Patent Publication Number: US-7904616-B2

Title: Storage apparatus and data transfer method

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     This is a continuation of U.S. application Ser. No. 12/175,763, filed Jul. 18, 2008 now U.S. Pat. No. 7,769,921. This application relates to and claims priority from Japanese Patent Application No. 2008-117634, filed on Apr. 28, 2008. The entirety of the contents and subject matter of all of the above is incorporated herein by reference. 
    
    
     BACKGROUND 
     The present invention generally relates to a storage apparatus and its data transfer method, and in particular relates to a storage apparatus configured from a controller comprising individual I/Os (referring to data transfer paths; hereinafter the same) to a cache memory, a host, and an HDD (Hard Disk Drive), and its data transfer method. 
     Conventionally, a storage apparatus configures a host interface, a cache memory, and a drive interface connected to an HDD, a controller for controlling the data transfer, an I/O for connecting the foregoing components, and a microprocessor for controlling the overall system as a single unit (module), and connects a plurality of modules with a dedicated I/O. Each module has an independent power supply, and host transfer data is subject to storing to a cache memory of the other module in order to improve the reliability against data loss (for instance, refer to Japanese Patent Laid-Open Publication No. 2005-44010). 
     Meanwhile, there are cases where the storage apparatus is configured to expand hosts and HDD drives by connecting a plurality of modules. With data transfer through the I/O, there are cases where the I/O between modules to be routed can be selected even if the transfer source and the transfer destination are the same. Accordingly, with a storage apparatus, an optimal path selection (routing) is possible according to the transfer status and failure status of each I/O. As one example of a path selection method, Japanese Patent Laid-Open Publication No. H11-313069 discloses a method of selecting the optimal path by weighting in accordance with the usage status in the connection of nodes; that is, the connection of controllers, and selecting the optimal path from the weighted sum between the nodes included between the transfer source and the transfer destination. 
     SUMMARY 
     A storage apparatus deals with scalability by increasing the number of I/Os with the host or the number of I/Os with the HDD drive. Thus, a storage apparatus deals with scalability by adding the number of I/Os with the host and number of I/Os with the HDD drive of the controller controlling the data transfer. Nevertheless, the reading and writing performed in the storage apparatus are simultaneously transferred with a plurality of I/Os. Thus, in a storage apparatus, in proportion to the increase in the number of I/Os, it is necessary to improve performance of the I/O switch (switch band), cache read and write operation to be performed in the controller. In particular, when configuring a large-scale storage apparatus, it is difficult to have the switch band and cache band for the required number of I/Os. In other words, with a storage apparatus, it is difficult to realize devices such as a controller comprising the required band. 
     Thus, with a storage apparatus, four or eight modules are connected by adding I/Os between modules for connecting the controllers in order to deal with scalability. In other words, a storage apparatus deals with scalability based on a multi controller architecture. Each controller comprises the switch band and cache band required for the number of I/Os, and it is thereby possible to have the number of I/Os required in a storage apparatus while having the transfer rate to the interface. 
     A multi controller architecture is not limited to cases where the I/O of the transfer destination and the I/O of the transfer source are contained in the same controller, and there are cases where they are contained in each controllers. In this case, data transfer is performed via an I/O between modules provided between the controllers. With a storage apparatus, there are cases when a plurality of transfer paths exist when transferring data via a plurality of controllers. Meanwhile, the inter-module I/O between controllers causes deterioration in the transfer rate due to a transfer failure or the transfer from a plurality of controllers being concentrated on the same I/O. With a storage apparatus, it is desirable to avoid this kind of I/O and select the optimal path for transfer. 
     As an example of path selection, Japanese Patent Laid-Open Publication No. H11-313069 discloses a method of calculating the weighted sum and performing path selection while consolidating the weighting between all nodes. Here, a node corresponds to a controller in the storage apparatus or an I/O between the controllers. There are the following problems when applying this method to a storage apparatus having a multi controller architecture. 
     Specifically, weighting that gives consideration to the occurrence of a failure in the interface is required. Moreover, since the weight of each node will change by the minute due to variation in the number of I/Os with the host or the number of I/Os with the HDD drive subject to a transfer, path determination during the transfer must also be performed in addition to the path selection before the transfer. 
     In addition, with an actual storage apparatus, the number of controllers in relation to one control means (microprocessor or the like) is limited in order to avoid deterioration in the transfer performance caused by the deficiency in the processing performance of the control means. Although the management of failures that occur during the transfer is desirably concentrated in a single control means, this is impossible with the foregoing storage apparatus. Accordingly, transfer path selection giving consideration to the management of failures in a storage apparatus having a multi controller architecture is required. 
     Furthermore, when deciding the transfer path based on the failure status and transfer status as weighting factors, and the number of controllers to be managed by the control means, it is necessary to perform weighting with each factor upon selecting the optimal transfer path. 
     The present invention was devised in view of the foregoing points. Thus, an object of the resent invention is to propose a storage apparatus and its data transfer method capable of improving access performance. 
     In order to achieve the foregoing object, the present invention provides a storage apparatus including a plurality of controllers connected to a host system and a storage device and for controlling transfer of data with the host system and the storage device, a transfer path of data for connecting the controllers, and a plurality of processors connected to the controllers and for controlling the controllers. Each of the plurality of controllers is connected to the same or a different processor. A first processor connected to a controller that received a transfer command from the host system determines, based on the transfer command, a first controller to become a transfer source of data and a second controller to become a transfer destination of the data, and determines a path belonging to the transfer path between the first controller and second controller. If there are a plurality of paths, a second processor connected to the second controller selects a path in which the number of controllers connected other than to the second controller becomes maximum as a specific path, and the plurality of processors transfer data between the first controller and the second controller in line with the specific path. 
     The present invention additionally provides a storage apparatus including a plurality of controllers connected to a host system and a storage device and for controlling transfer of data with the host system and the storage device, a transfer path of data for connecting the controllers, and a plurality of processors connected to the controllers and for controlling the controllers. Each of the plurality of controllers is connected to the same or a different processor. A first processor connected to a controller that received a transfer command from the host system creates, as header information of a packet of a first controller to become a transfer source, identifying information of the first controller and a second controller to become a transfer destination, identifying information of the transfer path between the first controller and the second controller, and identifying information of a controller to be routed upon transferring the data. 
     The present invention further provides a data transfer method of a storage apparatus including a plurality of controllers connected to a host system and a storage device and for controlling transfer of data with the host system and the storage device, a transfer path of data for connecting the controllers, and a plurality of processors connected to the controllers and for controlling the controllers. Each of the plurality of controllers is connected to the same or a different processor. A first processor connected to a controller that received a transfer command from the host system determines, based on the transfer command, a first controller to become a transfer source of data and a second controller to become a transfer destination of the data, and determines a path belonging to the transfer path between the first controller and second controller. If there are a plurality of paths, a second processor connected to the second controller selects a path in which the number of controllers connected other than to the second controller becomes maximum as a specific path, and the plurality of processors transfer data between the first controller and the second controller in line with the specific path. 
     Accordingly, if a plurality of paths can be selected for the data transfer from a transfer source to a transfer destination in a storage apparatus having a multi controller architecture, since such selection can be made appropriately, it is possible to efficiently access the intended data. 
     According to the present invention, it is possible to realize a storage apparatus and its data transfer method capable of improving access performance. 
    
    
     
       DESCRIPTION OF DRAWINGS 
         FIG. 1  is a configuration diagram of a storage apparatus to which the present invention is applied; 
         FIG. 2  is an internal configuration diagram of a controller configuring the storage apparatus; 
         FIG. 3  is an explanatory diagram of multiplicity, traffic volume, and dummy transfer detection of I/O ports; 
         FIG. 4  is an explanatory diagram of multiplicity, traffic volume, and dummy transfer detection of I/O ports; 
         FIG. 5  is an explanatory diagram showing the allocation of controllers managed by the microprocessor in the storage apparatus and a status sharing method based on a processor memory; 
         FIG. 6  is an explanatory diagram of a data transfer path in the storage apparatus; 
         FIG. 7  is an explanatory diagram of a data transfer path in the storage apparatus; 
         FIG. 8  is an explanatory diagram of a packet format to be used in data transfer in a controller and between controllers; 
         FIG. 9  is an explanatory diagram of a packet format to be used in data transfer in a controller and between controllers; 
         FIG. 10  is an explanatory diagram of a packet format to be used in data transfer in a controller and between controllers; 
         FIG. 11  is an explanatory diagram of a packet format to be used in data transfer in a controller and between controllers; 
         FIG. 12  is an explanatory diagram of a packet format to be used in data transfer in a controller and between controllers; 
         FIG. 13  is an explanatory diagram of a packet data transfer example in a controller and between controllers; 
         FIG. 14  is a flowchart explaining the main routine of the data transfer method according to the present invention; 
         FIG. 15  is a flowchart explaining a routing method to be applied during an occurrence of a controller failure; 
         FIG. 16  is a flowchart explaining a method of determining the routing from the respective statuses of packet multiplicity, traffic volume, dummy transfer detection of I/O between controllers; 
         FIG. 17  is a flowchart explaining a method of determining the routing to be performed during the data transfer; and 
         FIG. 18  is a second configuration diagram of a storage apparatus to which the present invention is applied. 
     
    
    
     DETAILED DESCRIPTION 
     An embodiment of the present invention is now explained in detail with reference to the attached drawings. This invention, however, shall not be limited to the following embodiments. 
     (1) First Embodiment 
       FIG. 1  is an apparatus block diagram showing a first embodiment of a storage apparatus  100  according to the present invention. In  FIG. 1 , a controller  1  ( 1   a  to  1   d ) (CTL  0  to CTL  3 ) is a controller for controlling the data transfer via the respective I/Os (referring to the data transfer paths; hereinafter the same) among a cache memory  4  ( 4   a  to  4   d ) (Cache  0  to Cache  3 ), a host interface  2  ( 2   a  to  2   d ) (Host I/F  0  to Host I/F  3 ), a drive interface  3  ( 3   a  to  3   d ) (Dry I/F  0  to Dry I/F  3 ), and a microprocessor  5  ( 5   a,    5   b ) (MP 0 , MP 1 ). The controllers  1  are connected each other with an I/O (Dual I/O, Cross I/O). 
     In  FIG. 1 , the controller  1   a  and the controller  1   b  are connected with a cross  0  I/O (Cross  0  I/O), the controller  1   a  and the controller  1   c  are connected with a dual  0  I/O (Dual  0  I/O), the controller  1   b  and the controller  1   c  are connected with a cross  1  I/O (Cross  1  I/O), and the controller  1   b  and the controller  1   d  are connected with a dual  1  I/O (Dual  1  I/O), thereby configuring loop-shaped architecture. Differences of using the dual I/O and the cross I/O will be explained later with reference to  FIG. 4  and  FIG. 5 . 
     The host interfaces  2   a  to  2   d  are host interfaces that control the data transfer in packet units according to a communication protocol with the host. The drive interfaces  3   a  to  3   d  are drive interfaces that control the data transfer in sector units according to a communication protocol with the hard disk drive. 
     The storage apparatus  100  of the present invention also comprises cache memories  4   a  to  4   d  for temporarily storing the transfer data from the host interfaces  2   a  to  2   d  and the drive interfaces  3   a  to  3   d.  By cache memories  4   a  to  4   d,  it is possible to prevent the loss of host transfer data during the writing of data into the drive and improve the host read performance based on a cache hit. Hard disk drives are connected to the drive interfaces  3   a  to  3   d,  and logical volumes  8   a  to  8   d  (LU 0  to LU 3 , LU: Logical Unit) as access space are also allocated. 
     The microprocessors  5   a,    5   b  are control means of the respective controllers  1   a  to  1   d,  host interfaces  2   a  to  2   d,  and drive interfaces  3   a  to  3   d.  The microprocessors  5   a,    5   b  are connected to the controllers  1   a  to  1   d  and the processor memories  7   a,    7   b  (MP Mem  0 , MP Mem  1 ) via the memory controller hubs  6   a,    6   b  (Memory Controller Hub: MCH). For instance, the memory controller hub  6   a  is connected to the controller  1   a  (CTL  0 ) and the controller  1   b  (CTL  1 ), and the microprocessor  5   a  directly controls the controller  1   a,  the controller  1   b,  the host interface  2   a,  the host interface  2   b,  the drive interface  3   a,  and the drive interface  3   b.  The memory controller hubs  6   a  and  6   b  are connected to the controllers  1   a,    1   b  and the controllers  1   c,    1   d  with an MP I/O (MP I/O). The storage apparatus  100  is configured from the foregoing elements. 
     The internal configuration of the controllers  1   a  to  1   d  is now explained.  FIG. 2  is a block diagram of the controllers  1   a  to  1   d.  In  FIG. 2 , as a data transfer method via the respective I/Os (MP I/O, cross I/O, host I/O (Host I/O), drive I/O (Dry I/O), dual I/O) connected to the controller  1 , for instance, PCI Express (PCI-Express) as a high-speed serial bus standard is adopted. 
     The DDR I/F  10  controls the data transfer between the cache memory  4  and the controller  1 . In this case, a high-speed memory, for example DDR (Double Data Rate Synchronous DRAM) is used as the cache memory  4 . The DDR I/F  10  and the cache memory  4  are connected with a cache I/O. 
     The PCI I/Fs  11   a  to  11   e  (PCI I/F) control the data transfer according to a communication protocol in the respective layers of a physical layer, a link layer, and a transaction layer. The data transfer at the transaction layer is conducted in packet units according to a TLP (Transaction Layer Packet) format. 
     The memory controller  12  (MEM CTL) controls the issue of various commands for DRAM such as the read, write and RMW (Read Modify Write) to the cache memory, and the refresh operation. The memory controller  12  is connected to an internal high-speed switch (SW)  15 . 
     Each bridge circuit  13   a  to  13   e  (BRG MP, BRG Cross, BRG D, BRG Dual, BRG D) is connected to the internal high-speed switch  15 , and is a bridge circuit including a reception buffer and send buffer of the transferred TLP. As a result of being equipped with a buffer, the bridge circuits  13   a  to  13   e  are able to retain and rewrite the header information configuring the TLP, and also comprise the packet conversion function suitable for the TLP transfer in the controller  1  and between the controllers  1 . This will be explained later with reference to  FIG. 8  to  FIG. 13 . 
     The DMA circuits  14   a,    14   b  (D DMA, H DMA) are connected to the bridge circuit  13   e  (BRG H) and the bridge circuit  13   c  (BRG D), and control the transfer operation as the requester via each I/O of the transfer source. The DMA circuit  20  (Multi DMA) controls the transfer with the microprocessor  5  and the cache memory  4  via each I/O. 
     The high-speed switch  15  multiplexes and transfers the TLP via the respective I/Os according to an arbiter  16  if there is a TLP transfer request (REQ) to the same I/O via different I/Os. 
     The arbiter  16  creates I/O selection commands according to a predetermined priority in response to simultaneously TLP transfer requests (REQ). The arbiter  16  also detects the TLP multiplicity on the transfer destination I/O from the number of simultaneously TLP transfer requests (REQ), or from the number of transfer I/Os that created a TLP transfer request (REQ) within a period and the number of TLP transfer requests (REQ) within a period. 
     A traffic monitor  17  (Traffic Mon.) calculates the traffic volume by measuring the occupancy time of the TLP transfer on the I/O of the transfer destination within a period. The traffic monitor  17  measures, for example, the time connected to the output I/O via the transfer source I/O. Measurement is realized by using a selection signal created in the arbiter  16 , or equipping the high-speed switch  15  with a status detection function that synchronizes the period of TLP transfer through the transfer destination I/O and shows the transfer status. 
     A dummy transfer monitor  21  (Dmy Xfer Mon.) is a function for detecting the transfer involving dummy data. The transfer with dummy data is the transfer (discharge) as a result of the received TLP data amount not satisfying the reception buffer size and not being able to receive data for a given time thereafter, and dummy data is added to satisfy the reception buffer size. 
     A failure monitor  18  (failure Mon.) comprises a function of detecting failures in the I/O (dual I/O and cross I/O). Examples of an I/O failure includes a reply (completion status) timeout upon performing Non-Posted TLP transfer, or a case of the receiving side detecting an error in the parity added during the PCI Express transfer. 
     MSI.MP I/F register  19  reflects the multiplicity, traffic volume, and dummy data transfer detected in the respective monitors  17 ,  18 ,  21  as well as the detection of failures, and transfers this to the microprocessors  5   a,    5   b  and the processor memories  7   a,    7   b  via the MP I/O. 
     Examples of the foregoing multiplicity, traffic volume, and dummy data transfer detection are now explained.  FIG. 3  shows the TLP transfer and multiplexing to the destination I/O (Dual I/O) via the respective transfer source I/Os (MP I/O, Host I/O, Dry I/O). 
     The bridge circuits  13   a,    13   c,    13   e  (BRG MP, BRG H, BRG D) of the transfer source I/Os (MP I/O, Host I/O, Dry I/O) are configured from a buffer for storing the received TLP, and a timer for measuring the TLP reception interval. The buffer capacity comprises, for instance, twice (1024 Byte, 1 KB) of the 512 Byte of the access unit (sector) of the hard disk drives. A buffer is used for minimizing the overhead upon multiplexing with other I/Os on the high-speed switch  15 , and is provided for continuing the TLP transfer of an I/O during the another transfer via a separate I/O. 
     The bridge circuits  13   a,    13   c,    13   e  of the transfer source I/O are configured for the TLP transfer from access in several byte units of the register access to several KB of the data. The bridge circuits  13   a,    13   c,    13   e  start up the timer at the timing of receiving the TLP, and measure the time interval until the subsequent TLP received. If the bridge circuits  13   a,    13   c,    13   e  detect a timeout, they force to satisfy the buffer capacity of 1 KB by adding dummy data to the received TLP, and transfer this to the high-speed switch  15 . The bridge circuits  13   a,    13   c,    13   e  simultaneously output a TLP transfer request (REQ) to the arbiter  16 . The arbiter  16  returns Acknowledge (ACK) to each bridge circuit  13   a,    13   c,    13   e  in the order that the TLP transfer request (REQ) is made. If the TLP transfer requests (REQ) are made simultaneously, the arbiter  16  returns ACK according to the priority. After receiving ACK, the bridge circuits  13   a,    13   c,    13   e  transfer the data in the buffer. The arbiter  16  multiplexes the TLP transferred to the high-speed switch  15  in the order that ACK was returned, and transmits this to the output I/O. 
     The arbiter  16  also detects the TLP multiplicity of the transfer destination I/O from the number of TLP transfer requests (REQ) that are made simultaneously, or from the number of transfer I/Os that created a TLP transfer request (REQ) within a period and the number of TLP transfer requests (REQ) that are made per period. The traffic monitor  17  calculates the traffic volume by measuring the occupancy time of the TLP transfer on the transfer destination I/O in given intervals. The dummy transfer monitor  21  is a function for detecting the transfer involving dummy data, and corresponds to the timeout detection of the timer configuring the bridge circuits  13   a,    13   c,    13   e.  The dummy transfer monitor  21  counts the timeout detection in a period and compares it with a threshold. The dummy transfer monitor  21  is able to detect wastage in the I/O band by measuring the frequency of the dummy data transfer in the comparison with the threshold. 
     Wastage of the I/O band is now explained with reference to  FIG. 4 .  FIG. 4  shows a case where the MP I/O receives a TLP with a small data amount, and the TLP is transferred together with dummy data e configuring the bridge circuits  13   a,    13   c,    13   e.  Although the dummy data is meaningless data in the transfer destination, wastage of the I/O band will occur since it exists as data on I/O. With the storage apparatus  100 , wastage of the I/O band will increase and the deterioration in the transfer rate of the other I/Os will occur if dummy data transfer is frequent. 
     Although  FIG. 4  illustrates an example using the MP I/O, transfer with dummy data will also occur if there is register access between the host interface  2  and the drive interface  3  connected to the host I/O and the drive I/O. Accordingly, in  FIG. 3 , all bridge circuits  13   a,    13   c,    13   e  comprise a timer and a dummy data addition function. 
     A method of sharing the respective statuses such as the detection of multiplicity, traffic volume, dummy data and failure is now explained. In this case, the microprocessor  5   a  (MP  0 ) acquires the status of the directly-controlled controllers  1   a,    1   b  (CTL  0 , CTL  1 ), host interfaces  2   a,    2   b  (Host I/F  0 ,  1 ), and drive interfaces  3   a,    3   b  (Dry I/F  0 ,  1 ) and accesses the control register based on the TLP transfer with the processor memory  7   a  (MP Mem  0 ) via the memory controller hub  6   a  (MCH  0 ) and the register access of the microprocessor (self system (CTL 0 / 1 ) status). The microprocessor  5   b  (MP  1 ), as with the microprocessor  5   a,  acquires the status of the controllers  1   c,    1   d  (CTL  2 , CTL  3 ), the host interfaces  2   c,    2   d  (Host I/F  2 ,  3 ), and the drive interfaces  3   c,    3   d  (Dry I/F  2 ,  3 ) and updates the processor memory  7   b  (self system (CTL 2 / 3 ) status). 
     Here, when viewed from a single microprocessor  5 , the directly controllable controller  1 , host interface  2 , and drive interface  3  are defined as a “self system,” and a controller on the other side with the dual I/O are defined as the “other system.” For instance, in the microprocessor  5   a,  the controllers  1   a,    1   b  are a “self system” and the controllers  1   c,    1   d  are the “other system,” and in the microprocessor  5   b,  the controllers  1   c,    1   d  are a “self system” and the controllers  1   a,    1   b  are the “other system.” 
     Meanwhile, there are cases where each microprocessor  5  needs to acquire the status of controller  1  or I/O of the “other system.” For example, this would be cases where the microprocessor  5   a  needs to acquire the status of the controller  1   c  or  1   d,  or access the host interfaces  2   c,    2   d  and the drive interfaces  3   c,    3   d  via the I/O. 
     In this case, the status is shared by the microprocessor  5   a  performing data transfer from the processor memory  7   b  of the “other system” to the processor memory  7   a  of the “self system” via the dual I/O. In addition, there may be cases that the microprocessor  5   a  directly accesses the register and I/O in the controllers  1   c,    1   d  of the “other system.” Here, with the storage apparatus  100 , two types of transfer paths exist when sharing the status in the communication between the processor memories  7   a,    7   b.  Thus, when transferring the status or the like from the processor memory  7   b  to the processor memory  7   a,  the microprocessor  5   a  selects the optimal dual I/O from the failure, multiplicity, traffic volume, and dummy data transfer detection in the dual I/O of the controllers  1   c,    1   d  of the “other system.” 
     Incidentally, the communication between the MSI.MP I/F register  19  and the microprocessor  5  is not limited to the communication via the MP I/O, and other cases may be considered where an interrupt signal to be directly connected to the microprocessor  5  with a hard wire or a dedicated register I/F bus is provided when using an MSI (Message Signaled Interrupt) (not shown) as the interrupt notice, this is transferred to the processor memory  7  as TLP via the MP I/O. The data transfer between the host and the HDD drive (LU  8   a ) to be performed is now explained.  FIG. 6  shows, as one example of the data transfer (LU read) to the host, the data transfer path when the LU  8   a  (LU  0 ) is reproduced, store into the cache memory  4   a  (Cache  0 ), and transferred from the cache memory  4   a  to the respective host interfaces  2   a  to  2   d  (Host I/F  0 - 3 ). The storing of the LU  8   a  into the cache memory  4   a  is performed by the microprocessor  5   a  (MP  0 ) controlling the controller  1   a  (CTL  0 ) to which the LU  8   a  belongs starting up the DMA circuit  14   b  (D DMA). 
     After the storing of the LU  8   a  into the cache memory  4   a,  the microprocessor  5  controlling the respective controllers  1  starts up the DMA circuit  14   a  (H DMA) in the controller to which the transfer destination host interface  2  belongs, and thereby transfers the data of the LU  8   a.    FIG. 6  shows the transfer path (xfer 0 , xfer 1 , xfer 2 , xfer 3 ) to the respective host interfaces  2 . In  FIG. 6 , there is a plurality of transfer paths in a transfer (xfer 3 ) in which the transfer source (cache memory  4   a ) and the controller  1   d  to which the transfer destination (Host I/F  3 ) belongs are not mutually adjacent. Here, the transfer path (xfer 3 ) between the controllers  1  that are not connected in dual I/O or cross I/O is referred to as a “diagonal path.” In  FIG. 6 , in the case of a diagonal path xfer 3  via the controller  1   c  (CTL  2 ), the DMA circuit  14   a  (H DMA) of the transfer destination controller  1   d  (CTL  3 ) is started up, and the controllers  1   c,    1   d  and data transfer are managed in the same microprocessor  5   b  (MP  1 ). Meanwhile, the controller  1   a  (cache memory  4   a  in which data of the LU  8   a  is stored) and the controller  1   c  are connected with a dual  0  I/O (Dual  0  I/O), and its I/O failure is managed with the transfer timeout of the received data on the controller  1   c.  Accordingly, comprehension of failures during the transfer can be consolidated in the microprocessor  5   b.  The storing of the LU  8   a  into the cache memory  4   a  is not limited to the example shown in  FIG. 6 , and, for example, such data may be transferred to the controller  1  to which the transfer destination host interface  3  belongs, and then stored into the cache memory  4  t. In this case also, as the transfer path from the LU  8   a  to the cache memory  4   d,  there is a path that routes the controller  1   b  or the controller  1   c.  The subject matter of the present invention is also applied to foregoing case. 
       FIG. 7  shows the data transfer (LU write) path from the host interface  2   a  to each LU  8 . In  FIG. 7 , the received data of the host interface  2   a  is stored to the cache memory  4   a  of the affiliated controller  1   a  and the cache memory  4   c  of the controller  1   c  in a symmetrical location via the dual  0  I/O. With the storage apparatus  100 , it is possible to avoid the loss of transfer data from the host based on above-cited method. After the storing, the microprocessor  5   a  transfers the data to the drive interface  3   a  in which the target LU  8   a  exists so as to update the LU  8   a.  The microprocessor  5   a  controls the transfer with the DMA circuit  14   b  (D DMA) of the transfer destination controller  1   a.  The storing of data received from the host interface  2   a  to the cache memory  4  is not limited to  FIG. 7 , and may also be stored to the cache memory  4   b,    4   d.    
     I/O access that passes through the dual I/O or cross I/O is now explained with reference to  FIG. 5  to  FIG. 7 . In the dual  0  I/O, dual  1  I/O, communication ( FIG. 5 ) involving the sharing of the processing memory  7  of the “other system” and access to the register of the controller  1 , the host interface  2 , and the drive interface  3  of the “other system,” the transfer data ( FIG. 7 ) from the host, and the data transfer ( FIG. 6 ) during the transfer to the host are multiplexed. In particular, since the register access of the microprocessor  5 , the host interface  2 , and the drive interface  3  involve the dummy data transfer explained in  FIG. 4 , in the dual I/O, the band is easily wasted due to the occurrence of such access. Meanwhile, the cross  0  I/O and cross  1  I/O do not entail the foregoing problems since they are used only in the data communication between the controllers  1 . 
     The method of transfer performed between the controller  1  and the microprocessor  5  is now explained regarding the data transfer between the host and the LU  8  explained with reference to  FIG. 6  and  FIG. 7 . 
       FIG. 8  to  FIG. 12  are diagrams showing the TLP format to be applied only for the data transfer in the controller  1  or between the controllers  1  of  FIG. 6  and  FIG. 7 . The TLP format is compliant to the TLP format of PCI Express, and is optimized for the TLP transfer in the controller  1  and between the controllers  1 . 
       FIG. 8  shows a write command  31  from the transfer source, which is configured from a header portion and write data.  FIG. 9  is a reply packet (write reply  32 ) requiring the status reply to the write command  31 , and is configured from a header and error status.  FIG. 10  shows a read command  33 , and is configured only from a header.  FIG. 11  is a read reply  34  (completion) to the read command, and is configured from a header and transfer data from the transfer source (completer). 
       FIG. 12  shows the portion optimized for the TLP transfer in the controller  1  or between the controllers  1 . In  FIG. 12 , the TLP format ( 31  to  34 ) is assigned, as information to be included in the complete ID and the requester ID, an LSI number for identifying the controller to which the completer or the requester belongs, and a Func. (Function) number for identifying the requester or the I/O to which the requester is to be connected. Moreover, the TLP format ( 31  to  34 ) is assigned a route ID 1  and a route ID 2  for identifying the routing controller between the controllers  1  to which the requester and the completer respectively belong for the purpose of selecting to the diagonal path (xfer 3 ) in  FIG. 6 . The completer ID, the requester ID, the route ID 1  and the route ID 2  are assigned by the microprocessor  5 . 
     The route ID 1  is assigned a controller number ( 0  to  3 ) of the controllers  1   a  to  1   d.  Nevertheless, a code value other than  0  to  3  for identifying the respective paths such as the transfer path (xfer  0 - 2 ) of  FIG. 6  and the transfer path (xfer  0 - 3 ) of  FIG. 7  is invalid; that is, there is no routing controller  1 . The route ID 2  defines the transfer source of the data in the routing controller  1 ; that is, it defines the input I/O (dual I/O or cross I/O) to the controller  1 . Contrarily, the route ID 2  may define the transfer destination; that is, the output I/O from the controller  1 . A tag (Tag) number is used identifying a reply packet to the requester request when taking over all tag numbers contained in the TLP with all read replies  34  in response to a certain read command. 
       FIG. 8  to  FIG. 12  are all compliant with the TLP format of PCI Express, and explanation regarding the respective bits configuring the header is omitted. 
     The TLP header explained above is not applied to the TLP transfer between the I/Os (Host I/O, Dry I/O, MP I/O) and controller  1 . A format that is fully compatible with the TLP of PCI Express is applied to the foregoing TLP transfer to maintain the transfer compatibility. TLP compatibility will be explained with reference to  FIG. 13 . 
       FIG. 13  is a diagram of the data transfer in the controller  1  and between the controllers  1  performed in the TLP format explained with reference to  FIG. 8  to  FIG. 12 , and shows, as one example, a case of application to the transfer path (xfer  3 ) (transfer via the controller  2 ) in  FIG. 6 . In other words,  FIG. 13  shows a case where the host interface  2   c  (Host I/F  3 ) receives a read command, data of the LU  8   a  (LU  0 ) is stored to the cache memory  4   a  (Cache  0 ), and cache data is transferred to the host interface  2   c  via the controller  2   b  (CTL  2 ). 
     In  FIG. 13 , when the host interface  2   c  receives a read command, the microprocessor  5   b  (MP  1 ) managing the controller  1   d  analyzes the transfer command from the host interface, and searches for the LU  8  to be reproduced (LU search will be explained later with reference to  FIG. 14 ). Meanwhile, a header is temporarily saved in the bridge circuit  13   e  (BRG H) of the controller  1   d.  Header is saved by transferring the header portion from the buffer configuring the bridge circuit  13   e  to the processor memory  7   b.    
     After the LU search, the microprocessor  5   b  stores the data of the reproduced LU  8   a  to the cache memory  4   a  based on the startup of the DMA circuit  14   b  (D DMA). Meanwhile, the microprocessor  5   b  converts the header information of the saved read command into a requester ID corresponding to the I/O that received the read command and the number ( 3 ) of the controller  1   c.  In addition, the microprocessor  5   b  also transfers the header information to be taken over including the tag number to the memory controller  12  of the controller  1   a  via the processor memory  7   b.    
     In other words, the microprocessor  5   b  assigns the number ( 3 ) of the controller  1   c  as the LSI number of the complete ID showing the transfer destination to the completion status (Cpl STS) in response to the read command, and assigns the DMA circuit  14   a  (HDMA) (0x13) showing the host interface  2  as the transfer destination I/O. The microprocessor  5   b  also assigns the number ( 0 ) of the controller  1   a  as the LSI number of the requester ID, and assigns the memory controller  12  (0x05) showing the cache as the transfer source I/O. The microprocessor  5   b  takes over the tag number. Moreover, the microprocessor  5   b,  based on the routing result, stores the routing controller number and the I/O (Route ID  1 - 2 ). In the case of  FIG. 13 , since this routes the controller  1   c,  the input in the controller  1   c  is a dual I/O, and the number ( 2 ) of the controller  1   c  is stored in the route ID 1  and the number (0x01) of the dual I/O is stored in the route ID 2 . Routing will be explained later with reference to  FIG. 14  onward. 
     When the completion status preparation in the controller  1   a  is complete, the microprocessor  5   b  starts up the DMA circuit  14   a  (H DMA) of the transfer destination controller  1   c,  and controls the cache data transfer in TLP units. The memory controller  12  (MEM CTL) of the controller  1   a  requests the high-speed switch  15  to transfer data to the transfer destination IO via the route ID 1  and the route ID 2 . The memory controller  12  of the controller la requests the transfer to the dual I/O as the transfer destination I/O since the route ID 2  is “0x01, ” and thereby performs the TLP transfer from the controller  1   a  (CTL  0 ) to the controller  1   c  (CTL  2 ). 
     In the controller  1   c,  the bridge circuit  13   d  (BRG Dual) that received the TLP via the dual  0  I/O detects that the route ID 1  is “0x02, ” detects the coincidence with the self controller number ( 2 ), and thereby identifies that it is a routing controller. The bridge circuit  13   d  recognizes the cross  1  I/O as the I/O between the controllers  1  as the transfer destination I/O based on the foregoing recognition result, that the route ID 2  is “0x01” and the input is “dual I/O.” Moreover, the bridge circuit  13   d  requests the high-speed switch  15  to transfer data to the transfer destination I/O (cross  1  I/O), and transfers data to the controller  1   d  (CTL  3 ) connected to the cross  1  I/O of the controller  1   c.    
     In the controller  1   d,  the bridge circuit  13   b  (BRG Cross) that received the TLP via the cross  1  I/O detects “0x03” as the controller identifying information based on the complete ID. The bridge circuit  13   b  detects the coincidence with the self controller number, and identifies that it is a reception controller of a completion status. Moreover, the bridge circuit  13   b  recognizes the host I/O as the transfer destination since the transfer destination I/O is the DMA circuit  14   a  (H DMA) (0x13). Then the bridge circuit  13   b  requests the high-speed switch  15  to transfer data to the host I/O, and transfers TLP to the host interface  2   d  (Host I/F  3 ). Meanwhile, the bridge circuit  13   e  (BRG H) compares the tag number retained during the reception of the read command and the tag number assigned to the transfer TLP, switches the header at the time of reception to those that coincide, and converts this into a completion status for external I/O reply. 
     Based on the above, the transfer between the controllers  1  uses the TLP format of  FIG. 8  to  FIG. 12 , and the transfer between the external I/Os uses the TLP fully compatible with PCI Express by switching the TLP header, when performing the data transfer. 
     The data transfer control explained in  FIG. 6  is now explained with reference to the flowchart of  FIG. 14 .  FIG. 14  shows the control of command reception and search for the reproduced LU the determination of availability of data transfer path, path selection processing, and data transfer. The flowchart of  FIG. 14  is realized by the microprocessor  5  controlling the controller  1  and the like. Foremost, the microprocessor  5  receives a transfer command issued by the host from the host interface  2  (S 801 ). Subsequently, the microprocessor  5  controlling the host interface  2  that received the transfer command detects the reception of the transfer command via interruption (MSI) from the host interface  2  or the controller  1 . Further, the microprocessor  5  transfers the received transfer command to the processor memory  7  and analyzes the command (S 802 ). 
     The LU search and cache store processing performed in the range of S 803  to S 807  are now explained. Subsequently, the microprocessor  5  detects an LU reproduction command after the processing at S 802 , and searches for the LU  8  managed by the controller  1  of the “self system” (S 803 ). When the LU  8  to be reproduced is detected (S 803 : Detected), the microprocessor  5  jumps to S 806 , starts up the DMA circuit  14   b  (D DMA) of the controller  1  to which the reproduced LU  8  belongs, and stores data of the reproduced LU  8  to the cache memory  4 . 
     Meanwhile, if there is no corresponding LU  8  at S 803  (S 803 : NA), the microprocessor  5  succeeds the processing to the microprocessor  5  of the “other system” (S 804 ). Specifically, the microprocessor  5  transfers the received command in the processor memory  7  of the “self system” to the processor memory  7  of the “other system.” The microprocessor  5  of the “other system” searches for the LU  8  managed by itself, detects the reproduced LU (S 805 ), starts up the DMA circuit  14   b  (D DMA) of the controller  1  to which the reproduced LU  8  belongs, and stores data of the reproduced LU  8  to the cache memory  4  (S 806 ). 
     In the example of  FIG. 6 , the microprocessor  5   b  (MP  1 ) controlling the host interface  2   d  (Host I/F  3 ) that received the transfer command searches for the LU  8   a  (LU  0 ) within the range of the managed LU  8   c,    8   d  (LU  2 , LU  3 ). Subsequently, the microprocessor  5   b  succeeds the processing to the microprocessor  5   a  (MP  0 ) since there is no corresponding LU  8   a,  the searches for the LU  8   a  within the range of the LU  8   a,    8   b  (LU  0 , LU  1 ) managed by the microprocessor  5   a.  The microprocessor  5   a  detects the LU  8   a  as a result of the search, and controls the storing thereof to the cache memory  4  (Cache  0 ). 
     Finally, the microprocessor  5  shares the write completion of the reproduced LU  8  into the cache memory  4  at S 807  as a status among the microprocessors  5 . The method explained in  FIG. 5  is used as the method for sharing the status. 
     The LU information to be managed by each microprocessors  5  may be shared using a part of the processor memory  7  or the cache memory  4 . Thus, the LU search processing (S 803  to S 805 ) may also be performed by single microprocessor  5 . 
     Subsequently, after storing the reproduced LU  8  to the cache memory  4 , the microprocessor  5  determines the data transfer path availability from the cache memory  4  to the transfer destination (S 808 ). With the transfer path (xfer  0 , xfer  1 , xfer  2 ) of the  FIG. 6 , the optimal path is uniquely decided so as long as there is no failure on the path. Nevertheless, since the transfer path (xfer  3 ) has the same path length; that is, since the number of controllers  1  to be passed through is the same and there are a plurality of transfer paths, the optimal transfer path can be selected according to the change in the operational status of the respective controllers  1 . 
     Subsequently, during the data transfer path availability determination at S 808 , the microprocessor  5  shares the information of the controller to which the cache memory  4  stored with data of the reproduced LU  8  and the host interface  2  that received the transfer command respectively belong with the processor memory  7 . The microprocessor  5  determines that the transfer path is “xfer  3 ” since the controllers  1  are not adjacent and are of a diagonal location regarding the cache memory  4   a  (Cache 0) and the host interface  2   d  (Host I/F  3 ), and performs the transfer path selection processing (S 808 : YES). 
     Subsequently, the microprocessor  5  sequentially performs the routing determination  1 ,  2  of S 809 , S 80 A, and determines the optimal transfer path based on the controller failure, and multiplicity, traffic volume, and dummy data transfer detection between the controller I/Os (S 809 , S 80 A). Upon transferring data, the microprocessor  5  transfers data of the reproduced LU  8  in the cache memory  4  according to the generation method of the completion status (TLP format) and the transfer method based on DMA startup explained with reference to  FIG. 8  to  FIG. 13  (S 80 B). 
     Meanwhile, the microprocessor  5  proceeds to the processing of S 80 B if the data transfer path selection at S 808  is not required (S 808 : NO) (correspond to  FIG. 6  transfer path “xfer  0 , xfer  1 , xfer 2 ”) (S 80 B). Incidentally, the microprocessor  5  may execute S 809  before executing S 80 B to deal with a failure in the I/O on the data transfer path. Although the microprocessor  5  processed the data transfer selection in this embodiment, other microprocessor might do this process as the same. Finally, the microprocessor  5  performs the routing determination  3  of S 80 C as processing corresponding to S 809  and S 80 A even during the completion status transfer, and selects the optimal path according to the ever-changing status of the controller  1  (S 80 C). 
     As a specific example, a case of applying the transfer path of “xfer  3 ” in the  FIG. 6  to the flowchart of  FIG. 14  is now explained. In  FIG. 6  and  FIG. 14 , the microprocessor  5   b  (MP  1 ) receives a transfer command from the host interface  2   d  (Host I/F  3 ) (S 801 ). Subsequently, the microprocessor  5   b  transfers the transfer command to the processor memory  7   b  (MP Mem  1 ) controlling the host interface  2   d.    
     Subsequently, the microprocessor  5   b  analyzes the content of the command and detects a reproduced LU  8   a  (S 802 ). Here, the microprocessor  5   b  determines that the management LU of the microprocessor  5   b  is LU  8   c,    8   d  (LU  2 , LU  3 ), and not available (S 803 : NA). Subsequently, the microprocessor  5   b  succeeds the processing to the microprocessor  5   a  (MP  0 ) of the “other system” (S 804 ). 
     Subsequently, the microprocessor  5   a  determines that the management LU of the microprocessor  5   a  is LU  8   a,    8   b  (LU  0 , LU  1 ), and detects the reproduced LU  8   a  (S 805 ). Subsequently, the microprocessor  5   a  starts up the DMA circuit  14   b  (D DMA) of the controller  1   a  (CTL  0 ), and stores the data of the reproduced LU  8   a  to the cache memory  4   a  (Cache  0 ) (S 806 ). Subsequently, the microprocessor  5   a  shares the store end status of the data of the reproduced LU  8   a  to the cache memory  4   a  with the processor memories  7   a,    7   b  (MP Mem  0 , MP Mem  1 ) (S 807 ). Subsequently, the microprocessor  5   a  determines that the cache memory  4   a  and the host interface  2   d  that received the transfer command are of a diagonal location (S 808 : YES), proceeds to the processing at S 809 , and selects “xfer  3 ” as the optimal transfer path (routing determination  1  (S 809 ), routing determination  2  (S 80 A)). Subsequently, the microprocessor  5   a  transfers data from the cache memory  4   a  to the reproduced LU  8   a  (S 80 B). Subsequently, the microprocessor  5   a  constantly selects the optimal transfer path based on S 80 C (routing determination  3  (S 80 C)). 
     The routing determination  1  at S 809  is now explained. The flowchart of  FIG. 15  shows the path determination method according to the failure status of the controller  1 . In particular, this is a determination method according to a failure of the I/O (dual I/O and cross I/O) between the respective controllers deeply related to the path selection. The microprocessor  5 , at S 901 , acquires the failure status of the I/O (dual I/O and cross I/O) between the respective controllers, and shares the failure status of the I/O between all controllers with the writing of data into the respective processor memories. As an example of a failure detection method, considered may be a case where a timeout arises during the reception of the reply packet based on a Non-Posted transfer, or when a communication error is detected between the controllers  1  in the self diagnosis of the I/O to be performed upon the startup of the storage apparatus. 
     Subsequently, the microprocessor  5 , at S 902 , confirms the I/O failure between controllers in the controller  1  of the “self system” based on the shared failure information. For example, in the case of “xfer  3 ” of  FIG. 6 , the microprocessor  4   b  of the “self system” confirms the I/O failure between the controllers  1   c,    1   d  (dual  0  I/O, cross  1  I/O). Subsequently, if there is no I/O failure between the controllers (S 902 : NO), the microprocessor  5   b  of the “self system” selects the routing of the controller  1   c  (S 903 ). 
     Thereby, with the storage apparatus  100 , the failure management of the selected path of “xfer  3 ” can be consolidated in the microprocessor  5   b  controlling the transfer destination host interface  2   d.  In other words, with the storage apparatus  100 , since from the inter-controller I/O of the transfer source I/O to the inter-controller I/O of the transfer destination are managed with a microprocessor  5   b,  the occurrence of a failure can be grasped promptly. Although sharing a failure with the processor memory  7  can also be considered, a failure in the dual I/O cannot be shared. Thus, the superiority of S 902  and S 903  should be understood. 
     Subsequently, if an inter-controller I/O failure occurs at S 902  (S 902 : YES), the microprocessor  5 , at S 904 , confirms the inter-controller I/O failure of the controller  1  of the “other system” (S 904 ). Specifically, at S 902 , the microprocessor  5   b  of the “self system” determines the dual  0  I/O, cross  1  I/O, and determines the availability of the routing of the controller  1   c  (S 902 ). At S 904 , the microprocessor  5   a  of the “other system” determines the dual  1  I/O, cross  0  I/O, and determines the availability of the routing of the controller  1   b  (S 904 ). 
     If the microprocessor  5  detects a failure at S 904  (S 904 : YES), it determines that it is impossible to have a path (S 905 ). Finally, the microprocessor  5  determines the retry from the S 901  (S 906 ). The microprocessor  5  determines the error if the retry count is exceeded or the retry processing is not performed (S 906 : NO). Meanwhile, if the microprocessor  5  determines that there is no inter-controller I/O failure at S 904  (S 904 : NO), it selects the routing of the controller  1   b  (S 907 ). 
     The routing determination  2  at S 80 A is now explained. The flowchart of  FIG. 16  shows the method of selecting the transfer path based on the detection of multiplicity, traffic volume, and dummy data of the inter-controller I/O explained in  FIG. 2  and  FIG. 3 . 
     The microprocessor  5 , at S 101 , confirms the existence of an inter-controller I/O failure from the result of the routing determination  1  (S 809 ). If the microprocessor  5  confirms a failed I/O (S 101 : YES), it determines that path selection is impossible, and sets the result of the routing determination  1  as the final result of the routing determination (S 102 ). If the microprocessor  5  confirms that there is no inter-controller I/O failure and the result of the routing determination  1  (S 809 ) is to be preferentially applied (S 101 : YES), it also sets the result of the routing determination  1  as the final result (S 102 ). In other words, with the storage apparatus  100 , since this corresponds to a case where the path determination at S 903  of the routing determination  1  (S 809 ) was performed, in the foregoing case, this is advantageous in the failure management of the transfer path as described above. 
     Meanwhile, if the microprocessor  5  is to proceed from S 101  to S 103  (S 101 : NO), it acquires the status information on the detection of the multiplicity, traffic volume, and dummy data of each inter-controller I/O (dual I/O and cross I/O) (S 103 ). Each information is directly acquired from the sharing to the processor memory  7  and from the MSI.MP I/F register  19  in the controller  1 . Subsequently, the microprocessor  5  determines the path from the acquired status information at S 104  to S 106 . 
     The microprocessor  5  determines the dummy data transfer in the dual I/O as the initial determination condition (S 104 ). Moreover, if the microprocessor  5  detects dummy data transfer at S 104  (S 104 : YES), at S 108 , it determines the I/O subject to the dummy data transfer (dual  0  I/O or dual  1  I/O), and selects a path to the dual I/O with no dummy data transfer (S 108 ). When applying “xfer  3 ” of  FIG. 6 , if the microprocessor  5  detects the dummy data transfer to the dual  0  I/O, it selects routing of the dual  1  I/O; that is, routing of the controller  1   b.    
     If the microprocessor  5  detects the dummy data transfer to both the dual  0  I/O and the dual  1  I/O, it repeats the update and acquisition of the status information, and selects a path of the dual I/O with a low detection frequency. Otherwise, the microprocessor  5  may also entrust the decision to the determination at S 105  onward. 
     Meanwhile, if the microprocessor  5  does not detect a dummy data transfer at S 104  (S 104 : NO), at S 105 , it compares the packet multiplicity of the dual  0  I/O and the dual  1  I/O (S 105 ). If there is a difference in the multiplicity (S 105 : YES), the microprocessor  5  proceeds to the processing at S 108 , and selects a dual I/O with small multiplicity (S 108 ). Thereby, the microprocessor  5  is able to select a path while giving consideration to the influence to the overhead, which arises upon multiplexing with the high-speed switch  15 , to the transfer. 
     Meanwhile, if there is no difference in the multiplicity at S 105  (S 105 : NO), the microprocessor  5 , at S 106 , makes its determination based on the traffic volume of the dual  0  I/O and the dual  1  I/O (S 106 ). If there is a difference in the traffic volume (S 106 : YES), the microprocessor  5 , at S 108 , selects the dual I/O with a small traffic volume (S 108 ). Thereby, the microprocessor  5  is able to select a transfer path that can be transferred another data transfer sufficiently with the selected dual I/O. Based on the above, the microprocessor  5  gives maximum weighting to S 104  (existence of dummy data transfer) regarding the path determination based on dual I/O, and thereafter sequentially gives weighting to S 105  (multiplicity difference), and S 106  (traffic volume difference). The reason why weighting is given more to S 105  than S 106  (multiplicity priority) is because the increase in the transfer overhead caused by the multiplexing of packets will deteriorate the transfer efficiency; that is, waste the band of the dual I/O. 
     Meanwhile, if there is no difference in the traffic volume at S 106  (S 106 : NO), the microprocessor  5  determines the routing of cross  0  I/O or cross  1  I/O from the same status information (multiplicity, traffic volume, etc.) in the cross I/O at S 107  (S 107 ). Here, the microprocessor  5  gives weighting of “(multiplicity)&gt;(traffic volume)” as with S 105  to S 106  regarding the priority of determination at S 107 . 
     In addition, since the cross I/O as no processor access or cache dual writing as with the dual I/O, the band will not be wasted. Accordingly, the microprocessor  5  sets “(Dual I/O)&gt;(Cross I/O)” upon selecting the path, and makes its determination by giving weighting to the dual I/O. 
     Based on the above, the microprocessor  5  is able to optimally select the path of “xfer  3 ” of  FIG. 6  by sequentially performing the routing determination  1  ( FIG. 15 ) and the routing determination  2  ( FIG. 16 ) explained above. 
     The routing determination  3  of S 80 C is now explained. The flowchart of  FIG. 17  shows the control method in a case where there is change in the status information after the startup of S 80 B of  FIG. 14 . The microprocessor  5  starts up the S 80 B, and thereafter foremost determines the existence of a failure in the dual I/O and cross I/O from the status information to each controller  1  (S 111 ). If there is a failure (S 111 : YES), the microprocessor  5  performs the routing determination  1  of  FIG. 15  (S 112 ). Subsequently, the microprocessor  5  proceeds to S 115  if routing that avoids the dual I/O and cross I/O that failed at S 113  (S 113 : NO), and error ends the routine if such routing is impossible (S 113 : YES). 
     Meanwhile, if there is no failure at S 111  (S 111 : NO), the microprocessor  5  performs the routing determination  2  of  FIG. 16  (S 114 ). Subsequently, the microprocessor  5 , at S 115 , makes a comparison with the previous routine result from the determination of S 114  or S 112  to S 113  (S 115 ). 
     If there is no difference (S 115 : NO), the microprocessor  5  proceeds to S 111 , and continues the subsequent processing (S 111  to S 115 ). Meanwhile, if there is a difference (S 115 : YES), the microprocessor  5  determines whether to temporarily stop the transfer (S 116 ). Upon temporarily stopping the transfer (S 116 : YES), at S 117 , the microprocessor  5  temporarily stops the DMA circuit  14 , selects a new transfer path by switching the route ID 1  and route ID 2  in a completion status, and restarts the DMA circuit  14  (S 117 ). Subsequently, if the microprocessor  5  is not to temporarily stop the transfer at S 116  (S 116 : NO), it ends the routing determination  3 , and performs the transfer while maintaining the path until the transfer in response to the received command is complete. 
     Based on the above, the microprocessor  5  is able to optimize the path according to changes in the operational status of the controller  1  even during the transfer. 
     (2) Second Embodiment 
       FIG. 18  shows another architecture example of the storage apparatus  100  according to the present invention. With the storage apparatus  100  of this embodiment shown in  FIG. 18 , host interfaces  2   a  to  2   d  and drive interfaces  3   a  to  3   d  are connected to the memory controller hubs (Memory Controller Hub: MCH)  6   a  to  6   d  in substitute for the controllers  1   a  to  1   d  in the architecture of the storage apparatus  100  shown in  FIG. 1 . In addition, microprocessors  5   a  to  5   d  are connected to the cache memories  4   a  to  4   d,  and the processor memories  7   a,    7   b  (MP Mem) are integrated with the cache memories  4   a  to  4   d.  The processor memory  7  is not limited to being integrated, and may also be connected to a different I/O of the microprocessors  5   a  to  5   d.    
     Moreover, with the storage apparatus  100  of this embodiment, MCH bridges  22   a  to  22   d  are provided for the purpose of controlling the TLP transfer between the memory controller hubs  6 . Connection between the memory controller hubs  6 , as with  FIG. 1 , is as follows. Namely, the memory controller hub  6   a  and the memory controller hub  6   b  are connected with the cross  0  I/O, the memory controller hub  6   a  and the memory controller hub  6   c  are connected with the dual  0  I/O, the memory controller hub  6   b  and the memory controller hub  6   d  are connected with the dual  1  I/O, and the memory controller hub  6   c  and the memory controller hub  6   d  are connected with the cross  1  I/O. Since the remaining configuration of the storage apparatus  100  in this embodiment is the same as  FIG. 1 , the same components are given the same reference numeral, and the detailed explanation thereof is omitted. 
     As an example of data transfer in the configuration of this embodiment,  FIG. 18  shows the data transfer from the LU  8   a  to the host interface  2   d.  This is the same transfer as “xfer  3 ” of  FIG. 6 , and shows that there are two transfer paths. Accordingly, as with the first embodiment, it is necessary to select the optimal transfer path. 
     In this case, as with the first embodiment upon selecting the optimal path, the microprocessor  5  foremost determines an I/O failure between the memory controller hubs  6  to be controlled by each of the microprocessors  5 . The microprocessor  5  detects the existence of a failure in the memory controller hub  6  and the I/O between all memory controller hubs based on the MCH bridge  22 . The microprocessor  5  selects a path without a failure based on the acquired failure information of the I/O between the memory controller hubs. 
     Meanwhile, the microprocessor  5  is able to detect the dummy data transfer, I/O multiplicity, and traffic volume explained with reference to  FIG. 3  in the first embodiment by providing a similar circuit in the memory controller hub  6 . Detection of the traffic volume may also be included in the MCH bridge  22 . The microprocessor  5  uses the detection information obtained from the memory controller hub  6  and the MCH bridge  22  to perform path selection according to the method explained with reference to  FIG. 16 . 
     The TLP flowing between the memory controller hubs  6  can be subject to packet transfer according to the selected transfer path by using the header configuration of  FIG. 8  to  FIG. 12 . Specifically, the TLP flowing between the memory controller hubs  6  can be handled, as in the first embodiment, by replacing the controller number with the identifying information of the memory controller hub  6  routed through the transfer source and transfer destination, and the identifying information of the I/O to be used in the transfer. 
     The switching of the packet header explained with reference to  FIG. 13  is performed according to the same method in the memory controller hub  6 . 
     The TLP transfer comprising the header configuration of  FIG. 8  to  FIG. 12  is performed only with the I/O between the memory controller hubs  6 . During the transfer to the microprocessor  5 , the host interface  2 , and the drive interface  3 , after the TLP that was transferred between the memory controller hubs  6  arrives, the TLP compatibility is maintained by switching to the header retained during the reception of the transfer command in each I/O. 
     Like this, with the storage apparatus  100 , the microprocessor  5  determines the data transfer path by analyzing the transfer command received from the host. If there are a plurality of data transfer paths, a data transfer path in which the number of controllers  1  connected to the microprocessor  5  becomes maximum is selected regarding the microprocessor  5  controlling the controllers  1  connected to the transfer destination transfer path, and data is transferred via such selected transfer path. 
     Accordingly, in a storage apparatus of a multi controller architecture comprising a plurality of controllers  1 , if a plurality of paths can be selected for the data transfer from the transfer source to the transfer destination, the data transfer path can be appropriately selected, and it is thereby possible to efficiently access the intended data. 
     The present invention can be broadly applied to storage apparatuses configured from controllers comprising individual I/O units for communicating with the cache memory, the host, and the HDD drive.