Abstract:
A network storage controller for transferring data between a host computer and a storage device, such as a redundant array of inexpensive disks (RAID), is disclosed. The network storage controller includes at least one channel interface module which is adapted to be connected to the host computer and storage device. The channel interface module is connected to a passive backplane, and selectively transfers data between the host computer and storage device and the passive backplane. The network storage controller also includes at least one controller memory module, attached to the passive backplane. The controller memory module communicates with the channel interface module via the passive backplane, and processes and temporarily stores data received from the host computer or storage device. In applications where redundancy is required, at least two controller memory modules and at least two channel interface modules are used. The controller memory modules may mirror data between one another using the passive backplane and a shared communication path on the channel interface modules, thereby substantially avoiding the use of host or disk channels to mirror data. The channel interface modules are operable to selectively connect the host computer or storage device to one or more controller memory modules. The controller memory modules may include a DMA engine to facilitate the transfer of mirrored data.

Description:
FIELD OF THE INVENTION 
     The present invention is related to network storage controllers, and in particular, to a network storage controller utilizing redundant modular components with the data processing functions independent of the I/O interface. 
     BACKGROUND OF THE INVENTION 
     Network storage controllers are typically used to connect a host computer system with peripheral storage devices, such as disk drives or tape drives. The network storage controller acts as an interface between the host computer and the peripheral storage devices. In many applications, the network storage controller performs processing functions on the data transferred between the host computer and peripheral devices. One common application of such a system is a Redundant Array of Independant Disks (RAID). A RAID system stores data on multiple disk drives to protect the data against disk drive failure. If one disk drive fails, then the RAID system is generally able to reconstruct the data which was stored on the failed drive from the remaining drives in the array. A RAID system uses a network storage controller, which in many cases includes a RAID controller, an interface between the host computer and the array of disk drives. 
     Many applications require a storage system to have very high availability. This high availability is a key concern in many applications, such as financial institutions and airline reservations systems, because the users rely heavily on the data stored on the RAID system. In these type of applications, unavailability of data stored on the RAID system can result in significant loss of revenue and/or customer satisfaction. Employing a RAID system in such an application enhances availability of the stored data, since if a single disk drive fails, data may still be stored and retrieved from the system. In addition to the use of a RAID system, it is common to use redundant RAID controllers to further enhance the availability of a storage system. In such a situation, two or more controllers are used in a RAID system, with each controller having failover capability, where if one of the controllers fails the other remaining controller will assume operations for the failed controller. Such a platform enhances the availability of a RAID system, however, it can lead to several disadvantages, as will be discussed below. 
       FIG. 1  shows a block diagram representation of a common current-day dual controller configured RAID network storage controller  10 , showing a fiber channel to fiber channel connection. That is, in this example, the host computer and the array of disk drives both communicate with the network storage bridge using fiber channel connections. While fiber channel is a common channel medium is such systems, it should be understood that other channels may also be used, such as, for example, Small Computer System Interface (SCSI) or Ethernet. The RAID system shown in  FIG. 1  includes two host ports, host port- 1   14  and host port- 2   18  and two disk ports, disk port- 1   22  and disk port- 2   26 . Each host port  14 ,  18  may be associated with different host computers, and each disk port  22 ,  26  may be associated with different disk arrays, as is common in RAID systems and is well known in the art. The network storage bridge  10  includes dual RAID controllers, controller-A  30 , and controller-B  34 . In a system employing zoning of controllers, controller-A  30  may be zoned to host port- 1   14  and disk port- 1   22 , and controller-B  34  may be zoned to host port- 2   18  and disk port- 2   26 . 
     As is understood in the art, systems which employ dual controllers with write back caching require data mirroring between controllers to maintain cache coherency. Each controller  30 ,  34 , must have a copy of the data and status of the other controller in order to maintain redundancy between the controllers and thus maintain operation of the RAID system if one controller fails. Mirroring data between controllers can decrease the performance of a RAID system because transferring data between controllers uses processing resources of the controllers, as well as channel bandwidth, as will be discussed in more detail below. 
     The controllers  30 ,  34  are connected to a fiber channel backplane  38 , which is connected to two IO modules, IO module- 1   42 , and IO module- 2   46 . Each controller  30 ,  34 , includes a CPU subsystem  50 , a memory  54  (e.g., double data rate), control logic  58 , a dual port fiber channel connection with two host ports  62   a ,  62   b  and a dual port fiber channel connection with two disk ports  66   a ,  66   b . The CPU subsystem  50  performs tasks required for storage of data onto an array of disks, including striping data, and initiating and executing read and write commands. The memory  54  is a nonvolatile storage area for data and other information. The control logic  58  performs several functions, such as interfacing with the CPU subsystem  50 , memory  54 , and the host ports  62   a ,  62   b  and the disk ports  66   a ,  66   b . The control logic  58  may also have other functions, including a parity generation function, such as an exclusive OR (XOR) engine. The host ports  62   a ,  62   b  and disk ports  66   a ,  66   b  provide communications with the fiber channel backplane  38 . The IO modules  42 ,  46  include link resiliency circuits (LRCs)  70 , also known as port bypass circuits, which function to connect each host port  14 ,  18  and each disk port  22 ,  26  to each controller  30 ,  34 . This allows both controllers  30 ,  34  to have access to both host ports  14 ,  18  and both disk ports  22 ,  26 . 
     In order to provide full redundancy, each controller must have a connection to each host port  14 ,  18  and each disk port  22 ,  26 . This way, if there is a failure of one of the controllers, the other controller can continue operations. As mentioned above, it is common for each host port  14 ,  18  to be associated with different host computers, and for each disk port  22 ,  26  to be associated with different disk arrays. In these cases, each controller  30 ,  34  is typically associated with one disk port and one host port, which helps to enhance the performance of a RAID system. However, in such a case, half of these ports are passive. For example, if controller-A  30  is associated with host port- 1   14  and disk port- 1   22 , then controller-A  30  receives all communications from host port- 1   14  and controls the disk array(s) on disk port- 1   22 . Likewise, controller-B  34  would be associated with host port- 2   18  and disk port- 2   26 . These techniques are well known in the art and can increase performance of the RAID system as well as simplify control and communications of the two controllers  30 ,  34 . In the example of  FIG. 1 , on controller-A  30  the host port connection  62   a  and disk port connection  66   a  are connected to host port- 1   14  and disk port- 1   22 , respectively, through the LRCs  70  of IO module- 1   42 . Because controller-A  30  is associated with host port- 1   14  and disk port- 1   22 , the host port connection  62   a  and disk port connection  66   a  actively communicate with host port- 1   14  and disk port- 1   22 . The remaining host port connection  62   b  and disk port connection  66   b  are connected to host port- 1   18  and disk port- 2   26 , respectively, through the LRCs  70  of IO module- 2   46 . These connections are typically passive connections, as controller-A  30  is not actively communicating with host port- 2   18  and disk port- 2   26 , so long as controller-B  34  does not fail. Likewise, controller-B  34  would be associated with host port- 2   18  and disk port- 2   26 . Thus, for controller-B  34 , the host port connection  62   b  and disk port connection  66   b  would communicate with host port- 2   18  and disk port- 2   26  through LRCs  70  of IO module- 2   46 . The remaining host port connection  62   a  and disk port connection  66   a  would be connected to host port- 1   14  and disk port- 1   22  through LRCs  70  of IO module- 1   42 . 
     As mentioned above, in typical redundant controller operations with write back caching data is mirrored between controllers. When mirroring data between controller-A  30  and controller-B  34 , it is common to transfer the mirrored data over the disk port connections. For example, controller-B  34  may receive data over host port- 2   18  that is to be written to an array of drives over disk port- 2 . Controller-B  34  would receive this data and store it in memory  54 . In order to maintain cache coherency, controller-B  34  must also communicate this data to controller-A  30 , thus both controllers have the data, and if one fails the other is still able to write the data. In a traditional system, this transfer of data is accomplished over several steps. First, controller-B  34  sends the data over the disk port connection  66   a  which connects to the LRC  70  connected to disk port- 1   22 . The data would transfer to the associated hardware on disk port- 1   22  and be transferred back to the LRC  70 , where it would then be received at the disk port connection  66   a  on controller-A. Controller-A would then store the data in memory  54 , providing a copy of the data that was originally sent to controller-B  34 . Controller-B  34  would then perform the appropriate steps to write the data to the disk array. Once the data is written to the disk array, controller-B  34  then communicates this to controller-A  30  using the same communication path as described above, and controller-A  30  then removes the record of the data write. Likewise, if controller-A  30  receives data to be written to the array of disks on disk port- 1   22 , it sends the data to controller-B  34  using the same mirroring technique. 
     While this technique may use the remaining disk port on each controller, the second host port on each controller remains unused, thus passive, during normal operation of the system. The passive ports on each controller adds a significant amount of hardware to the controller, and can add significant cost to the network storage controller  10 . Thus, it would be advantageous to provide a redundant network storage controller which maintains high availability while reducing cost and hardware associated with passive ports located on the controllers. 
     Additionally, mirroring data in such a system results in the mirrored data and storage data being sent over the same port for the controller that is receiving the mirrored data. Bandwidth to and from the disk array is consumed by the mirrored data, which can reduce the performance of the network storage bridge. Additionally, when mirroring data, processing resources within the controllers  30 ,  34  are consumed, because the controller sending the data has to put it into form to be transferred over the disk port, and the controller receiving the data must process the data received over the disk port. For example, in the fiber channel embodiment shown in  FIG. 1 , mirrored data is formatted pursuant to fiber channel protocol, which can require several interrupts and processing resources. Thus, it would be advantageous to have a network storage controller which consumes little or no channel bandwidth when mirroring data between controllers. It would also be advantageous to have a network storage controller which consumes less processing resources for mirroring data. 
     Furthermore, with the continual increasing of demand for data storage, RAID controllers often require upgrades with additional disk drives or faster bus interfaces. However, a RAID controller may not be configured to add additional bus interface capacity or may not support a new type of bus interface. Such controllers commonly have to be replaced when an upgrade is performed. This replacement of controllers can increase the cost of upgrading a RAID system. The replacement of an operational RAID controller represents a loss in value that may inhibit the decision to upgrade a RAID system. Thus, it would be advantageous to have a system which can support upgrades of capacity, as well as new interface types, with ease and reduced cost. 
     Accordingly, there is a need to develop an apparatus and method for use in a network storage controller which: (1) provides redundancy with reduced cost for passive components, (2) reduces the amount of mirrored data which is sent over the disk or host ports, (3) reduces the processing overhead involved with mirroring data, and (4) provides easily replaceable and upgradeable components. 
     SUMMARY OF THE INVENTION 
     In accordance with the present invention, an apparatus and method are provided involving connection of a host computer with at least one storage device. The apparatus includes a passive backplane having a number of data buses, including a first data bus and a second data bus. The apparatus includes at least two channel interface modules, which are connected to the passive backplane, the host computer and the storage devices. The channel interface modules are operable to send and receive storage data to and from the host computer and the storage devices, and selectively transfer the storage data to the data buses. The apparatus also includes at least two controller memory modules, which are connected to the passive backplane and communicate with the channel interface modules via the passive backplane, and which store and process the storage data transferred to and from the channel interface modules. 
     The channel interface modules include a communication path portion and a channel interface portion. The channel interface portion is operable to transfer the storage data between the host computer and/or the storage devices and the communication path portion. The communication path portion is operable to selectively transfer data between the channel interface portion and the passive backplane. In one embodiment, the first channel interface module includes a first bus port and a second bus port, and the second channel interface module includes a third bus port and a fourth bus port, with each of the bus ports being operable to connect the communication path portion to the passive backplane. 
     The controller memory modules include a bus interface portion that connects to the passive backplane, a memory for temporary storage of the storage data, and a processing portion that organizes and arranges the storage data. The bus interface portion includes at least one backplane interface that connects to the passive backplane, a memory interface that connects to the memory, a processing portion that connects to the processing portion, a bridge core that contains control logic operable to connect the processing interface, memory interface and backplane interface. The bus interface portion may also include an exclusive OR (XOR) engine that performs XOR function on data blocks. In one embodiment, the bus interface portion also includes a direct memory access (DMA) engine that provides a DMA connection to the passive backplane. In another embodiment, the first controller memory module includes a first bus interface and a second bus interface, and the second controller memory module includes a third bus interface and a fourth bus interface, with each bus interface being operable to connect the bus interface portion to the passive backplane. 
     The passive backplane contains at least first and second data buses, and in one embodiment also contains third and fourth data buses. The data buses on the passive backplane maybe next generation peripheral component interconnect (PCIX) buses. In one embodiment, the first bus port is connected to the first data bus and the second bus port is connected to the third data bus. The third bus port is connected to the second data bus, and the fourth bus port is connected to the fourth data bus. The first bus interface is connected to the first data bus, and the second bus interface is connected to the second data bus. The third bus interface is connected to the third data bus and the fourth bus interface is connected to the fourth data bus. 
     The communication path portion of the first channel interface module may have a first shared path, a first switched path and a second switched path, and the communication path portion of the second channel interface module may have a second shared path, a third switched path and a fourth switched path. In this embodiment, the first shared path is connected to the first bus port and the second bus port. The first switched path is connected to the first bus port and the channel interface portion. The second switched path is connected to the second bus port and the channel interface portion. The second shared path is connected to the third bus port and the fourth bus port. The third switched path is connected to the third bus port and the channel interface portion. The fourth switched path is connected to the fourth bus port and the channel interface portion. Each switched path is operable to enable and disable communications involving the channel interface portion. 
     In yet another embodiment, the network storage apparatus for connecting a host computer with at least one storage device includes at least first and second channel interface modules, each adapted to be connected to a host channel and a disk channel and are operational to send and receive data over the host channel and the disk channel. The host channel is connected to a host computer, and the disk channel is connected to the storage devices. The apparatus includes at least first and second controller memory modules that communicate with the channel interface modules and process data from the host computer to provide storage data for storage on the storage devices, and process data from the storage devices to provide retrieved data for delivery to the host computer. The apparatus also includes a passive backplane that is connected to each of the controller memory modules and channel interface modules and supports communication between the channel interface modules and controller memory modules. The passive backplane supports communication such that when data is being carried by the host and/or disk channel and at the same time data is being mirrored between the first controller memory module and the second controller memory module, neither the host channel nor the disk channel carries more than fifty per cent of the data being mirrored. 
     The method for transferring data between a host computer and one or more storage devices includes first transferring data from the host computer to a channel interface module using a first channel medium. The data is then transferred in a second transferring step from the channel interface module to a first controller memory module using a passive backplane. The controller memory module processes the data to define storage data. The storage data is transferred in a third transferring step to the channel interface module via the passive backplane. The storage data is transferred in a fourth transferring step to the storage device via a second channel medium. The storage data is mirrored between the first controller memory module and a second controller memory module substantially independently of the first channel medium and the second channel medium. 
     In one embodiment, the second transferring step includes employing a first bus of the passive backplane to transfer the data to the controller memory module. The third transferring step includes employing a second bus of the passive backplane to transfer the storage data to the channel interface module. In one embodiment, the mirroring step includes not using the first channel medium and not using the second channel medium. In another embodiment, the mirroring step includes limiting the first channel medium and the second channel medium to carry at least less than fifty percent of the storage data. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a block diagram representation of a conventional dual controller network storage bridge; 
         FIG. 2  is a block diagram representation of a network storage apparatus of the present invention; 
         FIG. 3  is a block diagram representation of a controller memory module of the present invention; 
         FIG. 4  is a block diagram representation of a channel interface module of the present invention; 
         FIG. 5  is a block diagram representation of a redundant network storage bridge of the present invention; 
         FIG. 6  is a block diagram representation of a redundant network storage bridge showing a failed controller memory module; 
         FIG. 7  is a block diagram representation of a redundant network storage bridge showing a failed channel interface module; 
         FIG. 8  is a block diagram representation of a redundant network storage bridge showing four channel interface modules; 
         FIG. 9  is a block diagram representation of a network storage bridge utilizing a 2 GB fibre channel interconnect channel; 
         FIG. 10  is a block diagram representation of a network storage bridge utilizing an Ultra320 SCSI channel; and 
         FIG. 11  is a block diagram representation of a network storage bridge utilizing a gigabit ethernet channel. 
     
    
    
     DETAILED DESCRIPTION 
     With reference now to  FIG. 2 , a block diagram of the network bridge  100  of the present invention is shown. The network bridge  100  includes one or more controller memory modules (CMMs). In the embodiment shown in  FIG. 2 , there are two CMMs, CMM-A  104 , and CMM-B  108 , although a single CMM maybe used in applications where no redundancy is required, and additional CMMs may be used in applications requiring additional redundancy or higher performance. Each CMM  104 ,  108  has two backplane interfaces  112 . The system has a passive bus backplane  116 , which has two buses for each CMM. In the embodiment shown, the passive bus backplane  116  uses next generation Peripheral Component Interconnect (PCIX) buses, although it will be understood that any bus technology may be used, including switched architectures such as Infiniband or RapidIO, as well as traditional bus architectures such as PCI local bus. The passive bus backplane  116  can have a first data bus  120 , a second data bus  124 , a third data bus  128 , and a fourth data bus  132 . The first data bus  120  and second data bus  124  connect to the backplane interfaces  112  on CMM-A  104  via CMM bus connections  134 , and the third data bus  128  and fourth data bus  132  connect to the backplane interfaces  112  on CMM-B  108  via CMM bus connections  134 . 
     In the embodiment shown in  FIG. 2 , PCIX buses are used in the passive backplane  116 . The use of PCIX buses allows relatively high performance bus interconnection components connected to the passive backplane  116  with well understood and relatively simple bus protocol. PCIX technology is a next generation technology which leverages the traditional PCI bus. PCIX technology enables systems and devices that can operate at bus frequencies of up to 133 MHZ using 64-bit or 32-bit bus width and having a bandwidth of up to 1066 Mb/s with 64-bit, 133 MHZ PCIX bus. The PCIX bus employs a register-to-register protocol, which eases timing constraints associated with high frequency traditional PCI bus, and allows higher frequency operation of the PCIX bus. In addition to the ability to increase bus frequency, the PCIX bus incorporates several technologies which enhance bus efficiency, including attribute phase, split transaction support, optimized wait states and standard block size movements. 
     The attribute phase uses a 36-bit attribute field that describes bus transactions in more detail than the conventional PCI specification allows. It follows immediately after the address phase and contains several bit assignments that include information about the size of the transaction, ordering of transactions, cache snooping requirements, and the identity of the transaction initiator. With a split transaction as supported in PCIX, the device requesting the data sends a signal to the target. The target device informs the requester that it has accepted the request. The requester is free to process other information until the target device initiates a new transaction and sends the data to the requester. Thus, split transactions enable more efficient use of the bus. Wait states are optimized in PCIX, which eliminates the use of wait states, used in traditional PCI bus protocol, except for initial target latency. When a PCIX device does not have data to transfer, it will remove itself from the bus so that another device can use the bus bandwidth. This provides more efficient use of bus and memory resources. With standard block size movements, adapters and bridges (host-to-PCIX and PCIX to PCIX) are permitted to disconnect transactions only on naturally aligned 128-byte boundaries. This encourages longer bursts and enables more efficient use of cache-line-based resources such as the processor bus and main memory. It also facilitates a more pipelined architecture within PCIX devices. 
     The network bridge  100  has one or more channel interface modules (CIMs). In the embodiment shown in  FIG. 2 , there are two CIMs, CIM- 1   136  and CIM- 2   140 , although it will be understood that this number may vary depending upon the configuration and application in which the network bridge  100  is used. Each CIM  136 ,  140  has two CIM bus interface ports  144   a ,  144   b . On each CIM  136 ,  140  one CIM bus interface port  144   a  connects to one bus which is connected to CMM-A  104 , and one CIM bus interface port  144   b  connects to one bus which is connected to CMM-B  108  via CIM bus connections  146 . In the embodiment shown in  FIG. 2 , CIM- 1   136  connects to the first data bus  120  and third data bus  128 , and CIM- 2   140  connects to the second data bus  124  and fourth data bus  132 . Each CIM  136 ,  140  has a host port  148 , which connects to a host channel  152 , which connects to a host computer (not shown). Each CIM  136 ,  140  also has a disk port  156 , which connects to a disk channel  158 , which connects to one or more storage devices (not shown). In alternative embodiments, as will be discussed in more detail below, a CIM may contain only host ports or only disk ports, depending upon the application and channel interface required. 
     When the host computer sends data, it is sent over the host channel  152  and is received at the host port  148  on the CIMs  136 ,  140 . This data is sent to the CMMs  104 ,  108  via the passive backplane  116 . The CMMs  104 ,  108  contain memory and processing units, as will be described in detail below, which arrange the data into an appropriate form for storage on the storage devices. For example, if the system is used in a RAID 5 disk array system, the CMMs  104 ,  108  will arrange the data into appropriate stripes of data to be written to the disks, and will compute a parity block for the stripe of data. Thus, the CMMs  104 ,  108  process the data and format it for storage. Once this is complete, the CMMs  104 ,  108  transfer the data, ready for storage, to the CIMs  136 ,  140  via the passive backplane  116 . The CIMs  136 ,  140  then send the data to the storage devices connected to the disk port  156 . As will be described in more detail below, data can be transferred between the CMMs  104 ,  108  using the CIMs  136 ,  140  and the passive backplane  116 . Additionally, as will also be discussed below, the CMMs  104 ,  108  and CIMs  136 ,  140 , may be zoned to specific drives or hosts. 
     This configuration provides a modular and redundant architecture in which the host channel  152  and the disk channel  158  need not necessarily be the same channel medium. The modularity of the CMMs  104 ,  108  and CIMs  136 ,  140  also allows for relatively low cost upgrades and easy replacement of failed units. The use of a passive backplane  116  to exchange data between CMMs  104 ,  108  also avoids the use of channel bandwidth of the disk channel  158  or the host channel  152 , as would be required for data mirroring in a traditional redundant controller environment, as will be discussed below. 
     With reference now to  FIG. 3 , a block diagram representation of a CMM  104  is shown. The CMM  104  contains several components, including a CPU subsystem  160 , a memory  164 , and an interface FPGA  168 . The CPU subsystem  160  maybe a standard type CPU, such as a widely used microprocessor, or may be an application specific processor. In one embodiment, the CPU subsystem  160  is an Intel Pentium (TM) class microprocessor. The CPU subsystem  160  communicates with the interface FPGA  168  using a standard bus, such as a PCI bus. The memory  164  allows for temporary storage of data within the CMM  104 . This storage is used during normal read and write operations for several purposes, such as storing queued data that is waiting to be written to the disk array. In one embodiment, a DDR memory DIMM is used, which communicates with the interface FPGA  168  using a bus interface. 
     The interface FPGA  168  contains a number of components. It will be understood that these components maybe combined into a single FPGA, or may exist on several components within the CMM  104 . In one embodiment, shown in  FIG. 3 , the interface FPGA  168  includes a PCI interface  172 , a memory interface  176 , an XOR engine  180 , a bridge core  184 , a DMA engine  188 , data FIFOs  192 , and two backplane interfaces  112 . The PCI interface  172 , acts as an interface between the CPU subsystem  160  and the other portions of the interface FPGA  168 . In the embodiment shown, this interface uses a standard PCI bus connection. The PCI interface  172  connects to a bridge core  184 , which in turn connects to the backplane interfaces  112 , which interface with the first data bus  120  and second data bus  124  located on the passive backplane  116 . 
     The memory interface  176 , acts as an interface between the memory  164  and the interface FPGA  168 . The XOR engine  180  serves to perform XOR operations on the data to be stored, in order to obtain parity information on the data which is to be written. The XOR engine  180  is also used in situations where the use of parity information is required to recover data from a failed drive in a disk array. The XOR engine  180  connects to the CPU subsystem  160  through the PCI interface  172 . The data FIFOs  192  connect to the memory interface  176  and bridge core  184 , and in turn connect to the backplane interfaces  112 . The data FIFOs serve as a queue which is used by the CMM  104  to manage read and write operations. The DMA engine  188  serves to provide and receive DMA data from another CMM when the CMMs are operating to provide redundancy, as will be discussed in detail below. 
     Referring now to  FIG. 4 , a block diagram representation of a CIM  136  is shown. The CIM  136  contains a switched/shared PCIX FPGA  200 , and a channel interface  204 . The switched/shared PCIX FPGA  200  contains a first switched PCIX path  208 , and a second switched PCIX path  212 , and a shared PCIX path  216 . Each switched PCIX path  208 ,  212  connects to a bus interface port  144 , which in turn connects to a PCIX bus on the passive backplane  116  via a CIM bus connection  146 . Each switched PCIX path  208 ,  212 , also has an enable input  214 . The enable input  214  is used to enable or disable the switched PCIX path  208 ,  212 . The shared PCIX path  216  contains a bridge core  220 , which determine which data is to be routed over the shared path  216 , and passes that data through the shared path  216  and to the other CIM bus connection  146 . Likewise, each of the switched PCIX paths  208 ,  212  also contain a bridge core  224 , which determine which data is to be routed over the switched path  208 ,  212 , and passes that data through to the channel interface  204 . 
     The channel interface  204  connects the switched/shared PCIX FPGA  200  to the host channel  152  and the disk channel  158 . The channel interface contains control registers  228 , an address steering portion  232 , a PCIX interface  236 , a host port  148 , and a disk port  156 . The control registers  228  are used to control communications over the host channel  152  or disk channel  158 . The address steering portion  232  is used to direct data to the proper addresses on either the host computer or the storage devices. The PCIX interface  236 , functions to communicate data from the switched/shared PCIX FPGA  200 , and communicate data to the host port  148 , and the disk port  156 . The PCIX interface  236  enables communication over the appropriate channel medium for the application. For example, if the host channel  152  and the disk channel  158  use fiber channel, the PCIX interface  236  would act as the interface between the switched/shared PCIX FPGA  200  and the fiber channel. Likewise, if the host channel  152  and the disk channel  158  use a SCSI channel, the PCIX interface  236  would act as the interface between the switched/shared PCIX FPGA  200  and the SCSI channel. If both the host channel  152  and the disk channel  158  use the same channel medium, the CIM  136  can use identical host ports  148  and disk ports  156  for communication with both the host channel  152  and the disk channel  158 . 
     In one embodiment, the disk channel  158  and the host channel  152  do not use the same channel medium. In this embodiment, a different CIM is used for each different channel medium. For example, if the host computer used a fiber channel, and the disk array used a SCSI channel, the host computer would connect to one CIM, using a fiber channel interface, and the disk array would connect to another CIM, using a SCSI channel interface. If redundancy were required, two or more CIMs could be connected to each channel medium. 
     In the embodiment shown in  FIG. 4 , the first switched PCIX path  208  communicates with the first data bus  120  and the second switched PCIX path  212  communicates with the third data bus  128  through the bus interface port  144  and CIM bus connection  146 . The shared PCIX path  216  may be used as a communication path for one CMM to communicate with another CMM, as will be discussed in detail below. It will be understood that a similar configuration is used for the remaining CIMs that are present on the network bridge. For example, in the embodiment shown in  FIG. 2 , CIM- 2   140  is connected to the second data bus  124  and the fourth data bus  132 , and thus the switched/shared PCIX FPGA  200  contained in CIM- 2   140  would have switched PCIX paths  208 ,  212  which communicate with the second data bus  124  and fourth data bus  132  respectively. Likewise, if more than two CIMs are present, they will be configured to communicate with the appropriate buses on the passive backplane  116  as required by the application. 
     Referring again to  FIGS. 2-4 , the enable input  214  is used to zone a CIM  136 ,  140 , to a particular CMM  104 ,  108 . In such a case, a CMM  104  or  108  has exclusive ownership of a CIM  136  or  140  by enabling access to that CIM  136  or  140  from the bus segment that the CMM  104  or  108  is connected to. For example, in one embodiment, CMM-A  104  is zoned to CIM- 1   136 , and CMM-B  108  is zoned to CIM- 2   140 . Thus, CMM-A  104  has exclusive ownership of CIM- 1   136 , and CMM-B  108  has exclusive ownership of CIM- 2   140 . This zoning is achieved by activating the enable input  214  on the first switched PCIX path  208  in CIM- 1   136 , and disabling the enable input  214  on the second switched PCIX path  212  in CIM- 1   136 . This results in only the first switched PCIX path  208  communicating with the channel interface  204 . As mentioned above, in CIM- 1   136 , the first switched PCIX path  208  communicates with the first data bus, and therefore CMM-A  104  is zoned to CIM- 1   136 . Likewise, for the zoning of CIM- 2   140  to CMM-B  108 , the enable input  214  on the second switched path  212  is activated and the enable input on the first switched PCIX path  208  is not activated. This results in only the second switched PCIX path  212  in CIM- 2   140  communicating with the channel interface  204 , and therefore CMM-B  108  is zoned to CIM- 2   140 . By allowing only one CMM  104  or  108  to control a CIM  136  or  140 , channel control, mapping and management are simplified. Zoning is particularly useful when two or more host channels or disk channels are present. For example, if two host channels are present and two disk channels are present, CMM-A  104  may be zoned to the first host channel and the first disk channel, and CMM-B  108  maybe zoned to the second host channel and the second disk channel. Thus, CMM-A  104  will send and receive data from the first host channel through CIM- 1   136 , and CMM-B  108  will send and receive data from the second host channel through CIM- 2   140 . The use of zoning in the CMMs simplifies control in an active-active application, because the CMMs do not have to perform coherency checks. A coherency check is required if zoning is not implemented, because both CMMs would own the same channel. In such a case, prior to performing any functions regarding data, such as a read or a write function, a CMM must verify that the other CMM has not performed any functions for that data. This coherency check can be complex to implement, and can degrade performance because of the additional overhead each CMM must perform. 
     With reference now to  FIG. 5 , a block diagram representation of a network bridge  100   a  containing redundant components is shown. In this embodiment, two CMMs are used, CMM-A  104  and CMM-B  108 . Two CIMs are used, CIM- 1   136  and CIM- 2   140 . CMM-A  104  and CIM- 1   136  are both connected to the first data bus  120  in the passive backplane  116 . CMM-A  104  and CIM- 2   140  are both connected to the second data bus  124  in the passive backplane  116 . CMM-B  108  and CIM- 1   136  are both connected to the third data bus  128  on the passive backplane  116 . CMM-B  108  and CIM- 2   140  are both connected to the fourth data bus  132  on the passive backplane  116 . 
     As will be understood by those of skill in the art, redundant controllers require mirroring of data between the two controllers attached to the storage subsystem. This is due to the use of a write back cache, where the controller receives data from the host computer, caches the data and sends a message to the host computer that the data has been written. Thus the host computer determines that the data has been written, when it is actually stored in the controller and is waiting there to be written to the drives in the disk array. To help ensure that this data is not lost in the event of a failure, redundant controllers mirror this data to the other controller, thus having another copy of the data on the other controller. This is known as cache coherency. In one embodiment, the CMMs  104 ,  108  mirror data to provide cache coherency to the network bridge  100   a . This can be done by implementing a DMA path between CMM-A  104  and CMM-B  108 . This can be accomplished by providing a DMA engine  188  in the interface FPGA  168 , as discussed above with respect to  FIG. 3 , and a shared path  216  which is located in the switched/shared path FPGA  200 , as discussed above with respect to  FIG. 4 . Each CMM  104 ,  108 , uses this DMA path to send and receive data from the other CMM. By utilizing the DMA path, the two CMMs  104 ,  108  can mirror data without the need to use the host channel  152 , or the disk channel  158 , thus channel bandwidth in the disk channel  158  or host channel  152  is not consumed by the CMMs  104 ,  108  mirroring data. Additionally, by employing a DMA path between the two CMMs  104 ,  108 , less processing resources are required from the CMMs  104 ,  108  to complete the mirroring than would be required to mirror using the host channel  152  or disk channel  158 . 
     There is also a failover reset link  240  present between CMM-A  104  and CMM-B  108 . The failover reset link  240  is used for communicating a failure of one of the CMMs  104 ,  108 . In one embodiment, the failover reset link  204  is a serial connection between CMM-A  104  and CMM-B  108 . In this embodiment, each CMM  104 ,  108  maintains a heartbeat signal which is communicated over the failover reset link  204 , and monitored by the other CMM. If a problem is detected in the heartbeat signal, a CMM  104 ,  108  can send a signal over the failover reset link to terminate the operation of the other CMM. For example, if CMM-B  108  has a failure, CMM-A  104  will detect that the heartbeat signal from CMM-B  108  is no longer active. After a preset time period in which no heartbeat signal is received, CMM-A  104  sends a termination signal to CMM-B  108 . When CMM-B  108  receives the termination signal, it discontinues operation. CMM-A then takes control of all read and write operations. Likewise, if CMM-A  104  failed CMM-B  108  would receive the indication over the failover reset link  240 , and take control of all read and write operations. Thus, the system is redundant and continues to operate when a CMM  104  or  108  fails. 
     Referring now to  FIG. 6 , the operation of the system when a CMM fails will now be described. As shown in  FIG. 6 , the network bridge  100   a  has CMM-A  104  and CMM-B  108 , a passive PCIX backplane  116 , and a CIM- 1   136  and a CIM- 2   140 . When CMM-A  104  fails, CMM-B  108  detects the failure over the failover reset link  240 , as described above, and terminates operations on CMM-A  104 . CMM-B  108  then assumes control of all memory and control operations formerly conducted by CMM-A  104 . When this happens, CMM-B  108  sends a command to CIM- 1   136  and CIM- 2   140 , to enable communications with CMM-B  108  only. In this case, CIM- 1   136  would receive this command, and disable the first switched path  208  connected to the first data bus  120 , and to enable the second switched path  212  connected to the third data bus  128 , thereby connecting CMM-B  108  to the host port  148  and disk port  156  on CIM- 1   136 . CIM- 2   140  also receives the command from CMM-B  108 , and performs the same function to disable the first switched path  208  connected to the second data bus  124 , and to enable the second switched path  212  connected to the fourth data bus  132 . In one embodiment, the passive backplane  116  contains control logic lines, which connect to the enable inputs  214  on the CIMs  136 ,  140 , and are connected to the CMMs  104 ,  108 . The CMMs  104 ,  108  can use these control logic lines to enable and to disable the switched paths  208 ,  212  on the CIMs  136 ,  140 . Alternatively, other embodiments may be used to enable and disable the switched paths  208 ,  212 , such as control logic within the CIM which receives command information via the PCIX buses on the passive backplane  116 , for example. 
     Referring now to  FIG. 7 , the operation of the system when a CIM fails will now be described. The CMMs  104 ,  108  periodically perform runtime diagnostics, which verify the proper operation of all components within the CIM  136 ,  140 . The results of these runtime diagnostics are communicated to the CMMs  104 ,  108  via the control logic lines. The CMM  104 ,  108  that is zoned to a CIM  136 ,  140  monitors these runtime diagnostics, and in the event of an error or a failure, communicates a command over the control logic lines to terminate the operation of that CIM  136  or  140 . As depicted in  FIG. 7 , CIM- 1   136  has a failure. CMM-A  104  determines that CIM- 1   136  has had a failure, and disables CIM- 1   136 . CMM-A  104  then communicates this information to CMM-B  108  via the shared path  216  on CIM- 2   140 . CMM-B  108  receives this information and assumes control of all communication between the host port  148  and disk port  156  on CIM- 2   140 . CMM-A  104  remains in a passive state until CIM- 1   136  has been replaced, or the failure is corrected. 
     Referring now to  FIG. 8 , a block diagram of a network bridge l 00   b  is shown in which four-CIM modules are present. In this embodiment, two CMMs, CMM-A  104  and CMM-B  108 , communicate to four CIMs, CIM- 1   136 , CIM- 2   140 , CIM- 3   300 , and CIM- 4   304 . In this embodiment, the CMM-A switched paths  208  on CIM- 1   136  and CIM- 2   140  are enabled. Likewise, in CIM- 3   300  and CIM- 4   304 , the CMM-B switched paths  212  are enabled. Thus, CIM- 1   136  and CIM- 2   140  provide the interface between CMM-A  104  and the host port  148  and the disk port  156 , and CIM- 3   300  and CIM- 4   304  provide the interface between CMM-B  108  and the host port  148  and disk port  156 . A network bridge of this embodiment is useful in several cases including, for example, when multiple hosts are present. In this embodiment, CIM- 1   136  and CIM- 2   140  provide communications to a first host, and CIM- 3   300  and CIM- 4   304  provide communications to a second host. The same arrangement can be done for multiple disk nodes, such as two separate RAID arrays. As will be understood, this configuration provides for a scalable system which can provide communications between one or more host nodes and one or more disk nodes, while also providing for redundant operation. Additionally, such an embodiment may be useful to connect hosts and/or disk arrays which use a different channel medium. For example, an existing system may have two CIMs and use fiber channel connections for both the host and disk channels. If a user wanted to upgrade the system to add another disk array which used a SCSI connection, additional CIMs could be added which enable communication with a SCSI channel, allowing the upgrade of the existing system without having to replace existing hardware. 
     Referring now to  FIGS. 9-11 , several alternative embodiments of a CIM are shown to provide an example of the different configurations a CIM may have, and the different channel mediums a CIM may connect to.  FIG. 9  shows a block diagram representation of a PCIX to gigabit interconnect (GBIC) configured CIM  136   a . Within the CIM  136   a , the switched/shared PCIX FPGA  200  connects to a dual poet 2 Gb fiber channel interface  400 . Each port of the dual poet 2 Gb fiber channel interface  400  connects to a serializer/deserializer (SERDES)  404   a ,  404   b . Each SERDES  404   a ,  404   b  connects to the channel medium using a 2 Gb fiber channel connection. In the embodiment shown in  FIG. 9 , one SERDES  404   a  connects to a GBIC host channel  152   a , and the other SERDES  404   b  connects to a GBIC disk channel  158   a.    
       FIG. 10  shows a block diagram representation of a PCIX to SCSI CIM  136   b . Within the CIM  136   b , the switched/shared PCIX FPGA  200  connects to a dual port Ultra320 SCSI interface  408 . Each port of the dual port Ultra320 SCSI interface  408  connects to a host or disk channel, and also has a termination  412  connection, as is required for SCSI systems. In the embodiment shown in  FIG. 10 , one port of the dual port Ultra320 SCSI interface  408  connects to a very high density interconnect (VHDIC) host channel  152   b , and one port of the dual port Ultra320 SCSI interface  408  connects to a VHDIC disk channel  158   b.    
       FIG. 11  shows a block diagram representation of a PCIX to Ethernet CIM  136   c , which employs quick switch connections  416   a ,  416   b  for use in the switched paths. The quick switch connections  416   a ,  416   b  are bus relays which contain enable inputs which act to enable and disable the quick switch connection  416   a ,  416   b . Each quick switch connection  416   a ,  416   b  connects to an interface connection  420 , which contains an acceleration FPGA and data FIFOs. The interface connection  420  connects to a gigabit Ethernet ASIC  424 , which performs proper functions to the data to communicate the data over an Ethernet connection. The gigabit Ethernet ASIC  424  connects to a MAC/physical converter  428  which converts the signal to a physical signal, which is then routed to a transformer  432  to output the signal at the proper voltage. In one embodiment, the transformer  432  connects to a GBIC connection to a disk channel  158   c . In the embodiment of  FIG. 11 , if a redundant system were required, shared paths would be provided on other CIMs. It will be appreciated that different channel mediums may be used in a single system using a combination of the different interface modules, such as those shown in  FIGS. 9-11 . For example, a host computer may connect to the network bridge using a fiber channel medium, and the network bridge may connect to a disk array using a SCSI channel medium. 
     The foregoing discussion of the invention has been presented for purposes of illustration and description. Further, the description is not intended to limit the invention to the form disclosed herein. Consequently, variations and modifications commensurate with the above teachings, within the skill and knowledge of the relevant art, are within the scope of the present invention. The embodiments described hereinabove are further intended to explain the best modes presently known of practicing the inventions and to enable others skilled in the art to utilize the inventions in such, or in other embodiments, and with the various modifications required by their particular application or uses of the invention. It is intended that the appended claims be construed to include alternative embodiments to the extent permitted by the prior art.