CROSS REFERENCE TO RELATED APPLICATIONS 
   This application is a continuation of U.S. patent application Ser. No. 11/183,461 filed Jul. 18, 2005, which issued on Feb. 6, 2007 as U.S. Pat. No. 7,174,423 which is a continuation of U.S. patent application No. 09/767,773 filed Jan. 23, 2001, which issued on Aug. 30, 2005 as U.S. Pat. No. 6,938,122. 


   BACKGROUND OF THE INVENTION 
   The invention relates to data storage systems, and in particular, to data storage systems with remote data mirroring capability. 
   Given the importance of the availability of information, several techniques have been developed for providing enhanced reliability and availability of data stored in a data storage system. Once such technique is data mirroring. In a “mirrored” system, the data stored on one data storage system is replicated on another data storage system. Thus, if one or more storage devices on one of the data storage systems fails, or a catastrophic system failure should occur, the data is readily available in the form of a mirrored copy from the mirrored data storage system. 
   Devices on a data storage system (or source) that are mirrored on the same remote, data storage system (or target) are referred to as a device group. Likewise, devices on the target that serve or mirror devices on the same source are referred to as a device group. Device groups are used, for example, to insure the consistency of blocks of data too large to be stored in a single area, during planned or unplanned interruptions such as link failures or planned back-ups. Device groups typically span a number of physical and logical volumes, and, in some instances, as when data striping is used, several device groups may be mapped to different portions of a physical or logical volume. 
   Typically, the source and target device groups are arranged as pairs on any two systems and each source/target device group pair is connected by two dedicated links (e.g., ESCON or Fibre Channel links), one for supporting writes during data backup operations and reads during data recovery operations, and the other for redundancy. Thus, a redundant arrangement of N data storage systems in which each data storage system is capable of supporting a mirrored configuration involving each of the N−1 other remote data storage systems requires N*(N−1) physical links, or an average of 2*(N−1) physical links per system. This link requirement becomes impractical when N&gt;3, as the number of ports on the data storage system is limited. 
   SUMMARY OF THE INVENTION 
   In one aspect of the invention, in a remote data mirroring arrangement of data storage systems, ports on a data storage system are connected to ports on other data storage systems. Each storage system is provided with configuration topology information. From a switch fabric that connects to ports of all of the data storage systems information identifying ports of the other data storage systems connected to the switch fabric is determined. The configuration topology information and the information obtained from the switch fabric are used to establish a logical link between a port on the storage system and a second port on a second storage system so that data residing on a device group supported by the port and a corresponding, mirrored device group supported by the second port can be exchanged between the data storage system and the second data storage system. 
   In another aspect of the invention, a system includes an arrangement of storage systems each adapted to control at least one group of devices that are supported in a mirrored configuration with a corresponding group of devices controlled by one of the other storage systems. For each device group and corresponding device group, first ports are associated with the device group and second ports are associated with the corresponding device group. The system further includes a switch element adapted to connect one of the first ports to at least one of the second ports so that data may be exchanged between the first and second ports for each device group and corresponding device group. 
   In yet another aspect of the invention, in a remote, mirrored arrangement of data storage systems, a data storage system includes a port adapted to control at least one device group and a switch element coupled to the port and ports in the other storage systems. The port uses the switch element to link the port to a selected one of the ports controlling a second device group that mirrors the device group controlled by the port. 
   The advantages of the invention include the following. The replacement of a dedicated one-to-one protocol such as ESCON with a switched protocol reduces the required number of ports from 2(N−1) to 2. Such increased connectivity provides for better performance (e.g., load balance) as well as increased redundancy. Also, because each processor on a controller of a data storage system is able to service multiple device groups, a system user can use full remote data mirroring connectivity without having to dedicate processors to supporting that connectivity. 
   Other features and advantages of the invention will be apparent from the following detailed description and from the claims. 


   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a block diagram of a system comprised of  FIGS. 1A and 1B . 
       FIG. 1A  is a first portion of a block diagram of the system of  FIG. 1  that includes data storage systems having remote data minoring capability relative to each other and that are connected to a Fibre Channel (FC) switch fabric. 
       FIG. 1B  is a second portion of a block diagram of the system of  FIG. 1  that includes data storage systems having remote data minoring capability relative to each other and that are connected to a Fibre Channel (FC) switch fabric. 
       FIG. 2  is a detailed block diagram of a disk director used to establish logical link connections over the switch fabric for an FC port. 
       FIGS. 3A-3D  are illustrations of configuration topology tables.  FIG. 3A  is an illustration of a remote systems table.  FIG. 3B  is an illustration of a device groups table.  FIG. 3C  is an illustration of a disk directors table.  FIG. 3D  is an illustration of a logical links table. 
       FIG. 4  is a format of a World Wide Name field. 
       FIG. 5  is a logical depiction of the system of  FIG. 1  with an exemplary configuration of controllers configured to support various device groups. 
       FIG. 6  is a depiction of a configuration topology graph corresponding to the system shown in  FIG. 5 . 
       FIG. 7  is a flow diagram of a connection initialization process performed by a processor in a disk director. 
       FIG. 8  is a flow diagram of a single link discovery process performed by a processor in a disk director. 
   


   DETAILED DESCRIPTION 
   The present invention features a system environment in which data identical to that stored on a data storage system is stored on a geographically remote data storage system. The remote data storage system is used to backup data stored on the data storage system and provide data recovery when the data storage system and its data are lost as a result of malfunction or disaster. 
   Referring to  FIGS. 1 ,  1 A and  1 B. a system  10  includes data storage systems  12   a ,  12   b ,  12   c  and  12   d . The data storage systems  12   a ,  12   b  and  12   c  are connected to at least one host computer (or host)  14   a ,  14   b  and  14   c , respectively. The host computer  14  may be, for example, a personal computer, workstation, or the like which may be used by a single user, or a multi-user system. The data system  12  receives data and commands from, and delivers data and responses to, the host computer  14 . 
   The data storage systems  12   a - 12   d  are mass storage systems having respective controllers  16   a - 16   d , each of which is coupled to pluralities of storage devices (or, simply, devices) shown as disks  18 . The controller  16 a is coupled to devices  18   a , devices  18   b  and devices  18   c . The controller  16   b  is coupled to devices  18   d , devices  18   e  and devices  18   f . The controller  16   c  is connected to devices  18   g , devices  18   h  and devices  18   i . The controller  16   d  is coupled to devices  18   j  and devices  18   k . Each of the devices  18  is logically divided, in accordance with known techniques, into one or more logical volumes. 
   Each controller  16  interconnects the host computer  14  and the devices  18 , and can be, for example, that made by EMC and known as the Symmetrix controller. The controllers  16   a - 16   c  receive memory write commands from respective host computers  14  directly from host buses  20   a - 20   c , respectively, for example, connected and operated in accordance with a SCSI protocol, and deliver the data associated with those commands to the appropriate devices  18  over connecting buses  22   a ,  22   b , . . .  22   k . Buses  22  also preferably operate in accordance with a SCSI protocol. The controllers  16   a - 16   c  also receive read requests from the host computer  14  over host bus  20 , and deliver requested data to the host computer  14 , either from a cache memory of the controller  16  or, if the data is not available in cache memory, from the devices  18 . 
   In a typical configuration, the controller  16  also connects to a service management console (not shown), which is used for maintenance and access to the controller  16  and can be employed to set parameters of the controller  16  as is well known in the art. 
   Typically, and as indicated above, each of the devices  18  is configured to store logical volumes (or devices). There can be a plurality of logical volumes, for example 4, 8, or more logical volumes, on a physical device. In a configuration in which multiple copies of a logical volume are maintained, that is, in particular, where two copies of data are separately stored for a logical volume, it is said that mirrors are maintained. (There could be more than two mirrored copies. Typically, however, mirrored pairs are employed.) The controller  16  can then respond to a read request by reading from either of the copies stored in the storage devices  18 . Mirroring may exist at both the logical volume and physical device level, or at the logical volume level only. Data mirroring configurations can occur on the same controller, or on different controllers, as in the case of remote data mirroring (or remote data facility, ORDF). 
   In operation, the host computer  14  sends, as required by the applications it is running, commands to the data storage system  12  requesting data stored in the logical volumes or providing data to be written to the logical volumes. Still referring to  FIGS. 1 ,  1 A and  1 B the controllers  16   a - 16   c  include host adapters  24   a - 24   c  for facilitating communications with the host computers  14   a - c , respectively. The host computer  14  typically connects to a port of the host adapter  24  over the SCSI host bus line  20 . 
   The controllers  16   a - 16   d  each further include a global memory  30 , shown as global memories  30   a - 30   d , respectively. The host adapter  24  in each controller connects over at least one system bus  31  to the global memory  30 . Also connected to the global memory system  30  in each of the controllers  16  are disk directors  32 , more specifically, disk directors  32   a - 32   b  in the controller  16   a , disk directors  32   c - 32   d  in the controller  16   b , disk directors  32   e - 32   f  in the controller  16   c  and disk directors  32   g - 32   h  in the controller  16   d . The disk directors  32  communicates with the host adapter  24  through the global memory  30 . Although not shown, the global memory  30  can include a cache memory for storing data, as well as various data structures for maintaining control information and for supporting communications between the hosts  14  and the devices  18 . 
   The disk directors  32  control the storage devices  18 . Each of the disk directors  32   a - 32   h  includes a respective one of ports  34   a - 34   h , respectively. In the described embodiment, the disk directors  32  are installed in the controller  16  in pairs. Thus, only two disk directors in each of the controllers  16  are shown. However, it will be understood that additional disk directors may be employed by the system  10 . 
   The disk directors in the controllers  16   a - 16   d  communicate with the respective global memories  30   a - d  over dedicated buses  36   a - 36   d , respectively. During a write operation, the disk director  32  reads data stored in the global memory  30  by the host adapter  24  and writes that data to the appropriate logical volumes. During a read operation and in response to a read command, the disk director  32  reads data from a logical volume and writes that data to the global memory  30  for later delivery by the host adapter  24  to the requesting host  14 . 
   In the system  10  of  FIGS. 1 ,  1 A and  1 B each of controllers  16  is connected to a switch fabric  38 . The switch fabric  38  includes a plurality of fabric ports (FP)  40 , shown as FP  40   a , FP  40   b , FP  40   c , FP  40   c , FP  40   d , FP  40   e , EP  40   f , FP  40   g  and FP  40   h . Each port in each disk director is connected to a different one of the fabric ports  40 . As shown, ports  34   a - 34   h  are coupled to the fabric ports  40   a - h , respectively, over connections or links  42   a - 42   h , respectively. The switch fabric allows each of the controllers  12  to be connected to other controllers  12  when the others controllers are serving as remote data storage systems that provide backup capability in the form of minored storage for the data stored in the data storage system  12 . With respect to any given one of the controllers  16 , a “remote” controller or system will be any other controller  16  that maintains mirrored copies of data stored on that given controller  16  or stores data for which the given controller  16  itself maintains mirrored copies. Thus, any controller  16  and another controller that is a remote controller (and thus provides RDF frmnctionality with respect to that controller) are said to be in a mirrored configuration or arrangement. Remote data mirroring facility and recovery procedures may be performed using known techniques, such as those described in the above-referenced U.S. Pat. No. 5,742,792. 
   In the described embodiment, the switch fabric  38  is a Fibre-Channel fabric. However, other one-to-many switch protocols, e.g., Gigabit Ethernet, can be used. 
   Other system implementations are contemplated. For example, it will be understood that the data storage systems  12  need not be limited to one host computer as shown. For instance the data storage system  12  could be coupled to more than one host computer. Alternatively, and as is illustrated with the data storage system  12   d , the data storage system  12  need not be coupled to a host device at all. Such a system data storage system may be used only to perform writes in connection with write requests received from one of the other data storage systems  12   a - c  so that updated information stored by the host adapter on that other data storage system is also stored on the remote data storage system  12   d , thereby maintaining the information stored in the other data storage system, e.g., data storage system  12   a , in a minored condition on the remote data storage system  12   d . Also, the host adapter  24  can include a plurality of host adapters, each supporting a different host bus/host computer. There could be more or few than the four data storage systems shown in  FIGS. 1 ,  1 A and  1 B. There could be more than two ports per data storage system. 
   Referring to  FIG. 2 , each disk director  32  includes a processor  50  coupled to a local, nonvolatile memory (NVM)  52  by an internal bus  54 . The processor  50  controls the overall operations of the disk director  32  and communications with the local memory  52 . The nonvolatile memory  52  stores firmware  56  and parameter data in a parameter store  58 . Also included in the local memory  52  are various data structures  60 , i.e., configuration topology tables  64 , which maintain configuration information for the disk director  32  as well as the system  10 . The tables  64  include a remote systems table  64   a , a device groups table  64   b , a directors (or processors) table  64   c  and a logical links table  64   d . The functionality of these data structures will be described later. 
   Still referring to  FIG. 2 , the firmware  56  includes a number of processes executed by the processor  50  to control data transfer between the host computer  14  (if present), the global memory  32  and the storage devices  18 . In addition, the firmware  56  requires processes to control communications between the port  34  and ports in other controllers via the switch  38 . To that end, the firmware  68  is adapted to include a connection initialization process  66  and a single link discovery process  68 , as will be described. The firmware  56 , data structures  60  and parameter store  58  are read each time the data storage system is initialized. The firmware  56  is copied to the volatile memory  52  at initialization time for subsequent execution by the processor  50 . 
   The various configuration topology tables  64  are illustrated in  FIGS. 3A-3D . Referring to  FIG. 3A , a remote systems table  64 a includes an entry  70  for each controller/data storage system that is involved in a mirrored arrangement with the data storage system. Each entry  70  includes a controller (system) serial number  72 ; a controller (system) model number  74 ; and a firmware (code) level identifier  76 . Other system-specific information can be included as well. 
   Referring to  FIG. 3B , the device groups table includes an entry for each device group  78 , that is,  78   a ,  78   b , . . .  78   k , that includes the following: a device group name  80 ; a pointer to a remote storage system that serves the device group  82 ; a list of device group devices  84  in association with characteristics (including RDF parameters)  86 . Referring to  FIG. 3C , the processor level table  64   c  includes an entry  90  for each disk director&#39;s processor  50 , and each entry  90  includes a corresponding name (e.g., WWN, or IP name)  92  and a list of pointers to the device groups supported by the processor  94 . Referring to  FIG. 3D , the logical links table  96  includes link entries  98 . Each includes the following: a link state  100 ; a pointer to one of the processors  102 ; and a pointer to one of the device groups  104 . 
   Referring to  FIGS. 3A-3D , the tables  64   a ,  64   b  and  64   c  are configured by the system user. The link table  64   d  is generated by the connection initialization and link discovery processes  66  and  68 , respectively (of  FIG. 2 ), as will be described. 
   Referring to  FIG. 4 , an exemplary format of a World Wide Name (WWN)  106  identified in the field  92  of the processors table  64   c  is shown. The WWN  106  provides to each processor that is involved in an RDF configuration a unique name. The WWN  106  includes a vendor-specific vendor number field  108  for identifying the manufacturer of the controller  16 , a serial number field  110  for identifying a serial number of the controller  16  and a director number  112  for identifying the port/processor. In the implementation shown, the lengths of the fields  108 ,  110  and  112  are 28 bits, 30 bits and 6 bits, respectively. 
   Referring to  FIG. 5 , an exemplary logical depiction of the system  10  (from  FIG. 1 ) is shown. In this example, there are ten device groups, S 1 , S 2 , S 3 , S 4 , S 5 , T 1 , T 2 , T 3 , T 4  and T 5 , which are also indicated by reference numerals  120   a ,  120   b ,  120   c ,  120   d ,  120   e ,  122   a ,  122   b ,  122   c ,  122   d , and  122   e , respectively. Five of the device groups, S 1  through S 5 , are source device groups, and device groups T 1  through T 5  are target device groups. In the example shown, the controllers are configured for remote data mirroring capability in the following manner: the controller  16   a  supports device groups S 1 , S 2  and T 4 ; the controller  16   b  supports device groups S 3 , S 5  and T 1 ; the controller  16   c  supports device groups S 4 , T 3  and T 5 ; and the controller  16   d  supports device group T 2 . Thus, the devices in source device group S 1  on the controller  16   a  are mirrored in devices in corresponding target device group T 1  on the controller  16   b , the devices in S 2  (in the controller  16   a ) are mirrored in T 2  (in the controller  16   d ), and so forth. Each controller has at its disposal  12  different possible logical links. For example, the controller  16   b  can use its two ports, port  34   c  and  34   d  to achieve the following logical links: port  34   c  to port  34   e , port  34   c  to port  34   f , port  34   c  to  34   g , port  34   c  to port  34   h , port  34   c  to port  34   a , port  34   c  to port  34   b , port  34   d  to port  34   e , port  34   d  to port  34   f , port  34   d  to  34   g , port  34   d  to port  34   h , port  34   d  to port  34   a  and port  34   d  to port  34   b . With the particular configuration illustrated in the figure, a port supporting a device group such as S 3  needs to be in communication with a port for the corresponding device group, in this case, T 3 . As shown in the figure, using the switch architecture and the two ports/processors per controller, a connection between two device groups, e.g., S 3  and T 3 , can be achieved with one of four possible logical links: port  34   c  to port  34   e , port  34   c  to port  34   f , port  34   d  to port  34   e  and port  34   d  to port  34   f.    
   Referring now to  FIG. 6 , an exemplary topology graph that would be maintained by the controller  16   b  for the system configuration of  FIG. 5  is depicted. At the first level, the remote system information  64   a  includes a representation of controllers  16   a  (Serial No. 012345678) and  16   c  (Serial No. 036357760), corresponding to entries in the remote systems table  64   a  ( FIG. 3A ). The next level shows device group T 1  pointing to controller  16   a  because controller  16   a  maintains S 1 , which is the RDF counterpart for the device group T 1 . Similarly, device groups S 3  and S 5  point to the controller  16   c , as that controller serves these device groups with respective mirrored device groups T 3  and T 5 . These device group representations each correspond to an entry in the device group table  64   b  (FIG.  3 B). The next level shows which of the processors (processors associated with ports  34   c  and  34   d ) is configured to support which device groups. As represented by the graph (which corresponds to table  64   c  of  FIG. 3C ), a processor for one of the directors  32   c - 32   d  is configured to support and thus points to device groups T 1  and S 3 . The processor for the other one of the directors  32   c - 32   d  is configured to support and thus points to S 5 . The bottom level shows the various logical links that are established (solid lines/arrows) or are desired (dashed lines/arrows). As shown, logical links are already established for T 1  and S 3 , and the arrows reflect the pointer data stored in the entries of the logical links table  64   d  ( FIG. 3D ). For these established links, the link state is active and the state value stored in the field  100  of table  64   d  is ‘0×ff’. 
   To establish logical links, such as the one between S 5  and T 5 , a port (more specifically, the associated disk director&#39;s processor) needs to execute the connection initialization process  66  and the single link discovery process  68 , as will be described below with reference to  FIGS. 7 and 8 , respectively. 
   Referring to  FIG. 7 , the connection initialization process  66  (process  66 ) begins by obtaining from the switch fabric  40  a list of ports connected to the switch fabric  40  (step  130 ). Preferably, that list includes the World Wide Name for each of those ports. The process  66  proceeds to examine the first item on the list (step  132 ). The process  66  determines if the WWN of the port is greater than or equal to the WWN of the port on which the process executes, hereinafter, “our_WWN” (step  134 ). If it is determined that the WWN is greater than or equal to our_WWN, the process  66  determines if there is another item on the list (step  136 ). If there are no more items on the list, the process  66  terminates (step  137 ). If there is another item to be examined, the process  66  proceeds to the next item on the list (step  138 ) and returns to step  134 . If the determination at step  134  indicates that WWN is less than our_WWN, the process  66  determines if the vendor number is equal to the vendor number for the port for which the process executes, or “our_vendor_number” (step  140 ). If it is not, the process  66  ignores that vendor number and proceeds to step  136 . If the vendor numbers are the same, the process  66  extracts the serial number and director number from the WWN (step  142 ). The process  66  determines which, if any, device group is served by the WWN (step  144 ). That is, it determines if the serial number extracted from the WWN matches the serial number pointed to by any device group. If there is no such match, the process  66  returns to step  136 . If there is a match, for each device group served by the serial number, the process  66  performs the following. The process  66  determines if a logical link already exists for the device group and serial number (step  146 ). If a logical link has already been established, the process  66  returns to step  136 . If no logical link exists, the process  66  adds a link to the link table by setting the state equal to one and adding pointers that point to a device group served by the matched serial number and the processor that points to that device group (step  148 ). The process  66  initiates the single link discovery process  68  to establish a link to the remote processor identified by the WWN director number (step  150 ). The process  66  then returns to step  136 . Thus, the process  66  performs steps  134  through  150  for each port represented on the list. The process  66  is performed as a background process and is repeated in accordance with a timing parameter set in the parameter store  58  ( FIG. 2 ). 
   Referring to  FIG. 8 , a detailed flow of the single link discovery process  68  (process  68 ) is shown. The process  68  initiates a login process for the port that is initiating the link discovery (initiator port) and the fabric (step  160 ). In the described embodiment for a FC switch, the login is performed using known FC login techniques. The process  68  of the initiator port initiates contact with the remote (responder) port by sending a synchronization message to the responder port (step  162 ). Synchronization may be achieved in this manner by resetting the responder port. If a timeout occurs prior to receiving a response or the link attempt is rejected by the fabric because the link from the fabric to the responder port is has failed (step  164 ), the process  68  retries synchronization after a predetermined retry delay has occurred (step  166 ). Otherwise, a response is received from the responder port and the synchronization message causes the responder port to perform its own FC login, as well as create a link entry for the port with a link state value of one as part of its own link discovery process (step  168 ). The initiator port reads the configuration information for the device group pointed to by the remote port and compares it to the configuration for the device group (step  170 ). It provides the device group to the responder port, which causes the responder group to determine if it supports the device group by reading the configuration data for the initiator port&#39;s device group and comparing it to configuration information stored for the remote device group (step  172 ). If the process does not detect or receive indication of a mismatch, that is the link discovery is successful (step  174 ), it causes the initiator port to advance its link state to a an ‘0×xff’ value (step  176 ). Alternatively, if the process  68  determines that link discovery is unsuccessful due to an RDF mismatch/rejection, the process  68  caches the information about the failure (notes the WWN of the remote port) so that the process  68  can avoid repeating attempts to establish a connection that cannot be established due to configuration limitations or mismatches (step  178 ). 
   Thus, during a single link discovery, the process  68  causes the state machines in both the initiator and responder ports to attempt to advance their state from ‘1’ to ‘0×ff’ with four possible results: ‘ff’ (link established); timeout (fail) with retry; “link down” rejection; or RDF rejection. The link is successfully established when both state machines have advanced the link state to ‘ff’. 
   It will be appreciated that numerous other modifications may be made in connection with the invention. The controllers  16  may further support a mirror service policy which identifies which disk directors in conjunction with the storage devices that they control are primarily responsible for satisfying requests from a host computer. This policy could be fixed at the beginning of a system set up, taking into account the expected loads. Preferably, however, the mirror service policy implementation is dynamic. That is, the mirror service policy can be modified, periodically, in response to statistics describing the nature of the read and write requests to the data storage system  12 , to change the mirror service policy during the operation of the data storage system  12 . An example of a dynamic mirror service policy (DMSP) is described in U.S. Pat. No. 6,112,257. 
   Additions, subtractions, and other modifications of the preferred embodiments of the invention will be apparent to those practiced in this field and are within the scope of the following claims.
A mechanism to support remote mirroring of storage devices by data storage systems in a one-to-many switched environment. Each data storage system includes a disk director that is adapted to control at least one device group that is supported in a mirrored configuration with a corresponding device group controlled by one of the other data storage systems. For each device group and corresponding device group, there are first ports associated with the device group and second ports associated with the corresponding device group. A switch element is adapted to connect one of the first ports to at least one of the second ports so that data may be exchanged between the ports for each device group and corresponding device group. The disk director selects which first port is to be connected to which second port via the switch element in the establishment of a logical link. Thus, each port connected to the switch is capable of achieving multiple connections to multiple destinations for increased connectivity, redundancy and performance (load balance) without additional hardware.