Abstract:
Disclosed are a system, a method, and a computer program product to optimize the overall latency of transferring data from peer computers to storage devices. The latency optimization occurs after a group of data is received and organized by the peer computers. The average amount of time to transfer data to each particular storage device from the peer computers is used to determine the assignment of data transfers between the peer computers. Each peer computer maintains continuously updated measurements of the average time to transfer data to each storage device. The updated transfer time measurements are based upon a moving average with adjustable weighting of past and present measurements enabling the system to adapt to changing conditions.

Description:
CROSS-REFERENCES TO RELATED APPLICATIONS  
       [0001]     The present application is related to application Ser. No. ______, entitled “Autonomic Link Optimization Through Elimination of Unnecessary Transfers”, Docket # TUC9-2002-0124 and to application Ser. No. ______, entitled “Autonomic Learning Method To Load Balance Output Transfers of Two Peer Nodes”, Docket # TUC9-2002-0125 both filed on an even date herewith, the disclosure of which is hereby incorporated by reference in its entirety. 
     
    
     TECHNICAL FIELD  
       [0002]     This invention concerns a system to maintain an optimized balance of outbound transfers between two peer nodes that are transferring data to one or more storage devices.  
       BACKGROUND OF THE INVENTION  
       [0003]     Data storage systems may maintain more than one copy of data to protect against losing the data in the event of a failure of any of the data storage components. A secondary copy of data at a remote site is typically used in the event of a failure at the primary site. Secondary copies of the current data contained in the primary site are typically made as the application system is writing new data to a primary site. In some data storage systems the secondary site may contain two or more peer computers operating together as a backup appliance to store the data in one or more storage devices. Each peer computer receives inbound data from the primary site and transfers the data to a storage controller, storage device(s), or other computers for backup storage of the data. This type of system could be used for a disaster recovery solution where a primary storage controller sends data to a backup appliance that, in turn, offloads the transfers to a secondary storage controller at a remote site. In such backup systems, data is typically maintained in volume pairs. A volume pair is comprised of a volume in a primary storage device and a corresponding volume in a secondary storage device that includes an identical copy of the data maintained in the primary volume. Typically, the primary volume of the pair will be maintained in a primary direct access storage device (DASD) and the secondary volume of the pair is maintained in a secondary DASD shadowing the data on the primary DASD. A primary storage controller may be provided to control access to the primary storage and a secondary storage controller may be provided to control access to the secondary storage.  
         [0004]     The backup appliance typically receives data transfers for specific volumes from a primary storage controller. The backup appliance maintains consistent transactions sets, wherein application of all the transactions to the secondary device creates a point-in-time consistency between the primary and secondary devices. For each consistent transactions set, there will be one data structure created that will contain information on all outbound transfers in the set. This structure will be maintained on both of the peer nodes of the backup appliance. The backup appliance will maintain consistent transactions sets while offloading the transactions sets to the secondary device asynchronously. Both peer nodes may transfer the data to any of the storage devices. To obtain the shortest transfer time it is necessary to divide the data transfers between the peers. An equal division of the data transfers between the two peers may not be optimal because the latency time to transfer data to a particular storage device may be different for each peer. This may result in one peer finishing before the other, and idle time for the other peer. A division of the data transfers between the two peers that results in both peers finishing simultaneously would reduce the total throughput latency for the system because both peers have to finish transferring the current consistent transactions set before beginning to transfer the subsequent one. There is a need to divide the data transfers between the peers to achieve an optimal minimum transfer time to transfer all of the data.  
       SUMMARY OF THE INVENTION  
       [0005]     It is an object of the present invention to provide a method to optimize the overall latency of transferring data from peer computers to storage devices. Disclosed are a system, a method, and a computer program product to provide for the optimization of the output transfer load balance between two peer computers transferring data to one or more storage devices. The peer computers receive, organize and transfer the data to storage devices. The data set received may be a consistent transactions set or other type of data set for storage on one or more storage devices. The data set is composed of a plurality of data transfers. Each data transfer is an equal size block of data. The number of data transfers may vary for each data set received. Each of the peer computers receives all of the data transfers in the set, so that each peer has access to the entire set of data. The present invention operates by managing the assignments of data transfers for each peer computer and no data is transferred between the peers as the assignments change. The latency optimization occurs after a group of data is received and organized by the peer computers. The data transfers to storage devices are divided between two peer computers to balance the load between the peers and provide the minimum amount of time to transfer all of the data in the consistent transactions set to the storage devices. An important facet of the present invention is the use of the average amount of time it takes to transfer data blocks to each particular storage device from the peer computers. In the present invention each peer computer maintains continuously updated measurements of the average time it takes to transfer a block of data to each storage device. The transfer time measurements are used to optimize the output transfer load balance between the peers.  
         [0006]     In addition the operation is autonomous and self-adjusting resulting in the peer nodes optimizing the assignments of the data transfers during the operation of the invention resulting in the minimization of idle time for either peer. The self-adjusting feature allows the system to react to changing conditions that affect data transfer rates to the storage devices.  
         [0007]     Data sent to a backup appliance comprised of two peer computers is transferred to one or more storage devices using an optimization technique that divides the output transfer load between the peers using average latency information for each storage device. Data grouped in a consistent transactions set is first sorted into portions corresponding to the particular storage device where each data portion will be stored. The average latency time for the storage of a standard size block of data is compiled for each storage device for each of the peer computers. If the difference between the average latency for a storage device for the two peers is less than a specified relatively small latency threshold, then the storage device is assigned as an equal latency storage device and either peer may transfer data to the storage device. Excluding the equal latency storage devices, each peer is assigned to transfer data to the storage devices that have the smallest average latency for that particular peer. After assignment of the storage devices to the peer with the smallest latency, the equal latency storage devices are assigned to balance the total transfer load between the peers to enable both peers to finish transferring all of the data portions for the consistent transactions set at approximately the same time. Assigned storage devices may be reassigned to either peer to enable the equal latency storage devices to adequately balance the output transfer load between the peers. The storage devices are reassigned in a manner to minimize the total time to transfer all data in the consistent transaction data set.  
         [0008]     The operation of the present invention is autonomic by continuously updating the average latency time for each storage device. The updated average latencies for each storage device are used each time a new consistent transactions set is transferred to the peer computers for storage. The updated average latency time is based upon a moving average with adjustable weighting of past and present measurements. This enables the present invention to adapt to changing conditions.  
         [0009]     For a more complete understanding of the present invention, reference should be made to the following detailed description taken in conjunction with the accompanying drawings. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0010]      FIG. 1  is a block diagrammatic representation of a data storage network with primary and secondary sites.  
         [0011]      FIG. 2  is a block diagrammatic representation of a portion of the components located at the primary and secondary sites.  
         [0012]      FIG. 3  is a flowchart of the method used to balance the outbound data transfers of two peer computers.  
         [0013]      FIG. 4  is a flowchart of the method used in the present invention to assign equal latency data storage devices to balance the outbound data transfers.  
         [0014]      FIG. 5  is a flowchart of the method used to reassign data storage devices to permit the equal latency data storage devices to balance the outbound data transfers. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0015]     This invention is described in preferred embodiments in the following description. The preferred embodiments are described with reference to the Figures. While this invention is described in conjunction with the preferred embodiments, it will be appreciated by those skilled in the art that it is intended to cover alternatives, modifications, and equivalents as may be included within the spirit and scope of the invention as defined by the appended claims.  
         [0016]     Data storage systems may maintain more than one copy of data at secondary data storage sites to protect against losing the data in the event of a failure of any of the data storage components at the primary site.  FIG. 1  shows a block diagram of a data storage system with a primary site  110  and secondary site  150 . Primary site  110  and secondary site  150  are data storage sites that may be separated by a physical distance, or the sites may be located in close proximity to each other. Both the primary site  110  and secondary site  150  have one or more host computers  111 ,  151 , a communication network within each site  112 ,  152 , storage controllers  113 ,  153 , and a communication network  115 , between the sites. The host computers  111 ,  151 , store and retrieve data with respect to the storage controllers  113 ,  153 , using the site communication network  112 ,  152 . The site communication network(s)  112 ,  152  may be implemented using a fiber channel storage area network (FC SAN). Data is transferred between the primary site  110  and secondary site  150  using communication network  115  through primary backup appliance  114  and secondary backup appliance  160 . A secondary copy of the data from the primary site  110  is transferred to and maintained at the secondary site  150 . In the event of a failure at the primary site  110  processing may be continued at secondary site  150 . Because the physical distance may be relatively large between the primary site  110  and secondary site  150 , the communication network  115  is typically slower than the communication network within each site  112 ,  152 . Because of the relatively slow communication network  115  between the sites, consistent transactions sets are sent from primary site  110  to the secondary site  150  to ensure a point in time consistency between the sites. Consistent transactions sets are described in application entitled “Method, System and Article of Manufacture for Creating a Consistent Copy”, Application # 10339957, filed on Jan. 9, 2003 of which is hereby incorporated by reference in its entirety. At the secondary site  150  the consistent transactions set is received and then transferred to various data storage devices for permanent storage.  
         [0017]      FIG. 2 . is a block diagrammatic representation of a portion of the components of  FIG. 1 . At the primary site  110 , host computer(s)  201  communicates with storage management device  208  using communication line(s)  202 . The storage management device(s)  208  may comprise any storage management system known in the art, such as a storage controller, server, enterprise storage server, etc. Primary backup appliance  114  is comprised of peer node A  204 , peer node B  205  and communication line(s)  206 . Primary backup appliance  114  may have more or less components than shown in  FIG. 2 . Storage management device(s)  208  communicates with peer node A  204  and peer node B  205  using communication line(s)  203 . Host computer(s)  201  may alternatively communicate directly with peer node A  204  and peer node B  205  using communication lines(s)  219 . Herein references to peer node(s), peer computer(s), and peer(s) all refer to the same device(s). Peer node A  204  and peer node B  205  communicate with each other using communication line(s)  206 . Communication lines  202 ,  203  and  206  may be implemented using any network or connection technology known in the art, such as a Local Area Network (LAN), Wide Area Network (WAN), Storage Area Network (SAN), the Internet, an Intranet, etc. Communication between any of the components may be in the form of executable instructions, requests for action, data transfers, status, etc.  
         [0018]     At the secondary site  150  host computer(s)  211  communicates with storage management device  218  using communication line(s)  212 . The storage management device(s)  218  may comprise any storage management system known in the art, such as a storage controller, server, enterprise storage server, etc. Secondary backup appliance  160  is comprised of peer node  1   214 , peer node  2   215  and communication line(s)  216 . Secondary backup appliance  160  may have more or less components than shown in  FIG. 2 . Storage management device(s)  218  communicates with peer node  1   214  and peer node  2   215  using communication lines  213 . Host computer(s)  211  may alternatively communicate directly with peer node  1   214  and peer node  2   215  using communication line(s)  220 . Peer node  1   214  and peer node  2   215  communicate with each other using communication lines  216 . Communication lines  212 ,  213  and  216  may be implemented using any network or connection technology known in the art, such as a Local Area Network (LAN), Wide Area Network (WAN), Storage Area Network (SAN), the Internet, an Intranet, etc. The communication may be one or more paths between the components and not limited to the number of paths shown in  FIG. 2 . Communication between any of the components may be in the form of executable instructions, requests for action, data transfers, status, etc.  
         [0019]     Primary site  110  and secondary site  150  communicate with each other using communication lines  207 . Communication lines  207  may exist over a relatively large physical distance compared to communication lines  202 ,  203 ,  206 ,  212 ,  213  and  216 . Because of the physical separation of the primary  110  and secondary  150  locations, the transfer rate or bandwidth of communication lines  207  may be relatively slow compared to communication lines  202 ,  203 ,  206 ,  212 ,  213  and  216 . Communication lines  207  may be implemented using any connection technology known in the art such as the Internet, an Intranet, etc.  
         [0020]     For the present invention, primary site host computer(s)  201  sends data for storage to storage management device  208  using communication line(s)  202 . The storage management device  208  transfers this data to primary backup appliance  114  to create one or more backup copies of the data at a remote site. Alternatively, primary site host computer(s)  201  sends data directly to primary backup appliance  114  using communication line(s)  219  and then sends the same data to storage management device  208  using communication line(s)  202 . Alternatively, primary site host computer(s)  201  sends data to storage management device  208  that passes through an intelligent switch that forwards a copy of the data to both primary backup appliance  114  and storage management device  208 . The data is grouped into a consistent transactions set by peer node  1   204  and peer node  2   205  as it arrives from either storage management device  208  over communication lines  203 , primary site host computer(s)  201 , or an intelligent switch. Upon accumulating an entire consistent transaction data set, peer node A  204  and peer node B  205  transfer the consistent transactions set to peer node  1   214  and peer node  2   215  at the secondary site  150  using communication lines  207 . Peer node  1   214  and peer node  2   215  transfer the entire consistent transactions set to storage management device  218  for storage using communication lines  213 . Host computer(s)  211  may retrieve data from storage management device  218  using communication line(s)  212 .  
         [0021]      FIG. 3  contains flowchart  300  detailing the operation of the system to balance the output transfer load for peer node  1   214  and peer node  2   215 . Storage management device(s)  218  each controls one or more storage devices. Each storage device used for the storage of data has an address specified as a Logical Unit Number (LUN). LUN is used as a shorthand notation in this description to identify a storage device or storage location for the storage of data; herein the terms LUN and storage device are used interchangeably. The present invention is not limited to the use of a LUN to identify a storage device or storage location and any terminology known in the art may be used. The index L is used in this description to number each storage device. The index, L, ranges from 1 to M, the total number of devices, M, used to store all of the data contained in a consistent transactions set. The present invention is not limited to the use of a LUN to identify a storage device or storage location or L to number the storage devices and any terminology known in the art may be used with out departing from the spirit of the invention. Furthermore, the present invention is not limited to the use of a LUN to carry out the processing of the invention and this processing could also be carried out using other logical groupings, including storage controllers, storage controller partitions, storage controller adapter cards, storage controller adapter card ports, etc.  
         [0022]     Referring to  FIG. 3 , at step  303  peer node  1   214  and/or peer node  2   215  determine the number of data transfers, N(L), for each LUN, for the consistent transactions set to be stored. Each data transfer is an equal size block of data. The result of this determination is a 1 dimensional matrix of values with index L ranging from 1 to M, where M is the total number of LUNs needed to store all of the data in the present consistent transactions set. M may vary with each consistent transactions set to be stored. For example for the first LUN, L=1, N(1)=100 equal size blocks of data, second LUN, L=2, N(2)=1500 equal size blocks of data, continuing until N(M)=110 equal size blocks of data. The number of data transfers, N(L), and M will vary for each consistent transactions set. At step  304  first average latency, AL1(L), is calculated for all LUNs with index, L, ranging from 1 to M. First average latency, AL1(L), is the average latency to transfer a block of data from peer node  1   214  to the LUN identified by index L. At step  305  second average latency, AL2(L), is calculated for all LUNs with index, L, ranging from 1 to M. Second average latency, AL2(L), is the average latency to transfer a block of data from peer node  2   215  to the LUN identified by index L. First average latency, AL1(L), is calculated by applying an averaging algorithm to measurements of past data transfers from peer node  1   214  to the LUN identified by index L. Second average latency, AL2(L), is calculated by applying an averaging algorithm to measurements of past data transfers from peer node  2   215  to the LUN identified by index L. The averaging algorithm used to calculate first average latency, AL1(L), and second average latency, AL2(L), may be an equal weight average of past measurements of the appropriate latencies or more weight may be applied to the most recent latency measurements. The average latencies may be calculated by either peer node  1   214  or peer node  2   215  or both or may be calculated external to peer node  1   214  or peer node  2   215 . The latency values used in the calculation of first average latency, AL1(L), and second average latency, AL2(L), are measured each time a data transfer occurs from one of the peer computers to a secondary storage controller. This latency measurement may occur at step  360  (described below) or the measurement may occur any time data is transferred to a storage device. The measurement is accomplished by measuring a first amount of elapsed time for transferring all data from peer node  1   214  to the LUNs assigned to peer node  1   214 . This first amount of elapsed time is further decomposed into the elapsed time to transfer a block of data for each LUN resulting in a latency measurement for each LUN. For example, every time a data transfer is started, the peer node that is transferring the data determines the current time when the transfer begins. When the data transfer completes, the peer node determines the time the data transfer ended. The elapsed time is calculated by taking the difference between the two time values. The elapsed time is the latency for the data transfer. First average latency, AL1(L), is then recalculated by use of the decomposition of the measured first amount of elapsed time.  
         [0023]     A second amount of elapsed time for transferring all data from peer node  2   215  to the LUNs assigned to peer node  2   215  is measured. This second amount of elapsed time is further decomposed into the elapsed time to transfer a block of data for each LUN. Second average latency, AL2(L), is then recalculated in the manner described above for first average latency, AL1(L).  
         [0024]     Each peer node saves its latency values in its memory or associated memory. A variety of methods for calculating the average latency from the measured first and second amount of elapsed time for each peer may be used. A simple average of all the previous latency measurements taken for each LUN may be used.  
         [0025]     A second method for calculating the average latency employs a moving average of the previous W latency measurements. The number W is chosen by an administrator, user or dynamically by the system. W may be modified at any time. The moving average will be calculated using only the most current W latency measurements. In this way only the most recent measurement data is used and older measurement data is discarded. This enables the invention to dynamically adapt to changing conditions.  
         [0026]     In a third method a weighted average of the previous W latency measurements is used. In this method, a weight value is assigned to each of the W latency measurements. Each latency measurement is multiplied by its corresponding weight value and the average is taken over all of the products. The weights are chosen such that the oldest latency measurements have the smallest weights and the newest latency measurements have the greatest weights. The weighted average method can encompass the first two methods described above by a suitable choice of W and the weight value at each calculation interval. This provides a moving weighted average of the measured latencies that may be dynamically adjusted to provide an averaging technique of any of the three methods described above that may provide optimal operation of the present invention. Other averaging techniques may be used to further optimize the operation of the invention.  
         [0027]     In the absence of latency measurement from either peer node  1   214  or peer node  2   215  to any LUN, estimates of the average latency may be calculated based on the available measurements of the latencies of any or all of the LUNs. Because there may not be any measurement data for a particular LUN, equal estimates of the latency for each peer could be used. The estimated latency may be derived from the use of the average of all the latencies for all the LUN measurements that are available, or the average of all the latencies for all the LUNs that are contained in the same storage controller as the unknown LUN. Either way, peer node  1   214  and peer node  2   215  must be assigned equal values for the LUN latencies in the absence of actual latency measurement data. When data is available for the LUN, the estimated values will be discarded and the actual values will be used.  
         [0028]     At step  307  an average latency difference is calculated by subtracting second average latency, AL2(L), from first average latency, AL1(L), for all LUNs with index, L, ranging from 1 to M. At step  310  the absolute value of the average latency difference is compared to a latency threshold X. Latency threshold X is chosen as a small number that can be modified, if necessary, to tune the algorithms performance. X may be calculated using a specified percentage, Y, of the average latency difference that is considered to be small enough that the latency to transfer data to the selected LUN from either peer computer is approximately the same. A typical value of 5% for Y would be appropriate in most cases; however, values greater or less than 5% could be used to further optimize the operation of the present invention. The following equation could be used to calculate latency threshold X,  
       X   =       Y   100     ⁢       ∑     L   =   1     M     ⁢           AL   ⁢           ⁢   1   ⁢     (   L   )       +     AL   ⁢           ⁢   2   ⁢     (   L   )         2     .             
 
         [0029]     Alternatively latency threshold, X, may be chosen to be different for each LUN, this provides an advantage because a fixed percentage of the latency difference is used to determine the equal latency LUNs. The resulting latency threshold, X increases if first average latency, AL1(L), and second average latency, AL2(L), increase. This may be advantageous when there is a large variation in first average latency, AL1(L), and second average latency, AL2(L). The following equation could be used to calculate latency threshold X(L), 
    X(L)=(Y/100)*(AL1(L)+AL2(L))/2, for index, L, ranging from 1 to M., from specified percentage, Y.    
 
         [0031]     At step  315  those LUNs that have absolute values of the average latency difference less than latency threshold X (L) for index, L, ranging from 1 to M are assigned as equal latency LUNs. Latency threshold X(L), may or may not vary with index, L, as explained above. The LUNs that are not assigned as equal latency LUNs are divided into two categories at step  320 . At step  320  second average latency, AL2(L), is compared to first average latency, to determine which of the two peers has the shortest latency for all LUNs excluding the equal latency LUNs. At step  325 , the LUNs with a second average latency, AL2(L), that is greater than the sum of first average latency, AL1(L), and latency threshold X(L), are assigned to peer node  1   214 , for all LUNs excluding the equal latency LUNs. The following equation may be applied at step  325 , AL2(L)&gt;X(L)+AL1(L), or all LUNs with second average latency, AL2(L), that is greater than first average latency, AL1(L) excluding the equal latency LUNs may be used to assign the LUNs to peer node  1   214 . As explained above latency threshold X(L), may or may not vary with index, L.  
         [0032]     At step  330 , the LUNs with a first average latency, AL1(L), that is greater than the sum of second average latency, AL2(L), and latency threshold X(L), are assigned to peer node  2   215 , for all LUNs excluding the equal latency LUNs. The following equation may be applied at step  325 , AL1(L)&gt;X(L)+AL2(L) or all LUNs with first average latency, AL1(L) that is greater than second average latency, AL2(L), excluding the equal latency LUNs may be used to assign the LUNs to peer node  2   215 . At the conclusion of step  320 , all of the LUNs for the present consistent transactions set are assigned as equal latency LUNs, assigned to peer node  1   214 , or assigned to peer node  2   215 .  
         [0033]     At step  340  first peer latency, T1, is calculated by addition of all latencies for all of the LUNs assigned to peer node  1   214 , using the number of transfers N(L), for each LUN for the present consistent transactions set. The following equation could be used to calculate first peer latency:  
           T   ⁢           ⁢   1     =       ∑     L   =   1     M     ⁢     AL   ⁢           ⁢   1   ⁢     (   L   )     *     N   ⁡     (   L   )             ,       
 
 for index, L, of all of the LUNs assigned to peer node  1   214 . This calculation performs a summation for only the index values L, for LUNs that are assigned to peer node  1   214 . 
 
         [0035]     At step  343  second peer latency, T2, is calculated by addition of all latencies for all of the LUNs assigned to peer node  2   215 , using the number of transfers N (L), for each LUN for the present consistent transactions set. The following equation could be used to calculate second peer latency:  
           T   ⁢           ⁢   2     =       ∑     L   =   1     M     ⁢     AL   ⁢           ⁢   2   ⁢     (   L   )     *     N   ⁡     (   L   )             ,       
 
 for index, L, of all of the LUNs assigned to peer node  2   215 . This calculation performs a summation for only the index values L, for LUNs that are assigned to peer node  2   215 . 
 
         [0037]     At step  345  equal latency, Teq, is calculated by addition of all latencies for all of the LUNs assigned as equal latency LUNs, using the number of transfers N(L), for each LUN for the present consistent transactions set. The following equation could be used to calculate equal latency:  
         Teq   =       ∑     L   =   1     M     ⁢       Max   ⁡     (       AL   ⁢           ⁢   2   ⁢     (   L   )       ,     AL   ⁢           ⁢   1   ⁢     (   L   )         )       *     N   ⁡     (   L   )             ,       
 
 for index, L, of all of the LUNs assigned as equal latency LUNs. The function Max(AL2(L), AL1(L)) selects the maximum value of either AL2(L) or AL1(L) and uses that maximum value for the calculation. This calculation performs a summation for only the index values L, for LUNs that are assigned as equal latency LUNs. Alternatively, the invention could use a minimum function, Min(AL2(L),AL1(L)), in this calculation. 
 
         [0039]     At step  350 , latency difference, Td, is calculated by subtracting second peer latency, T2, from first peer latency, T1, using the following equation: 
 
 Td=T 2− T 1. 
 
         [0040]     If the absolute value of latency difference, Td, is less than or equal to equal latency, Teq, then step  357  is executed. If the absolute value of latency difference, Td, is greater than equal latency, Teq, then step  355  is executed. At step  355 , the LUNs that were assigned to peer node  1   214 , and to peer node  2   215  at steps  325  and  330  are reassigned to peer node  1   214 , and to peer node  2   215 , so that the absolute value of the latency difference, Td, when recalculated with the reassignment is less than or equal to the equal latency, Teq. This reassignment of LUNs enables the use of equal latency LUNs to balance the output transfer load between the two peers. One implementation of Step  355  is further explained below after a description of step  357 .  
         [0041]     At step  357 , the equal latency LUNs are assigned to peer node  1   214 , and to peer node  2   215  to minimize the latency difference, Td, between peers. The minimization of the latency difference, Td, has the practical effect of balancing the output transfer load between the two peers. It may not be possible to exactly balance the loads because of the finite nature of the transfers that are being reassigned, or other conditions, therefore, a minimization of the latency difference, Td, is sufficient to balance the output transfer loads between the two peers. If step  357  is executed as a result of first executing step  355  then step  357  can be accomplished by reassignment of all equal latency LUNs to the peer with the smaller latency. This is because after first executing step  355  the latency difference, Td, between peers is approximately equal to Teq.  
         [0042]     One implementation of step  357  is further detailed in flowchart  400  shown in  FIG. 4 . This implementation may be used in either the case of step  357  being executed as a result of first executing step  355  or if step  357  is executed directly after step  350 . Step  357  is composed of a series of operations beginning at step  401  and ending at either step  450  or step  451 . At step  405 , first peer latency, T1, and second peer latency, T2, are compared to determine the largest latency. At step  410 , a decision is made to determine which step to execute next. If second peer latency, T2, is greater than or equal to first peer latency, T1, then step  420  is executed. If first peer latency, T1, is greater than second peer latency, T2, then step  421  is executed. The process starting with step  420  is described first followed by a description of the process starting at step  421 .  
         [0043]     At step  420 , first equal latency, Tne1, is calculated using, Tne1=(T2−T1+Teq)/2. First equal latency, Tne1, is the total latency that must be moved from the equal latency LUNs to peer node  1   214 , so that the equal latency LUNs will balance the output transfer load between the two peers.  
         [0044]     At step  425 , one of the LUNs assigned as equal latency LUNs is selected from the available equal latency LUNs and an identification number, I, is obtained for the equal latency LUN selected. In the preferred embodiment, at each execution of step  425 , the LUN chosen should be the LUN with the largest first average latency, AL1(L), however this is not a requirement for the operation of the present invention. At step  430 , first equal transfers, Nne1(I), is calculated using, Nne1=Tne1/AL1(I), for the equal latency LUN selected and identified by identification number, I. First equal transfers, Nne1(I), is the number of data transfers that need to be reassigned from the selected equal latency LUN to peer node  1   214 .  
         [0045]     At step  435 , first equal transfers, Nne1(I), is compared to the number of transfers, N(I), for the selected LUN identified by identification number I. If the number of first equal transfers, Nne1(I), is more than the number of transfers, N(I), available for the LUN identified by identification number I, then step  440  is executed. At step  440  the selected LUN identified by identification number, I, is reassigned to peer node  1   214 , first equal latency, Tne1, is recalculated using, Tne1=(Nne1(I)−N(I))*Al 1 (I) and step  425  is executed again using the recalculated first equal latency, Tne 1 . First equal latency, Tne1, is recalculated to remove the latency that was reassigned from the equal latency LUNs to peer node  1   214  and other equivalent calculations for the equation given above may be used. At step  425 , an equal latency LUN is selected from the remaining equal latency LUNs in the same manner as previously described for step  425 . As an example, the first time step  425  is executed after the execution of step  440 , the LUN that was reassigned, to peer node  1   214 , is removed from the available equal latency LUNs. Step  430  is executed after step  425  using the next equal latency LUN selected with the next identification number, I. Identification number, I changes each time step  425  is executed after executing step  430 . Steps  435 ,  440 ,  425 , and  430  are repeated until first equal transfers, Nne1(I), is less than or equal to the number of transfers, N(I), available for the LUN identified by identification number I.  
         [0046]     If at step  435 , first equal transfers, Nne1(I), is less than or equal to the number of transfers, N(I), for the selected LUN identified by identification number I, then first equal transfers, Nne1(I), for the selected LUN identified by identification number I, are assigned to peer node  1   214  at step  445 . The remaining transfers, (N(I)−Nne(I)) remain assigned to the equal latency LUN identified by identification number I.  
         [0047]     At step  447 , all remaining equal latency LUNs not already assigned to peer node  1   214 , and the remaining transfers N(I)-Nne(I) (if any are available) are assigned to peer node  2   215 . After execution of step  447  control flows to step  450  where the process ends and returns to execute step  360 .  
         [0048]     If at step  410 , first peer latency, T1, is greater than second peer latency, T2, then step  421  is executed. At step  421 , second equal latency, Tne2, is calculated using, Tne2=(T1−T2+Teq)/2. Second equal latency, Tne2, is the total latency that must be moved from the equal latency LUNs to peer node  2   215 , so that the equal latency LUNs will balance the output transfer load between the two peers.  
         [0049]     At step  426 , one of the LUNs assigned as equal latency LUNs is selected from the available equal latency LUNs and an identification number, I, is obtained for the equal latency LUN selected. In the preferred embodiment, at each execution of step  426 , the LUN chosen should be the LUN with the largest AL2(I), however this is not a requirement for the operation of the present invention. At step  431 , second equal transfers, Nne2(I), is calculated using, Nne 2 =Tne2/AL2(I), for the equal latency LUN selected and identified by identification number, I. Second equal transfers, Nne2(I), is the number of data transfers that need to be reassigned from the selected equal latency LUN to peer node  2   215 .  
         [0050]     At step  436 , second equal transfers, Nne2(I), is compared to the number of transfers, N(I), for the selected LUN identified by identification number I. If the number of second equal transfers, Nne2(I), is more than the number of transfers, N(I), available for the LUN identified by identification number I, then step  441  is executed. At step  441  the selected LUN identified by identification number, I, is reassigned to peer node  2   215 , second equal latency, Tne2, is recalculated using, Tne2=(Nne2(I)−N(I))*Al2(I) and step  426  is executed again using the recalculated, second equal latency, Tne2. Second equal latency, Tne2, is recalculated to remove the latency that was reassigned from the equal latency LUNs to peer node  2   215  and other equivalent calculations for the equation given above may be used. At step  426 , an equal latency LUN is selected from the remaining equal latency LUNs. As an example, the first time step  426  is executed after the execution of step  441 , the LUN that was reassigned, to peer node  2   215 , is removed from the available equal latency LUNs. Step  431  is executed after step  426  using the next equal latency LUN selected with the next identification number, I. Identification number, I changes each time step  426  is executed after executing step  431 . Steps  436 ,  441 ,  426 , and  431  are repeated until second equal transfers, Nne2(I), is less than or equal to the number of transfers, N(I), for the selected LUN identified by identification number I. Identification number, I changes each time step  426  is executed after executing step  431 .  
         [0051]     If at step  436 , second equal transfers, Nne2(I), is less than or equal to the number of transfers, N(I), for the selected LUN identified by identification number I, then second equal transfers, Nne2(I), for the selected LUN identified by identification number I, are assigned to peer node  2   215  at step  446 . The remaining transfers (N(I)−Nne2(I)) remain assigned to the equal latency LUN identified by identification number I.  
         [0052]     At step  448 , all remaining equal latency LUNs not already assigned to peer node  2   215 , and the remaining transfers (N(I)−Nne2(I)) are assigned to peer node  1   214 . After execution of step  448  control flows to step  451  where the process ends and returns to execute step  360 . At step  360  all of the data for the present consistent transactions set is transferred to the LUNs assigned to peer node  1   214 , and to peer node  2   215 . Both peers will finish transferring data at approximately the same time because of the load balancing of the present invention.  
         [0053]     One implementation of step  355 , is now described. Step  355 , is executed if the LUNs that were assigned to peer node  1   214 , and to peer node  2   215  at steps  325  and  330  need to be reassigned to peer node  1   214 , and to peer node  2   215 , so that the equal latency LUNs will balance the output transfer load between the two peers.  FIG. 5  shows a flowchart  500  of one implementation of step  355 , starting at step  501 . At step  505 , first peer latency, T1, and second peer latency, T2, are compared to determine the peer with the largest total latency. At step  510 , a decision is made to determine which step to execute next. If second peer latency, T2, is greater than first peer latency, T1, then step  520  is executed. If first peer latency, T1, is greater than second peer latency, T2, then step  521  is executed. The case of first peer latency, T1, being equal to second peer latency, T2, is not encountered for this process because that would be determined at step  350 , with the result of step  355  not being executed. The process starting with step  520  is described first, followed by a description of the process starting at step  521 .  
         [0054]     At step  520 , first latency ratio, R1(L), is calculated using, R1(L)=(AL1(L)/AL2(L), for index, L, for all of the LUNs assigned to peer node  2   215 . This calculation determines first latency ratio, R1(L), for only the index values L, for LUNs that are assigned to peer node  2   215 . At step  525 , the LUN that is assigned to peer node  2   215 , with the smallest value of first latency ratio, R1(L), is selected and an identification number, K, is obtained for LUN selected.  
         [0055]     At step  530 , the number of second peer transfers, Nn2(K), is calculated using, Nn2(K)=(T2−Teq−T1)/((1+R1(K))*AL2(K)), for the LUN selected and identified by identification number, K. The number of second peer transfers, Nn2(K), is the number of data transfers from the selected LUN (identified by identification number, K, and assigned to peer node  2   215 ) that need to be reassigned from peer node  2   215  to peer node  1   214 , so that the equal latency LUNs will balance the load between the peers.  
         [0056]     At step  535 , the number of second peer transfers, Nn2(K), is compared to the number of transfers, N(K), for the selected LUN identified by identification number K. If the number of second peer transfers, Nn2(K), is more than the number of transfers, N(K), available for the LUN identified by identification number K, then step  540  is executed. At step  540  the selected LUN identified by identification number, K, is reassigned to peer node  1   214 . First peer latency, T1, is recalculated by addition of all latencies for all of the LUNs assigned to peer node  1   214  (including the selected LUN identified by identification number, K, that was reassigned to peer node  1   214 ), using the number of transfers N(L), for each LUN for the present consistent transactions set. The following equation could be used to recalculate first peer latency,  
           T   ⁢           ⁢   1     =       ∑     L   =   1     M     ⁢     AL   ⁢           ⁢   1   ⁢     (   L   )     *     N   ⁡     (   L   )             ,       
 
 for index, L, of all of the LUNs assigned to peer node  1   214 . This calculation performs a summation for only the index values L, for LUNs that are assigned to peer node  1   214 . Alternatively, the first peer latency could be recalculated by adding the reassigned LUN transfer latency, N(K)*AL1(K), to the previous first peer latency. Second peer latency, T2, is recalculated by addition of all latencies for all of the LUNs assigned to peer node  2   215 , using the number of transfers N(L), for each LUN (the selected LUN identified by identification number, K, that was reassigned to peer node  1   214  is removed), for the present consistent transactions set. The following equation could be used to recalculate second peer latency,  
           T   ⁢           ⁢   2     =       ∑     L   =   1     M     ⁢     AL   ⁢           ⁢   2   ⁢     (   L   )     *     N   ⁡     (   L   )             ,       
 
 for index, L, of all of the LUNs assigned to peer node  2   215 . This calculation performs a summation for only the index values L, for LUNs that are assigned to peer node  2   215 . Alternatively, the second peer latency could be recalculated by subtracting the reassigned LUN transfer latency, N(K)*AL2(K), from the previous second peer latency. After the recalculation of first peer latency, T1, and second peer latency, T2, step  525 , is executed again. At step  525 , the LUN that is assigned to peer node  2   215 , (the selected LUN identified by identification number, K, that was reassigned to peer node  1   214  is removed) with the smallest value of first latency ratio, R1(L), is selected and the next identification number, K, is obtained for the LUN selected. Step  530  is executed after step  525  using the next LUN selected with the next identification number, K. Identification number, K changes each time step  525  is executed after executing step  530 . Steps  535 ,  540 ,  525 , and  530  are repeated until the number of second peer transfers, Nn2(K), is less than or equal to the number of transfers, N(K), available for the LUN identified by identification number K. 
 
         [0059]     If at step  535 , the number of second peer transfers, Nn2(K), is less than or equal to the number of transfers, N(K), available for the LUN identified by identification number K, then the number of second peer transfers, Nn2(K), for the selected LUN identified by identification number K, are reassigned from peer node  2   215  to peer node  1   214  at step  545 . The remaining transfers N(K)−Nn2(K) remain assigned to peer node  2   215 . At step  550  the process ends and returns to execute step  357 .  
         [0060]     If at step  510 , first peer latency, T1, is greater than second peer latency, T2, then step  521  is executed. At step  521 , second latency ratio, R2(L), is calculated using, R2(L)=(AL2(L)/AL1(L), for index, L, for all of the LUNs assigned to peer node  1   214 . This calculation determines second latency ratio, R2(L), for only the index values L, for LUNs that are assigned to peer node  1   214 . At step  526 , the LUN that is assigned to peer node  1   214 , with the smallest value of second latency ratio, R2(L), is selected and an identification number, K, is obtained for LUN selected.  
         [0061]     At step  531 , the number of first peer transfers, Nn1(K), is calculated using, Nn1(K)=(T1−Teq−T2)/((1+R2(K))*AL1(K)), for the LUN selected and identified by identification number, K. The number of first peer transfers, Nn1(K), is the number of data transfers from the selected LUN (identified by identification number, K, and assigned to peer node  1   214 ) that need to be reassigned from peer node  1   214  to peer node  2   215 , so that the equal latency LUNs will balance the load between the peers.  
         [0062]     At step  536 , the number of first peer transfers, Nn1(K), is compared to the number of transfers, N(K), for the selected LUN identified by identification number K. If the number of first peer transfers, Nn1(K), is more than the number of transfers, N(K), available for the LUN identified by identification number K, then step  541  is executed. At step  541  the selected LUN identified by identification number, K, is reassigned to peer node  2   215 . First peer latency, T1, is recalculated by addition of all latencies for all of the LUNs assigned to peer node  1   214  (the selected LUN identified by identification number, K, that was reassigned to peer node  2   215  is removed), using the number of transfers N(L), for each LUN for the present consistent transactions set. The following equation could be used to recalculate first peer latency,  
           T   ⁢           ⁢   1     =       ∑     L   =   1     M     ⁢     AL   ⁢           ⁢   1   ⁢     (   L   )     *     N   ⁡     (   L   )             ,       
 
 for index, L, of all of the LUNs assigned to peer node  1   214 . This calculation performs a summation for only the index values L, for LUNs that are assigned to peer node  1   214 . Alternatively, the first peer latency could be recalculated by subtracting the reassigned LUN transfer latency, N(K)*AL1(K), from the previous first peer latency. Second peer latency, T2, is recalculated by addition of all latencies for all of the LUNs assigned to peer node  2   215 , (including the selected LUN identified by identification number, K, that was reassigned to peer node  2   215 ), using the number of transfers N(L), for each LUN for the present consistent transactions set. The following equation could be used to recalculate second peer latency,  
           T   ⁢           ⁢   2     =       ∑     L   =   1     M     ⁢     AL   ⁢           ⁢   2   ⁢     (   L   )     *     N   ⁡     (   L   )             ,       
 
 for index, L, of all of the LUNs assigned to peer node  2   215 . This calculation performs a summation for only the index values L, for LUNs that are assigned to peer node  2   215 . Alternatively, the second peer latency could be recalculated by adding the reassigned LUN transfer latency, N(K)*AL2(K), to the previous second peer latency. After the recalculation of first peer latency, T1, and second peer latency, T2, step  526 , is executed again. At step  526 , the LUN that is assigned to peer node  1   214 , (the selected LUN identified by identification number, K, that was reassigned to peer node  2   215  is removed) with the smallest value of second latency ratio, R2(L), is selected and the next identification number, K, is obtained for the LUN selected. Step  531  is executed after step  526  using the next LUN selected with the next identification number, K. Identification number, K changes each time step  526  is executed after executing step  531 . The steps  536 ,  541 ,  526 , and  531  are repeated until the number of first peer transfers, Nn1(K), is less than or equal to the number of transfers, N(K), available for the LUN identified by identification number K. 
 
         [0065]     If at step  536 , the number of first peer transfers, Nn1(K), is less than or equal to the number of transfers, N(K), available for the LUN identified by identification number K, then the number of first peer transfers, Nn1(K), for the selected LUN identified by identification number K, are reassigned from peer node  2   215  to peer node  1   214  at step  546 . The remaining transfers N(K)−Nn1(K) remain assigned to peer node  2   215 . At step  551  the process ends and returns to execute step  357 .  
         [0066]     For the present invention, the description and the claims that follow, variables, matrices, and indexes are assigned names and letters to facilitate an understanding of the invention. The variables, matrices, indexes, assigned names, and letters are chosen arbitrarily and are not intended to limit the invention.  
         [0067]     While the preferred embodiments of the present invention have been illustrated in detail, the skilled artisan will appreciate that modifications and adaptations to those embodiments may be made without departing from the scope of the present invention as set forth in the following claims.