid state. In the specific embodiment of storage controllers as used by the assignee of this invention, the local storage controllers have identified all the data that has been modified since the suspension and not transferred to the remote storage controllers. Consequently, the resume module additionally enables a remote copy program to make those transfers and update all the data in the local remote storage controllers independently of the local or remote hosts. When the data is updated, synchronism is restored and the consistency operation may be enabled. 
     Thus, in accordance with several objects of this invention, there has been disclosed a method and apparatus for maintaining consistency of data organized to be distributed across multiple storage controllers connected to provide redundant copies over independent paths. When the transmission of data over any one path is interrupted, all subsequent transfers to the redundant copy are inhibited. Operations continue between the application program and the local storage controllers so that this sequence of events is transparent to the application and does not interrupt operations with the local copy of the data. As will be apparent, the consistency module is particularly useful in monitoring single write operations and dependent write operations that are common to operations involving databases. 
     This invention has been described in terms of a particular embodiment.  FIGS. 1 through 3  depict specific configurations of local and remote hosts and local and remote storage controllers; the balance specific implementations. It will be apparent that a number of variations can be made. Each of those figures additionally discloses a maximum plurality of two local or two remote storage controllers. Data may be distributed over more than two storage controllers. The foregoing description assumes that a single device can only be included in one consistency group. It will be apparent that modifications could be made to enable a single device (CUU) to be included in multiple consistency groups. The remote storage controllers typically will be physically removed from the local storage controllers. However, they may also be collocated with the local storage controllers. Even more modifications can be made to the disclosed apparatus without departing from the invention. Therefore, it is the intent of the appended claims to cover all such variations and modifications as come within the true spirit and scope of this invention.