Patent Publication Number: US-6983353-B2

Title: Method and apparatus for enhancing operations in disk array storage devices

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   This invention relates to data processing systems with disk array storage devices and more specifically to methods and apparatus that enhance operations in such disk array storage devices, particularly with applications that operate on such data processing systems and that produce dependent input/output sequences with disk array storage devices. 
   2. Description of Related Art 
   A conventional data processing system that handles large quantities of data generally includes one or more hosts and a disk array storage device, or DASD. Other systems may include multiple disk array storage devices. A host generally includes one or more control processors, main memory and input and output devices. A host executes programs, also called applications, that operate on data transferred to the main memory from the disk array storage devices as known in the art. Disk array storage devices, such as those manufactured and sold by the assignee of this invention, include many physical storage devices, or magnetic disk drives, organized as logical storage volumes or logical devices. 
   A disk array storage device operates with a host adapter or equivalent module that receives a variety of input/output commands from a host over a channel in a host dependent format. The host adapter translates those host input/output commands, or host requests, into a format that disk adapters, or equivalent structures, in the disk array storage device recognize and use to direct operations at a logical device level. When an operation completes in the disk array storage device, the host adapter transmits a status word or message to the host to report either the success of the operation or the reason for a failure. Two specific commands that are important to this invention are “read” and “write” commands. A read command from a host effects a transfer of data from the disk array storage device to the host; a write command, a transfer to the disk array storage device from the host. 
   Significant efforts have been made to enhance the operation and performance of disk array storage devices in response to read and write commands from primary applications, such as are used in airline or banking operations. Also, efforts have been directed to enabling disk array storage device operations that are ancillary to the primary applications. A backup application is one example of such an ancillary application. Particular emphasis has been placed upon enabling such ancillary applications to operate without interrupting any primary applications running on the host. For example, it is obviously desirable to allow a database backup without interrupting any of the transactions underway with the various users of a primary application. 
   U.S. Pat. No. 6,101,497 to Ofek for a Method and Apparatus for Independent and Simultaneous Access to a Common Data Set, assigned to the same assignee as this invention, discloses a concept for achieving such operations. In accordance with that disclosure, certain physical disk drives in a disk array storage device are configured to be available to the primary applications. These are called “standard devices”. Other logical devices are configured to act either as a mirror for a standard logical device or to be split to provide a copy of the data on the standard device for some other purpose such as interaction with an ancillary application. In the context of the systems manufactured by the assignee of this invention, the second logical devices are called “BCV devices”. When BCV devices are incorporated, a BCV device initially attaches to a standard device to act as a mirror. Generally speaking, anytime after the BCV device has achieved synchronism with the standard device, the BCV device can be split, or detached, from the standard device. The data on the BCV device is then available to an ancillary application. This allows the ancillary application to act on the data on the BCV device independently of and simultaneously with the continued operation of a primary application with data stored on the standard device. 
   As the use of such data processing systems has grown, certain issues that impact the splitting of a BCV device from its corresponding standard device have appeared. These include an issue of pending write data operations. Disk array storage devices of many manufacturers, including those of the assignee of this invention, utilize cache memory to enhance performance, particularly for write operations. When a host issues a write command, the data to be written transfers only to the cache memory before a host adapter signals a host that the write operation is complete. The newly written data actually remains in the cache for some interval before that data, or overwritten data to the same location, transfers to the logical device storage medium. During that transient interval in the cache, the operation is complete with respect to the host, but pending with respect to physical disk device. Such cache entries are called “write pending” entries. The process of transferring a “write pending” entry to a logical device storage medium is called “destaging”. 
   Efforts have been made to manage write pending entries during operations of BCV devices. The objective of such efforts has been to assure that a BCV device, after it is split, accurately reflects the data on the standard device at the time of the split, including any write pending entries at the time that the split occurred. It is also an objective to minimize the impact of any such split operation on the main operations. For example, U.S. Pat. No. 6,370,626 to Gagne et al. discloses a Method and Apparatus for Independent and Simultaneous Access to a Common Data Set that reduces any interruption by implementing an “instant split” operation. When an “instant split” command is received, a corresponding BCV device immediately detaches from the standard device and becomes accessible to an alternate application. This occurs under a lock condition that lasts in the order of microseconds during which certain control operations are accomplished but no data is transferred and no write pending entries are managed. Immediately thereafter the lock is released. Various processes in the disk array storage device thereafter manage the write pending entries in an orderly fashion even as the primary application interacts with the standard device and an ancillary application, such as a backup application, interacts with the BCV device. 
   Applications and related data storage requirements constantly increase in complexity and volume. In the past some applications and associated data were stored entirely on a single standard device. Now such applications and associated data may be stored on multiple standard devices in a single disk array storage device. Other applications even require a storage capacity that exceeds the capacity of a single disk array storage device necessitating the distribution of a single application and its data over two or more disk array storage devices, each with up to hundreds of standard devices. 
   A database application is one example of an application in the last category. That is, one set of standard devices may contain the database data while another set of standard devices contains an associated log file, and these standard devices may be in different disk array storage devices. With these applications it may also be desirable to obtain a copy of the data and log files for independent processing. One approach uses an “instant split” operation as described in U.S. Pat. No. 6,370,626. However, with multiple standard devices in separate disk arrays, a host has to issue one instant split command to each set of BCV devices. Moreover, each discrete instant split operation is dispatched separately. Therefore the order in which the instant split commands are processed for different BCV devices is unpredictable. 
   For example, dependent input/output (I/O) sequences in database applications involve three write operations. The first write operation transfers an entry through a cache to a log file establishing the fact that data is to be written. The second write operation transfers the data to the cache for destaging to a standard device. The third write operation transfers another log entry to the cache for the log file; this entry indicates that the operation is complete. If multiple instant split commands issue so that the instant split for the logical volume containing the data is completed first, the data file may be updated without updating the log file in the BCV devices. In that event, the log file in the BCV device would not contain a record of the data change in another split BCV device. Alternatively if the log files were destaged and updated before the data file was updated, the log file could indicate the completion of an operation without the data actually having been transferred to the split BCV devices. Under either scenario, the data in the split BCV device is inconsistent. 
   It is difficult at best to identify any such inconsistent data. Consequently it became necessary to enable such instant split and other similar BCV device operations to occur while maintaining consistent data on multiple split BCV devices. U.S. patent application Ser. No. 09/613,118 filed Jul. 10, 2000 to Douglas E. LeCrone and Eugene D. Pflueger for a Method and Apparatus for Enhancing Recovery Operations in a Disk Array Storage Device discloses one such solution. In accordance with the disclosure, a special command to split the BCV devices is translated into one or more request data structures that identify the BCV devices as being involved in a consistent split operation. The command establishes a system lock over those particular BCV devices to be split. A host generates the lock. 
   The lock raises the I/O interrupt level to block user I/O operations to all logical devices connected to a host. That, is, while I/O interrupt level is raised, user applications cannot interact with any logical device by reading or writing data even with logical devices for which consistency is not a concern. Although every effort is made to minimize the duration of such interruptions, there still is a finite interruption interval. In certain applications, that interruption interval can be excessive. 
   This process may not always be effective in maintaining data consistency in multiple host environments where applications in different hosts can interact with data in one or more common logical volumes. As an example, assume that first and second host applications can interact with data in at least one common logical volume. Assume further that the first host has established a consistency group including any common logical volumes. When the first host initiates a splitting operation, it issues a lock to prevent that host from issuing any input/output operations to any logical volume. However, nothing prevents an application in the second host from transferring data to any of the common logical volumes. If that occurs, the data in the logical volumes may lose its consistency. 
   Therefore what is needed is a method and system for establishing consistency groups with certain additional criteria. For example, such a consistency group should be capable of including logical volumes from one or more data storage facilities. A consistency group should be capable of maintaining consistent data even when applications in multiple hosts have access to data in any common logical volume. Moreover, the establishment of a consistency group should have a minimal impact on normal data processing operations, including minimizing any interval during which access to data in any logical volume is blocked by a host lock or other like operation. 
   SUMMARY 
   Therefore it is an object of this invention to provide a method and apparatus for enhancing operations with a disk array storage device by minimizing certain delays that can be encountered in operations with consistency groups. 
   Another object of this invention is to provide a method and apparatus for minimizing certain delays in the operation of a disk array storage device with logical volumes in consistency groups, particularly with respect to operations involving dependent I/O sequences. 
   Still another object of this invention is to provide a method and apparatus for assuring data consistency during certain operations of a disk array storage device with logical volumes that can be accessed from host applications in multiple hosts. 
   Yet another object of this invention is to provide a method and apparatus for assuring consistency of data during certain operations of a disk array storage device during the execution of a dependent I/O sequence. 
   Still another object of this invention is to provide a method and apparatus for assuring data consistency in disk array storage devices during certain operations of the disk array storage devices involving dependent I/O sequences. 
   Yet still another object of this invention is to provide a method and apparatus for establishing consistency groups that are capable including logical volumes from one or more data storage facilities. 
   Still yet another object of this invention is to provide a method and apparatus for establishing consistency groups that are capable of maintaining consistent data even when applications in multiple hosts have access to data in any common logical volume. 
   Yet another object of this invention is to provide a method and apparatus for establishing a consistency group that has a minimal impact on normal data processing operations, particularly minimizing any interval during which access to data in any logical volume is blocked. 
   In accordance with this invention, data consistency in a group of logical devices in one or more disk array storage devices is maintained by establishing a plurality of logical devices as a local consistency group with a reject window having an open state that extends for a predetermined interval and a closed state. Each write operation to a logical device in the defined consistency group is blocked while the reject window is in the open state. When a write operation is blocked, a predetermined message is returned to the host processor indicating the blocked write operation. 

   
     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 typical data processing system adapted for utilizing this invention;  FIG. 1A  depicts a memory in a host in  FIG. 1 ; and  FIG. 1B  depicts a system memory in a disk array storage device of  FIG. 1 ; 
       FIG. 2  is a flow diagram depicting operations in a host computer in response to a SET command; 
       FIGS. 3A and 3B  depict a response to the SET command in a disk array storage device; 
       FIG. 4  is a flow diagram that depicts the operation of a host to a write command; 
       FIG. 5  is a flow diagram that depicts the operation in a disk array storage device in response to a write operation; 
       FIGS. 6A and 6B  constitute a flow diagram that depicts the response of a disk array storage device to an EXTEND command; and 
       FIGS. 7A and 7B  constitute a flow diagram that depicts the response of a disk array storage device to a RESET command. 
   

   DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS 
     FIG. 1  represents a multiple host, multiple disk array storage device system  10 . For purposes of illustration the system  10  includes two hosts  11  and  12 . Each host, as known, includes a host processor, memory and peripheral devices. Each host connects to a bus  14  that can extend to other hosts. The bus  14  may comprise a single communication path or parallel paths. 
   The bus  14  also connects to multiple disk array storage devices.  FIG. 1  discloses a disk array storage device (DASD)  15  and a DASD  16  by way of example. Each DASD has the same basic structure. Using the DASD  15 , as an example, a host adapter  17  acts as an interface with the bus  14  and includes processor-operated circuits or modules for performing various functions and controlling the other elements of the DASD  15 . The host adapter communicates over an internal DASD bus  18  with other components including a system memory  20 . The system memory  20  acts a global memory for various modules in the DASD  15  and as a cache memory for data. 
   The DASD  15  also includes a disk adapter  21  and logical devices  22  controlled by the disk adaptor  21  as standard devices. A disk adapter  23  connects to a one or more devices configured as BCV or like devices  24 . The disk adapters  21  and  23  connect to the host adapter  17  and system memory  20  by means of the internal DASD bus  18 . The internal DASD bus  18  can also connect to additional disk adapters and their related logical devices. 
   A logical device, such as the logical devices  22 , comprises a number of contiguous tracks or data blocks on one or more physical disk drives. That is, a logical device can include a portion or subset of the tracks on a physical device, all the tracks on a physical device or tracks on multiple physical disk drives. 
   The DASD  16  has a structure that is analogous to the DASD  15 . That is, the DASD  16  includes a host adapter  25  and a system memory  26  interconnected by an internal DASD bus  27  that attaches to a disk adapter  28  with its standard logical devices  30  and a disk adapter  31  with its BCV logical devices  32 . The internal DASD bus  27  may also attach to other disk adapters and their logical standard or BCV devices. 
   As known, during general operations one of the hosts  11  and  12  can issue a write command to a specific location within a standard logical device. A host adapter such as host adapter  17  or  25 , responds to that write command by performing a number of functions. The primary functions include transferring the data included with the write command to cache memory, such as the system memory  20  and thereafter, according to some protocol destaging that data from the system memory  20  to a specified track or tracks in the identified standard logical device. For a read command from a host, a host adapter transfers the data from the cache memory in the system if the data is present. Otherwise, the host adapter transfers data from the specified standard logical device to the cache memory and then from the cache memory back to the requesting host. 
   Each host has a host memory.  FIG. 1A  depicts a typical host memory  33  as might be found in the host  11  or the host  12  of  FIG. 1 . The host memory  33  in  FIG. 1A  includes user or primary applications  34 . The user applications are characterized by producing dependent I/O sequences. 
   LOCAL CONSISTENCY GROUP 
   In accordance with this invention, the host memory  33  in  FIG. 1A  also includes a stall write application  35  that operates in accordance with the parameters in a data structure  36 . The stall write application  35  has the capability of issuing a number of commands, like API commands, that the host adapters, like the host adapters  17  and  25 , can receive and process. A SET command is one such command. 
   The parameters contained in the data structure  36  include a group list  37 . The group list  37  identifies each logical device to be included in a local consistency group. The list may have any form. A window size parameter  40  establishes the duration of a reject window. A sense key parameter  41  and sense code parameter  42  provide the basis for a return message that can be sent by the host adapter to a host. The structure and use of sense key and sense code parameters is generally known in the art. A reject delay parameter  43  establishes a delay between the processing of a write operation and the transmission of a message with the sense key and sense code parameters  41  and  42 . An LCG flag  44  is set to indicate that a first write command after the operation should clear the local consistency group established by a SET command. The function of each parameter will be described more fully later. 
   When a host adapter, such as the host adapter  17  or host adapter  25  in  FIG. 1 , receives a SET command with the parameters shown in  FIG. 1A , the host adapter establishes certain data structures within the DASD system memory, such as the system memory  20  or system memory  26 . As shown in  FIG. 1B , each system memory includes a global section  50  and a logical device section for each logical device. In  FIG. 1B  the system memory includes a logical device section  51  for Logical Device (N−1) and a logical device section  52  for Logical Device (N). Each logical device section has the same structure so only the logical device section  51  is explained in detail. 
   The logical device section  51  includes a local consistency group data structure  53  that has several entries. As known, some disk array storage devices include a clock. A time stamp entry  54  receives the local time of clock provided by the associated DASD when a command, such as a SET command, is received. A window size entry  55  receives the window size parameter  40  from  FIG. 1A . Likewise a sense key entry  56 , a sense code entry  57 , a reject delay entry  60  and an LCG flag entry  61  receive the sense key parameter  41 , sensed code parameter  42 , reject delay parameter  43  and LCG flag  44 , respectively. Each logical device section also includes extended message flags, like extended message flag  62  in Logical Device (N−1) Section  51 . One entry in the extended message flags is a Local Consistency Group Exists flag, such as the flag  63 . 
   The global section  50  includes a Global LCG counter  64 . This counter identifies the number of logical devices in a DASD that are included in the defined Local Consistency Group. 
   One host adapter  17  in each DASD receives a SET command with the parameters shown in the data structure  36 . In response, the host adapter generates a Local Consistency Group Data Structure, such as the data structure  53  shown in  FIG. 1B  and a Global LCG counter  64 . From the receipt of the SET command until the expiration corresponding to the Window Size parameter in the entry  55  of  FIG. 1B , all host adapters, such as the host adapter  17 , block or stall any response to a write command. Each host adapter also returns the sense key and sense code entries  56  and  57  to the requesting host. Generally the requesting host processes this message and retries the write operation. Retries continue, with certain restraints, until the reject window closes. Thus in response to the SET command each host adapter establishes a plurality of logical devices as a consistency group with a reject window having a predetermined duration established by the window size parameter  55 . This reject window opens when the SET command is received and ends at a time calculated by combining the values in the time stamp entry  54  and window size entry  55 . Although responses to write commands to logical devices in the defined consistency group are blocked while the reject window is open, write operations are not blocked to logical devices outside the local consistency group. Moreover, read requests are not blocked to any logical device. When a block does occur, the host may retry the write operation. 
   DETAILED DESCRIPTION 
   With this as background, a stall write application  35  in host memory includes modules associated with different commands to produce a number of operating sequences in both the host and in a host adapter that receives a command. The first command is the SET command. The SET command establishes a data structure, such as the data structure  36  shown in  FIG. 1A . As shown in  FIGS. 1A and 2 , a SET command module  70  processes the SET command. As an initial step, the SET command module  70  defines a list of the logical devices to be included in a local consistency group in step  71  in  FIG. 2 . 
   Assume for example, that the logical devices in DASD  15  and DASD  16  are designated as standard logical devices “ 0 ” through “ 100 ” and that it is desired to use the data in the corresponding BCV devices corresponding to standard logical devices  0  through  5 ,  25  through  40  and  75  through  80  with an ancillary application. The defined list of logical devices obtained in  71  then will identify those specific standard logical devices. In one specific embodiment, the Group List syntax could be [ 0 – 5 ,  25 – 40 ,  75 – 80 ] wherein each group of consecutively numbered standard logical devices constitutes a “run” (e.g.  0 – 5 ); this list comprises three runs. Step  72  establishes a command that incorporates the list of logical devices to be included in the local consistency group and other parameters such as the reject window or window size parameter  40 , the sense key and sense code parameters  41  and  42 , a reject delay parameter  43  and the LCG flag  44 . Step  73  transmits that SET command onto the bus  14 . 
   The host adapter that receives the SET command generates a status message that indicates the success or non-success of the SET command. If the host receives a status message from each DASD in a timely fashion, step  74  transfers control to step  75  that determines whether all the received messages indicate success. If they do, the host response to the SET command is complete. If either of the tests in step  74  or step  75  fails, control transfers to step  76 . Step  76  terminates any consistency group dependent operations and displays an error message indicating that no consistency group could be formed. Step  76  may, for example, generate a reset command, described later. 
   When step  73  in  FIG. 2  transmits the SET command to one host adapter on each disk array storage device, a host adapter, such as the host adapter  17 , responds by processing a SET module  77  shown in  FIG. 1B  and discussed in detail with respect to  FIGS. 3A and 3B . Specifically, step  80  in  FIG. 3A  represents the receipt of the SET command. In step  81  the host adapter parses the SET command, particularly the group list parameter, to identify all the logical devices attached to the host adapter in the local consistency group. Step  82  selects an initial logical device in the local consistency group that is attached to the host adapter. A procedure  83  shown in  FIG. 3B  processes this initial logical device and each logical device in the DASD and the local consistency group. When procedure  83  ends, step  84  returns a message to the host; the message is dependent on the operations within the procedure  83 . 
   Referring now to  FIG. 3B , step  85  is a first step in the procedure  83  that tests for the existence of a prior local consistency group. Specifically step  85  examines the Local Consistent Group Exists flag, such as the flag  63  corresponding to logical device (N−1) in  FIG. 1B . If the flag is set, a prior local consistency group exists. Step  86  transfers control to step  87  that prepares a “Prior Consistency Group Exists” error message. Control passes back to step  84  in  FIG. 3A  to transfer this error message back to the host. 
   If no prior consistency group exists, step  86  transfers control to step  88  that tests the parameters from the SET command. If they are acceptable, step  88  stores them in the system memory. For example, if the SET command identifies a logical device (N−1), step  88  stores parameters shown in the local consistency group data structure  53 . Step  90  sets and records the local time of the disk array storage device. Step  91  increments a Global LCG counter, such as counter  64  in  FIG. 1B . On successful completion of the procedure  83 , the Global LCG counter  64  identifies the total number of logical devices for that disk array storage device that are in the local consistency group. 
   Step  92  performs various validity tests on the logical device to assure its proper operation. For example, each logical device number and configuration information for a DASD identifies a maximum number of logical devices. These validity tests could assure that the logical device number was within the range of allowed device numbers as established by the configuration information. Still other tests will be performed. 
   If those tests are not successful, step  93  transfers control to step  94  that prepares an Invalid Logical Device error message. Control then transfers to step  84  in  FIG. 3A . Otherwise, step  93  transfers control to step  95  that sets the Local Consistency Group Exists flag, such as the flag  63  in  FIG. 1B  for logical device (N−1). If more logical devices exist, step  96  transfers control to step  97  that selects another logical device in the disk array storage device. Then control transfers back to step  88 . 
   If all the logical devices are processed successfully, step  96  transfers control to step  98 . Step  98  prepares a “Command Processed Successfully” message. Control then transfers back to step  84  in  FIG. 3A . 
   After the SET command is sent, because no lock is placed on input/output operations, any host can initiate and generate a write command to a logical device in the local consistency group as represented by procedure  100  in  FIG. 4 . Step  101  represents the process by which a write command is sent to an identified logical device. The host then waits for a timely response in step  102 . When a host adapter receives a write command, it initiates a write command procedure  103  shown in  FIG. 1B  and detailed in  FIG. 5 . Step  104  represents the receipt of the write command. As a first operation the host adapter checks the Local Consistency Group Exists flag, such as flag  63  in  FIG. 1B , for determining the existence of a local consistency group. If one of those flags does not exist, no additional processing in accordance with this invention is necessary. Control transfers to step  107  that performs the write operation after which the host prepares a Successful Write Operation message in step  110 . The host adapter then transfers the prepared message back to the host in step  111 . 
   Referring again to  FIG. 4 , in that case and assuming that the Successful Write Operation message is received in a timely fashion, step  102  transfers control to step  112 . In this situation, the write command was not rejected by the host adapter. Consequently, step  112  considers the operation to be done because the write command was successful. 
   Returning to  FIG. 5 , if the corresponding local consistency group exists, its corresponding flag is set. Step  106  transfers control to step  114  to determine whether the write command was received within a reject window; that is, during an open state of the reject window. When a disk array storage facility includes a clock, step  114  essentially compares the clock time to determine if it is within the window that starts of the time of the time stamp entry  54  and extends for an interval determined by the window size entry  55  in  FIG. 1B . If the write command is received during this open state of the reject window, step  114  tranfers to step  115 . Step  115  determines whether a delay corresponding to a value in the reject delay entry  60  has elapsed. The delay in the reject window can be any value greater than or equal to zero. If the reject delay has elapsed, control transfers to step  116  that prepares a Retry Write message. This message indicates the write command was rejected because it was received during a reject window. In one embodiment this Retry Write message utilizes the information stored in the sense key and sense code entries  56  and  57  of  FIG. 1B . 
   When this message is received in the host, step  112  in  FIG. 4  transfers control to step  117  that determines whether another retry should be attempted. Typically, a host identifies a maximum number of permitted retry operations. If step  117  allows another retry, control transfers back to step  101  to send the write command again. Otherwise step  117  transfers control to step  120  that prepares a Process Retry error message. The host responds to such a message with appropriate recovery or diagnostics sequences. 
   Referring again to  FIG. 5 , whenever a first or subsequent write command is received after the reject window closes, that is when the DASD clock provides a time that is later than the time obtained by combining the values in time stamp entry  54  and the window size entry  55 , control transfers to step  121 . Step  121  determines whether the data structure should be cleared by examining the corresponding LCG flag, such as the LCG flag  61 . If that flag is set, the response to the write command step  122  clears the corresponding data structure, such as data structure  53 . In either case the control transfers to step  107  to perform the write operation and return a Successful Write Operation message. Obviously if an error occurs during a normal write operation, an error message would be produced; it is not shown for purposes of clarity. 
   A further understanding of the operation and advantages can be attained by reviewing conventional operations and operations in accordance with this invention while a reject window is open. In accordance with certain objectives of this invention, the stall write application assures data consistency, particularly in connection with write commands involved with dependent I/O sequences. Assume that primary applications are interacting with the standard devices and that BCV devices are attached to each standard device and are in synchronism with those devices. Assume further that it desired to split a number of logical devices from their respected standard devices, for some ancillary application, such as a backup application. First, a host, as a requesting host, issues the SET command identifying those logical devices and then issues the split command. 
   Without this invention or other type of consistency methodology, the dependent I/O sequences can produce corrupted data because each command to a split or detach a BCV device is sent independently to each logical device. Consequently, different host adapters will not process the command simultaneously. When a dependent I/O sequence occurs, first write-to-write-log, write-to-data and a second write-to-write-log operations occur in sequence. The write-to-data and second write-to-write-log operations require the completion of the prior operation or operations. When this sequence occurs, even during a splitting operation, the standard devices will properly record the information. However, the transfer of this information to the BCV devices may or may not occur prior to the process of detaching the BCV devices as previously described. 
   This invention only blocks write commands to each logical device in a defined local consistency group. Primary applications can continue to read data from all the standard devices. Updates to standard devices continue to transfer to BCV devices that are not in the deferred local consistency group. Moreover even when a write command to a device in the defined local consistency group is blocked, a return message identifies a stall write condition so the host automatically retries the write command. This retry process continues until either a maximum number retries has been reached or the reject window returns to a closed state. When the reject window returns to a closed state, the next attempt to process the write command is successful. 
   In many mixes of read and write operations, merely blocking selected write commands for a few seconds is acceptable. Moreover, data consistency exists for dependent I/O sequences because the second or third write commands in a sequence can not occur until the first and second write commands, respectively, are successfully, processed. 
   Referring specifically to  FIGS. 4 and 5 , the host generates a SET command and a subsequent write command to a logical device in a local consistency group. The write command is not blocked at the host, but is sent to a host adapter. If a consistency group exists, step  114  determines whether the write operation is occuring while the reject window is in an open state. If it is, step  116  prepares the Retry Write message for return to the host in step  111 . If that message is received in sufficient time, as defined by step  102 , and if the retry maximum has not reached, the same write command is again sent to the same logical device. Once the write command received after the reject window is closes, the host adapter  121  eventually performs the write operation and signals the successful completion. 
   Consider a situation in which a host sends first write-to-log command to a logical device and that host, or another host, issues a SET command. When the host sends the write-to-data command as the second command in the dependent I/O sequence, it is likely to coincide with the open state for the reject window. Thus the write-to-data command can not be processed until after the reject window returns to its closed state, so it can not transfer to the standard device or its corresponding BCV device. If the BCV devices are then split, they will only contain the first write-to-log command. Consequently, any application using the data on the BCV devices will “know” that this operation is incomplete. 
   As previously indicated and shown in  FIG. 1A , a SET command includes a reject delay parameter  43  with a value greater than or equal to 0. This value defines the time that elapses between the receipt of a write command and the transmission of a “Retry Write” message at step  116  in  FIG. 5 . More specifically, if the reject delay parameter is zero, step  115  in  FIG. 5  introduces no delay. Steps  116  and  111  send the “Retry Write” message back to the host immediately. In such situations there is a minimal delay until the host retries the write operation so the retry rate is high. In some systems such a high retry rate may cause a retry counter to reach a maximum value so step  117  in  FIG. 4  encounters a Process Retry error. In other systems the added load introduced by write retries conducted at a high retry rate may introduce an unacceptable load on system resources. 
   If, however, the reject delay parameter is set to a greater value, such as one second, step  115  in  FIG. 5  delays the transmission of the “Retry Write” message for that one-second interval. As will be apparent, any value above zero will significantly reduce the rate at which the host retries to write data to the blocked logical device. Generally speaking, some systems will benefit from delays; others will not. When a non-zero value is used, the actual value should be less than time interval established by step  102  in  FIG. 4  to avoid a Process Timeout error message. 
   As will be apparent, it is possible that the window size parameter in a SET command will open a reject window for too short a time. Disk array storage devices also include monitoring systems that can detect such a situation. In a number of situations where the window size is underestimated, an EXTEND command can increase the time the reject window remains in the open state. In one embodiment, a host adapter processing module  130  shown in  FIG. 1A  and in detail in  FIGS. 6A and 6B  performs this function. Referring to  FIG. 6A , step  131  represents the receipt of the EXTEND command from the host and step  132 , the selection of a first logical device from the group list. Step  133  compares the sense key and sense code parameters  41  and  42  of  FIG. 1A  in the EXTEND command and the values stored in the corresponding local consistency group data structure, such as the sense key and sense code entries  56  and  57  for logical device (N−1) in  FIG. 1B . This comparison assures that the EXTEND command does not alter the sense key and sense code sent back in response to a blocked write operation. If no match exists, step  134  transfers control to steps  135  and  136  in  FIG. 6B  to prepare a Reject Non-Match error message for transmission back to the host. The host responds to this message and terminates further operations, as by generating a RESET command described later. 
   If a match exists, step  134  in  FIG. 6A  transfers control to step  137  that clears internal Opened Reject Window and Closed Reject Window flags. Step  138  compares the time at the selected logical device and the interval established by the time stamp and window size entries in the corresponding data structure. If the reject window is in its open state, step  140  sets the internal Opened Reject Window flag. Then step  141  changes the value in the time stamp entry  54  to reflect the current time provided by the disk array storage device containing the logical device. Now the window will remain open for an additional interval using the original window size value. If the reject window is in its closed state, step  138  transfers control to step  142  that sets the internal Closed Reject Window flag and again changes the base time value in step  141 . 
   Steps  143  and  144  provide a loop control and logical device selection to assure that steps  133  through  142  process each logical device in the disk array storage device and on the list in the SET command. After all the logical devices have been selected, either or both of the Opened Reject Window and Closed Reject Window flags may be set. Step  145  in  FIG. 6B  is an initial step in analyzing these internal flags to determine whether the reject windows associated with the logical devices were all opened, or all closed or whether some of the reject windows were opened and closed. Step  145  tests the Opened Reject Window flag. If it is set, the reject window in at least one logical device was in the open state at the time the EXTEND command was processed. Step  146  therefore transfers control to step  147  to test the internal Closed Reject Window flag. If that flag is not set, then the reject windows in all the logical devices were opened. Step  150  transfers control to step  151  that prepares an All Open message for transmission back to the host in step  136 . If the internal Closed Reject Window flag is set, then the reject windows in some logical devices were in the open state and others were in the closed state. Control passes then to step  152  to prepare a “Mixed Open” message for transmission back to the host in step  136 . 
   If the internal Opened Reject Window flag is reset, then all the reject windows in all the logical devices were closed. To verify this, step  153  in  FIG. 6B  tests the internal Closed Reject Window flag. Step  154  should transfer control to step  155  to prepare an “All Closed” message for transmission back to the host in step  136 . If the internal Closed Reject Window flag were cleared, an error condition would exist for handling in step  156 . The systems and procedures for handling such errors are well known in the art. 
   Thus, in accordance with this aspect of the invention, the procedure  130  shown in  FIGS. 6A and 6B  allows a an extension of the time by updating the time stamp value in the entry for each logical device data structure the logical consistency group data structure. 
   A stall write application may also include a RESET command that directs a reset module  160  shown in  FIG. 1A  and in detail in  FIGS. 7A and 7B  to terminate a local consistency group immediately. Generating such a RESET command could be an option invoked by step  76  in  FIG. 2  as part of a termination of local consistency group dependent operations. A RESET command may also be part of other error recovery schemes. A RESET command following the closure of a reject window provides an express or directed clearing of all related data structures. In essence, when a host generates a RESET command, it positively clears the data structures in the system memory associated with each logical device such in as shown in  FIG. 1B  as opposed to waiting for a write command to a logical device after a reject window closes when the LCG flag is set. The RESET command and a Reset Module  160  in  FIGS. 1A ,  7 A and  7 B clear all the data structures for all the logical devices. 
   As shown in  FIGS. 7A and 7B , the reset procedure  160  begins with the receipt of the RESET command from the host in step  161  and the selection of a first logical device from the list in step  162 . Step  163  test the Local Consistency Group Exists flag for the logical device. For example, if logical device (N−1) in  FIG. 1B  is selected, step  163  test the logical consistency group exist flag  63 . If it is not set, there is no logical consistency group, hence there is no need to reset the group. So step  164  transfers control to step  165  to send a “No Consistency Group” message. This message indicates that there is a conflict between the logical devices identified in the list and the existence of a logical consistency group. 
   If the Local Consistency Group Exists flag is set, step  164  in  FIG. 7A  transfers control to step  166  that tests the reject window state. This test is performed by comparing the time at the logical device with the time obtained by combining the values in the time stamp and window size entries. If the reject window is in its closed state, control transfers to step  171  to prepare and send a “Reset Non-Active Consistency Group” message before the data structure is cleared in step  172 . If the reject window is still in its open state, step  170  bypasses step  171  and clears the data structure in step  172  without sending any message back to the host. 
   Steps  164  through  172  define three paths or three different procedures. When any of those is finished, control transfers to step  173  to assure that all the local logical devices in the local consistent group have been reset. If another exists, control passes through step  174  to select a next logical device to step  163 . As the loop comprising steps  163  through  174  tests each logical device in a local consistency group, any unusual states merely generate a corresponding message. The procedure  160  does not abort. However, each message defines abnormalities that may exist in the data for subsequent analysis. 
   When all the logical devices have been processed in accordance with  FIG. 7A , step  173  transfers control to step  175  in  FIG. 7B . If all the reject windows were in their open states at the time of the RESET command, step  176  transfers control to step  177  that prepares an “All Good” message. Step  178  then sends that prepared message to the host. If, however, one of the logical devices was characterized by a reject window in its closed state, step  176  passes control to  180  that prepares a “Reject Window Closed” message that is sent to the host with step  178 . Consequently, if host receives a “Reject Window Closed” message, the host can initiate appropriate recovery procedures. 
   Thus, in accordance with this invention, procedures are provided that facilitate and provide a simple method and apparatus for assuring data consistency in BCV and like devices made as copies in information in standard devices, particularly during dependent I/O sequences. This is readily accomplished by blocking only write operations to logical devices in a local consistency group; that is, only to those logical devices that contain related data such as database data and log files. The process is transparent to a user and will minimize any interruption to normal processing activities between a user application program and data in a disk array storage device. More specifically, this invention does not require the termination of all read/write operations during a splitting or like operation of BCV devices from standard devices or equivalent operations in other disk array storage devices. 
   This invention has been described in terms of a specific disk array storage device with well-known characteristics and in terms of specific procedures for implementing the invention in that disk array storage device. The exact step sequence illustrated in each of those procedures is shown by way of example. Alternative sequences could be substituted, particularly in adapting this invention to other types of disk array storage devices. It will be apparent that many 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.