Abstract:
In order to provide redundancy of host computers and storage array controllers, and thereby protect operation of a RAID system against host computer or storage array controller failure, two single RAID subsystems are conventionally provided in a conventional active-active configuration. This means each storage array controller has electrical access to each disk in its array as well as each disk in the other controller&#39;s array. The resultant interference between the controllers in accessing the disk channels causes the active-active system to normally function at approximately 130% of the speed of a single RAID system, rather than the optimum 200% of the speed of a single RAID system. The system of this invention, FULL-SPEED ACTIVE-ACTIVE redundant RAID system, contains a normally open switch or repeater which allows access by each storage array controller only to that controller&#39;s array of disks when both host computers and storage array controllers are operating normally. This provides a speed of 200% of the speed of a single RAID system. When one host computer or storage array controller fails, the switch or repeater is automatically closed, allowing the storage array controller of the functioning subsystem to control all of the disks of both single RAID subsystems.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     Not Applicable. 
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
     Not Applicable. 
     MICROFICHE APPENDIX 
     Not Applicable. 
     BACKGROUND OF THE INVENTION 
     (1) Field of the Invention 
     This invention relates to systems in which multiple controllers are used to control an array of storage devices. 
     (2) Description of Related Art Including Information Disclosed Under 37 CFR 1.97 and 37 CFR 1.98. 
     The acronym RAID refers to systems which combine disk drives for the storage of large amounts of data. In RAID systems the data is recorded by dividing each disk into stripes, while the data are interleaved so the combined storage space consists of stripes from each disk. RAID systems fall under 5 different architectures, plus one additional type, RAID- 0 , which is simply an array of disks and does not offer any fault tolerance. RAID  1 - 5  systems use various combinations of redundancy, spare disks, and parity analysis to achieve conservation reading and writing of data in the face of one and, in some cases, multiple intermediate or permanent disk failures. Ridge, P. M.  The Book Of SCSI: A Guide For Adventurers . Daly City Calif. No Starch Press.    1995   p. 323-329. In this application, a RAID system consisting of one host computer, one controller, and an array of multiple channels, each channel consisting of several direct access storage devices in serial electrical connection, will be termed a “single RAID subsystem”. 
     Conventional RAID systems guard against failure of a controller by the active-active system. This system consists of two single RAID subsystems, each with a host computer, a controller, and an array of direct access storage units. The direct access storage units, in the most common case, disks, are arranged in channels in which the disks are connected in a series. A common arrangement is for one controller to control six channels of five disks in each channel. In the active-active system, each channel of one system is connected electrically to another channel in another system. This means that, in the event of the failure of one controller, the other controller can serve all 10 disks in each “double” channel. Unfortunately, during normal operation when both controllers are operating there is interference associated with the fact that two controllers are simultaneously accessing a double channel of ten disks. This interference reduces the speed of a normally acting active-active system to about 130% of the speed of a single RAID subsystem rather than the 200% of a single RAID subsystem expected from the operation of two single RAID subsystems. 
     U.S. Pat. No. 5,768,623 discloses a system for storing data for several host computers an several storage arrays which are linked so that each storage array can be accessed by any host computer. The system uses dual ported disks and involves serial communication channels. No switches or repeaters are used to isolate the disk arrays during normal functioning of host computer and storage array controllers. 
     U.S. Pat. No. 5,729,763 discloses a system for storing data in which each of a number of disk interfaces is coupled to a corresponding disk drive by unidirectional channels. Each disk interface includes a unidirectional switch. Use of the switches allows a defective disk drive or switch to be removed without requiring shut-down of the entire system. 
     The RAID systems of the prior art do not provide the advantages of the present invention, that of increasing the overall speed of N same-speed single RAID subsystems to N times the speed of a single RAID system under normal conditions while providing for the sharing of multiple storage devices during conditions in which a host computer or storage array controller fails. 
     The system of the present invention is like the conventional active-active system except it incorporates a switch or repeater which isolates the channels of the two or more single RAID subsystems when all the host computers and controllers are functioning properly. If three same speed single RAID subsystems are included, for example, the system functions at 300% the speed of a single RAID subsystem during the vast preponderance of the time when all of the host computers and storage array controllers are functioning properly. In the case of a host computer or storage array controller failure, however, the bidirectional switch or bidirectional repeater closes and establishes electrical connection between the single RAID subsystem with the failure and the single RAID subsystem adjacent to it in the system. In this configuration the system has the speed expected of a conventional active-active system, after a host computer or storage array controller failure, about 100% of the speed of an individual RAID subsystem for the two affected single RAID subsystems. The remaining unaffected single RAID subsystems continue to operate at the unhindered maximum speed. 
     BRIEF SUMMARY OF THE INVENTION 
     The redundant RAID system of this invention extends the protection of the operation of a RAID system from providing for disk failure to providing for host computer or storage array controller failure. This invention consists of two or more (N) single RAID subsystems which are linked through the disk channels by a bidirectional switch or bidirectional repeater which is normally in the open position. Thus the system normally functions as (N) independent single RAID subsystems and functions at the speed of one single RAID subsystem multiplied by N if the single RAID subsystems all have the same speed. If the speed of the single RAID subsystems vary, the system normally functions at a speed which is the sum of the single RAID subsystems. In the event of a host computer or storage array controller failure, the bidirectional switch or repeater between two adjacent single RAID systems is changed to the closed position and the channels of disks of the functioning controller are electrically linked to the channels of disks of the disabled system. The functioning controller thus takes over the function of the disabled controller and provides continuing service, albeit at a reduced speed. The unaffected single RAID subsystems of the redundant RAID system of this invention continue to function unhindered. 
     In the normal operating mode the present invention enables each storage array controller to communicate with a set of disks independently of any other controller, thus operating the redundant RAID system at the speed of N single RAID subsystems. In the event of failure of one of the host computers or storage array controllers of a component single RAID subsystem, the system automatically assumes the configuration of a conventional active-active system with respect to the affected single RAID subsystem and the adjacent unaffected single RAID subsystem. The redundant RAID system continues to operate with access by the functioning adjacent RAID subsystem host computer and storage array controller to all of the disks of the failed and the functioning single RAID subsystems, although at a reduced speed. 
     Two advantages are associated with the present invention. 
     Firstly, a host computer and storage array controller redundant RAID system with a normal speed much higher than the conventional active-active host computer and storage array controller redundant systems is provided. In the event of failure of a host computer or storage array controller the speed of the system is no lower than that of a conventional host computer and storage array controller redundant system. If greater than two single RAID subsystems are included in the redundant RAID system, the speed of the system under nearly all conditions is greater than the conventional redundant system. 
     Secondly, the use of bidirectional repeater switching means allows the use of relatively long cables linking the disk channels, and provides additional flexibility in the physical location of the single RAID subsystem components of the invention. 
     The objective of this invention is to provide a host computer and storage array controller redundant RAID system which continues to operate despite the failure of a single host computer or storage array controller. 
     Another objective of this invention is to provide a N host computer and storage array controller redundant RAID system which operates at the speed of N single RAID subsystems if all have the same speed in the absence of failures, yet provides protection against host computer or storage array controller failure. 
     Another objective of this invention is to provide a N host computer and storage array controller redundant RAID system which continues to operate at a reduced speed during a host computer or storage array controller failure while the system continues to operate at the speed of N-2 single RAID systems if all subsystems have the same speed. 
     Another objective of this invention is to provide a N host computer and storage array controller redundant RAID system which continues to operate as long as fewer than or equal to N/2 of the single RAID subsystems suffer a failure of the host computer or storage array controller and each single RAID subsystem with a failed host computer or storage array controller is adjacent to a single RAID subsystem without a failure. 
     Another objective of this invention is to provide a host computer and storage array controller redundant RAID system which has repeater connections between the single RAID subsystem channels which allow extended physical separation between the single RAID subsystem components. 
     A final objective of this invention is to provide a host computer and storage array controller redundant RAID subsystem which is inexpensive, resistant to failure, easy to maintain, and is without harmful effects on the environment. 
    
    
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
     FIG. 1 is a diagrammatic representation of a single RAID subsystem. 
     FIG. 2 is a diagrammatic representation of a conventional prior art active-active RAID system with two controllers and two host computers. 
     FIG. 3 is a diagrammatic representation of one embodiment of the FULL-SPEED ACTIVE-ACTIVE redundant RAID system of the present invention. 
     FIG. 4 is a diagrammatic representation of a second embodiment of the FULL-SPEED ACTIVE-ACTIVE redundant RAID system of the present invention. 
     FIG. 5 is a flow chart of the process of operation of the first embodiment invention. 
     FIG. 6 is a flow chart of the process of operation of the second embodiment invention. 
     FIG. 7 is a diagrammatic representation of a core. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 1 is a schematic of the external view of a RAID system referred to in this application as a “single RAID subsystem”. The single RAID subsystem comprised a single host computer  10 , a storage array controller  30 , and an array of direct access storage devices (DASD). The host computer  10  is electrically connected to the disk array controller  30  by connector means  20 . 
     The connector means may be a wire or cable connector or a SCSI bus. 
     In all of the Figs. the convention is followed of depicting connectors which are not electrically connected as lines which cross perpendicularly. An electrical connection is indicated by a line which terminates perpendicularly at another line or at a symbol for a component. Thus in FIG. 1 the host computer  10  is electrically connected to disk array controller  30  by connector  20 . Connector  401  is electrically connected to disk array controller  30  and to DASD  1 A  40  and to DASD  1 B  41  but is not electrically connected to connectors  402  to  406 . 
     DASD may be disks, tapes, CDS, or any other suitable storage device. A preferred DASD is a disk. 
     All the DASD and connectors in a system taken as a whole is referred to as an “array” of DASD. The DASD are arranged in channels which consist of a number of DASD which are electrically connected to each other and to the disk array controller by connector means. The channels are designated in FIG. 1 as  1  to  6 . The number of channels may vary. A preferred number of channels is 6. 
     A channel, for example channel  1 , consists of connector  401 , DASD  1 A  40 , and DASD  1 B  41 . Although only two DASD are depicted in channel  1  of FIG. 1, there may be as many as 126 DASD in a channel. A preferred number of DASD in a channel is five. 
     A group of DASDs served by separate channels across which data is striped is referred to as a “tier” of DASDs. A DASD may be uniquely identified by a channel number and a tier letter, for example DASD  1 A is the first disk connected to channel  1  of the controller. 
     A preferred storage array controller  30  is the Z-9100 Ultra-Wide SCSI RAID controller manufactured by Digi-Data Corporation, Jessup Md. 
     Additional tiers of DASDs may be used. 
     Any suitable host computer  10  may be used. A preferred host computer  10  is a Pentium-based personal computer available from multiple vendors such as IBM, Research Triangle Park, N.C.; Compaq Computer Corp, Houston, Tex., or Dell Computer, Austin, Tex. 
     FIG. 2 shows the prior art active-active redundant host computer and storage array controller RAID system. This system comprises two single RAID subsystems of FIG. 1, system  11  and system  111  in FIG. 2 which are electrically connected through the disk array controllers and through the arrays of DASD. 
     FIG. 2 shows system  11  which comprises host computer  10 , connected by connector  20  to disk array controller  30 , and the system  11  array of which channels  1  to  6  consisting of connectors  401  to  406 , respectively, and associated DASD  40 - 60 , respectively. Only one DASD of each channel is depicted on FIG.  2 . 
     FIG. 2 also shows system  111  which comprises host computer  110 , connected by connector  120  to disk array controller  130 , and the system  111  array of which channels  1  to  6  consisting of connectors  401  to  406 , respectively, and associated DASD  141 - 161 , respectively. Only one DASD of each channel is depicted on FIG.  2 . Note that in both system  11  and system  111  the arrays are electrically connected bidirectionally to each system. For example, array  1  of system  11  is connected by connector  401  to array  1  of system  111 . 
     The disk array controller  30  of system  11  is connected to the disk array controller  130  of system  111  by a bidirectional connector which is depicted in FIG. 2 as connectors  300  and  310 . Disk array controller  30  contains internal software which generates a binary signal termed a “normal operating signal” or a “heartbeat” at an interval of a few milliseconds when the disk array controller  30  and host computer  10  of subsystem  11  are operational. When the host computer or disk array controller is in a defective condition, the emission of the normal operating signal ceases. The normal operating signal is emitted from disk array controller over connector  300  to the disk array controller  130  of subsystem  111 . Similarly, when the host computer  110  and disk array controller  130  of subsystem  111  are operating normally, a normal operating signal is emitted from disk array controller  130  over connector  310  to disk array controller  30  of subsystem  11 . 
     When one disk array controller no longer receives the normal operating signal because the host computer or disk array controller of the other system is defective, the operational disk array controller begins to assume the tasks of the defective array of the system containing the defective component. For example, if disk array controller  30  of subsystem  11  ceases to receive a normal operating signal from disk array controller  130  of subsystem  111 , disk array controller  30  will assume the control and service of not only its own DASD,  40 - 60  in FIG. 2, but also of the DASD of subsystem  111 ,  141 - 161 . Connector  20  also connects host computer  10  with disk array controller  130 . Similarly connector  120  connects host computer  110  with disk array controller  30 . Connectors  20  or  120  are used to transfer information from the host computer of a single RAID subsystem which has a faulty host computer or disk array controller to the disk array controller of the functional single RAID subsystem. This protects each component of the active-active RAID system from failure of any one host computer or disk array controller and allows each DASD to be read to or written from. 
     Unfortunately, the protection against failure in the system of FIG. 2 is achieved at a cost in speed of operation. An interference condition is created in any channel  401 - 406  of FIG. 2 because two disk array controllers are using a single connector to address the DASD of two single RAID subsystems. Each disk array controller must wait until the conductor is free before addressing its DASD. The net effect is a considerable reduction of speed in normal operation. If the speed of a single RAID subsystem is 100% (relative speed), then the relative speed of the active-active system of FIG. 2 under normal operating conditions is about 130%, rather than the 200% expected of two single RAID subsystems (which, however, do not enjoy the fault-tolerance associated with the redundant host computers and disk array controllers). 
     The present invention is designed to overcome the lack of performance associated with the active-active RAID system under normal conditions while retaining the fault-tolerance under conditions of failure of a host computer or disk array controller. 
     A first embodiment of the present invention, termed the FULL-SPEED ACTIVE-ACTIVE redundant RAID system, is depicted in FIG.  3 . 
     The system in FIG. 3 is identical to that in FIG. 2 with the exception of the addition of a normally open switch means between the channels which are connected in FIG.  2 . and the means to control the switch means. In FIG. 3 the electrical connector  401  between channel  1  of subsystem  11  and channel  1  of subsystem  111  is intercepted by core  70 . The core  70  consists of connections to channel  1  of subsystems  11  and  111  with normally open switch means, in this case a normally open repeater  90  electrically connected to and interposed between the segments of connector  401 , which has been segmented into connector  401  and  411 . When repeater  90  of core  70  is in the open position, there is no electrical connection between channel  1  of subsystem  11  and channel  1  of subsystem  111 . Similarly, switching means or repeaters  91 - 95  are interposed in the connections between channels  2 ,  3 ,  4 ,  5 , and  6 , respectively, and while the switching means or repeater  91 - 95 , respectively, are in the open position, there are no electrical connections between channels  2 ,  3 ,  4 ,  5 , and  6  of subsystem  11  and channels  2 ,  3 ,  4 ,  5 , and  6  of subsystem  111 , respectively. The core  70  is a container which contains and supports the switching means and the connection means for attaching switching means to a channel. Additional detail on the core  70  is found in FIG.  7 . 
     Any suitable switching means may be used such as a switch or a repeater. A preferred repeater is model SYM53C141 manufactured by LSI Logic Corp., Milpitas, Calif. The use of a repeater provides the advantage of amplifying the signal, thus making possible a greater physical distance between the single RAID subsystems of the redundant RAID system. 
     A preferred disk is a single port disk model ST39102LW manufactured by Seagate Technology, Inc. Scotts Valley, Calif. 
     When the switching means of the core are closed the electrical connections between the channels of subsystem  11  and subsystem  111  are formed. Under the conditions of closed switch means the system of FIG. 3 is electrically equivalent to that of the active-active system of FIG.  2 . 
     The core  70  is electrically connected to disk array controller  30  by connector  420 . The core  70  is also electrically connected to disk array controller  130  by connector  430 . 
     In operation, the switching means  90 - 95  in core  70  are normally open while each host computer and disk array controller is functioning normally. Under these normal conditions the channels of subsystems  11  and  111  are electrically isolated from each other. The relative speed achieved by the system is 200% of the speed of a single RAID subsystem. 
     In the rare event of failure of one host computer or disk array controller the normal operating signal or heartbeat emitted from a disk array controller is stopped. When the other disk array controller does not receive a normal operating signal it emits a closure signal to the core. The normally open switching means are now closed and the electrical connections between the channels of the functional and non-functional systems are closed, allowing the functional system to control the DASD of both subsystems. 
     In FIG. 3, if the host computer  110  or disk array controller  130  of subsystem  111  fails, the normal operating signal or heartbeat emitted from disk array controller  130  to disk array controller  30  would cease. Disk array controller  30  would emit a closure signal to the core  70  via connector  420 . The switching means  90 - 95  in core  70  would close, establishing electrical connection between channels  1 - 6  of subsystems  11  and  111 , respectively. Disk array controller  30  would then control the read and write function of the DASD of both subsystem  11  and  111 . In analogous fashion, failure of host computer  10  or disk array controller  30  of subsystem  11  would result in cessation of the normal operating signal or heartbeat from disk array controller  30  to disk array controller  130 . Disk array controller  130  would emit a closure signal to each core  70 - 75  via connector  430 . The switching means  90 - 95  in core  70  would close, establishing electrical connection between channels  1 - 6  of subsystems  11  and  111 , respectively. Disk array controller  130  would then control the read and write function of the DASD of both subsystems  11  and  111 . Connector  20  also connects host computer  10  with disk array controller  130 . Similarly connector  120  connects host computer  110  with disk array controller  30 . Connectors  20  or  120  are used to transfer information from the host computer of a single RAID subsystem which has a faulty host computer or disk array controller to the disk array controller of the functional single RAID subsystem. Under these fault conditions the relative speed expected is 100% of that expected from a single RAID subsystem. 
     The advantage of the present invention is that it achieves a relative speed of 200% under normal conditions in the absence of fault in host computer or disk array controller. Under the rare conditions of fault, the present invention has a relative speed of 100%. This is to be contrasted to the conventional active-active RAID system which has a relative speed of 130% under normal conditions and 100% under fault conditions. Finally, it is to be contrasted with a “system” consisting of two unconnected single RAID subsystems, which have a normal speed of 200%, but under conditions of fault in one host computer or disk array controller, is unable to access the DASD served by that host computer or disk array controller. 
     FIG. 4 is a diagrammatical representation of a second embodiment of the present invention. In FIG. 4 the FULL-SPEED ACTIVE-ACTIVE redundant RAID system is shown with 3 single RAID subsystems,  1 ,  111 , and  211 . This may be extended to N subsystems, where N is a number greater than two, by the addition of single RAID subsystems. In the second embodiment invention the number of cores is the same as the number of single RAID subsystems and cores. In the second embodiment the normal operating signal of one disk array controller is received by the adjacent disk array controller. There is no bidirectional exchange of normal operating signals between one set of two disk array controllers as in the first embodiment of the invention. In the second embodiment, the system normally functions at a speed which is the sum of the speed of the component single RAID subsystems. When a fault in a host computer or disk array controller occurs, the single RAID subsystem adjacent to the single RAID subsystem with the fault takes over the control of both its DASD and those of the single RAID subsystem with the faulty component. The remaining single RAID subsystems continue to operate normally. 
     The system depicted in FIG. 4 is the same as than in FIG. 3 except as noted below. This second embodiment comprises 3 single RAID subsystems,  11 ,  111 , and  211  which are arranged so that subsystem  11  is adjacent to subsystem  211 , subsystem  211  is adjacent to subsystem  111 , and subsystem  111  is adjacent to subsystem  11 . This may be extended to include N subsystems. In FIG. 4 only two channels in each single RAID subsystem are shown for convenience. A preferred number of channels in each single RAID subsystem is six. 
     Under normal conditions disk array controller  30  sends a normal operating signal or heartbeat via connector  320  to disk array controller  230 . Disk array controller  230  sends a normal operating signal via connector  330  to disk array controller  130 . Disk array controller  130  sends a normal operating signal to disk array controller  30  via connector  310 . 
     In normal operation, the switching means  90  and  91 ,  190  and  191 , and  290  and  291  in cores  70 ,  170 , and  270 , respectively, are normally open while each host computer and disk array controller is functioning normally. Under these normal conditions the channels of subsystems  11 ,  111  and  211  are electrically isolated from each other. The relative speed achieved by the system is the sum of the N single RAID subsystems or, in FIG. 4, 300% of the speed of a single RAID subsystem when the speed of the subsystems are equal. 
     In the rare event of failure of one host computer or disk array controller the normal operating signal or heartbeat emitted from a disk array controller is stopped. When the adjacent disk array controller does not receive a normal operating signal it emits a closure signal to the cores which link the channels of the functioning adjacent subsystem with the faulty subsystem. The normally open switching means are now closed and the electrical connections between the channels of the functional and non-functional systems are closed, allowing the functional system to control the DASD of both systems. 
     In FIG. 4, if the host computer  110  or disk array controller  130  of subsystem  111  fails, the normal operating signal or heartbeat emitted from disk array controller  130  to disk array controller  30  via connector  310  would cease. Disk array controller  30  would emit a closure signal to core  70  via connector  440 . The switching means  90 - 91  in cores  70  would close, establishing electrical connection between channels  1 - 2  of subsystems  11  and  111 , respectively. Disk array controller  30  would then control the read and write function of the DASD of both subsystem  11  and  111 . Under these fault conditions the relative speed expected of the two involved subsystems is 100% of that expected from a single RAID subsystem. 
     In FIG. 4, channel  1  of one subsystem is shown as connected by the switching means in the core to channel  1  of the adjacent RAID subsystem. It is not necessary that channels having the same numbers are served by a single switching means. It is necessary, however, that channel identifiers are included in the information sent to and received from each channel in order to allow the active disk array controller to identify the correct channel when the active disk array controller is controlling channels from two RAID subsystems. 
     In analogous fashion, failure of host computer  10  or disk array controller  30  of subsystem  11  would result in cessation of the normal operating signal or heartbeat from disk array controller  30  to disk array controller  230 . Disk array controller  230  would emit a closure signal to core  270  via connector  450 . The switching means  290 - 291  in core  270  would close, establishing electrical connection between channels  1 - 2  of subsystems  11  and  211 , respectively. Disk array controller  230  would then control the read and write function of the DASD of both subsystem  11  and  211 . Under these fault conditions the relative speed expected of the two involved subsystems is 100% of that expected from a single RAID subsystem. 
     In an alogous fashion, failure of host computer  210  or disk array controller  230  of subsystem  211  would result in cessation of the normal operating signal or heartbeat from disk array controller  230  to disk array controller  130  via connector  330 . Disk array controller  130  would emit a closure signal to core  1   70  via connector  460 . The switching means  190 - 191  in cores  170  would close, establishing electrical connection between channels  1 - 2  of subsystems  111  and  211 , respectively. Disk array controller  130  would then control the read and write function of the DASD of both subsystems  111  and  211 . 
     Connector  20  also connects host computer  10  with disk array controller  130 . Similarly connector  120  connects host computer  110  with disk array controller  230 . Finally, connector  220  connects host computer  210  with disk array controller  30 . Connectors  20 ,  120  or  220  are used to transfer information from the host computer of a single RAID subsystem which has a faulty host computer or disk array controller to the disk array controller of the functional single RAID subsystem. Under these fault conditions the relative speed expected of the two involved subsystems is 100% of that expected from a single RAID subsystem. 
     Under the normal operating condition of a system with N single RAID subsystems, the expected speed is the sum of the speeds of the single RAID subsystems or (N)(100%) of a single RAID subsystem if all the single RAID subsystems have the same speed. Under conditions of fault in the host computer or disk array controller of one or more single RAID subsystems, the expected speed is (N−F)(100%) when F is the number of single RAID subsystems with faults and all single RAID subsystems have the same speed. 
     FIG. 5 is a flow chart showing the process in a first embodiment redundant RAID system comprising a first and a second single RAID subsystem and a core which follows the failure of one storage array controller. When the storage array controller of the second single RAID subsystem fails to function normally, the normal operating signal or heartbeat ceases to be emitted by the storage array controller  510 . The storage array controller of the first single RAID subsystem notes the cessation of the normal operating signal and emits a closure signal to the switching means in the core  520 . The switching means closes, thereby establishing electrical connection between the channels which comprise the arrays of both the first and second single RAID subsystems  530 . The storage array controller of the functional first single RAID subsystem appropriates the flow of data to and from the host computer of the defective second single RAID subsystem  540 . This occurs after the second host computer makes several futile attempts at I/O operations with the faulty second storage array controller. Finally, storage array controller of the functional first single RAID subsystem serves the channels of both the functional first and second host computers  550 . 
     FIG. 6 is a flow chart showing the process in a second embodiment redundant RAID system comprising N single RAID subsystems and N core which follows the failure of one storage array controller. When the storage array controller of a second single RAID subsystem fails to function normally, the normal operating signal or heartbeat ceases to be emitted by the storage array controller  610 . The storage array controller of the first single RAID subsystem, which is adjacent to the failed second subsystem, notes the cessation of the normal operating signal and emits a closure signal to the switching means in the core  620 . The switching means closes, thereby establishing electrical connection between the channels which comprise the arrays of both the first and second single RAID subsystems  630 . The storage array controller of the functional first single RAID identifies the channels of the second single RAID subsystem which are electrically connected to the channels of the first single RAID subsystem by the switching means  640 . The storage array controller of the functional first single RAID subsystem appropriates the flow of data to and from the host computer of the defective second single RAID subsystem  650 . Finally, the storage array controller of the functional first single RAID subsystem serves the channels of both the functional first and second host computers  660 . 
     FIG. 7 is a diagrammatic representation of a core  70  as used with the first embodiment invention. The components of the core  70  are mounted in a case  71 . Six repeaters  90 ,  101 ,  102 ,  103 ,  104 , and  105  are mounted in the case. Connectors or channel connection means  121 ,  122 ,  123 ,  124 ,  125 , and  126  are used to make electrical connection with the channels of one single RAID subsystem,  11  in FIG. 3, with the repeaters,  90 ,  101 ,  102 ,  103 ,  104 , and  105 . Connectors or channel connection means  131 ,  132 ,  133 ,  134 ,  135 , and  136  are used to make electrical connection with the channels of the other single RAID subsystem,  111  in FIG. 3, with the repeaters,  90 ,  101 ,  102 ,  103 ,  104 , and  105 . In this arrangement, there is no electrical connection between the channels of single RAID subsystems  11  and  111  when the repeaters are in the open position. There is electrical connection between the channels of single RAID subsystems  11  and  111  when the repeaters are in the closed position. Connector  420  receives the closure signal from one disk array controller,  30  in FIG. 3, and connector  430  receives the closure signal from the other disk array controller,  130  in FIG.  3 . 
     The core  70  of FIG. 7 is also used in the second embodiment of the invention with the exception that only one connector is used to receive the closure signal from a storage array controller. 
     It will be apparent to those skilled in the art that the examples and embodiments described herein are by way of illustration and not of limitation, and that other examples may be used without departing from the spirit and scope of the present invention, as set forth in the claims.