Abstract:
A method and apparatus for managing a loop network, the loop network ( 200 ) including at least one loop ( 206, 208 ), a plurality of devices ( 210 ) connected to the at least one loop ( 206, 208 ) via ports ( 211, 212 ), wherein at least two of the devices are initiators ( 207, 209 ). The method includes each initiator ( 207, 209 ) sending a frame to all other initiators ( 207, 209 ) in the loop network ( 200 ) identifying any ports ( 211, 212 ) which should not be used. Each initiator ( 207, 209 ) merges the information from all other initiators ( 207, 209 ) with its own information identifying any ports ( 211, 212 ) which should not be used resulting in all the initiators ( 207, 209 ) generating a single list of ports ( 211, 212 ) to be used which is consistent across all the initiators ( 207, 209 ). Each initiator ( 207, 209 ) applies an algorithm ( 300 ) to determine a common set of ports ( 211, 212 ) to be used by all the initiators ( 207, 209 ) and to balance port accesses across the loop network ( 200 ).

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
   This application is a continuation of U.S. patent application Ser. No. 11/735,568, now U.S. Pat. No. 7,400,574 B2, filed Apr. 16, 2007, which claims priority from U.S. patent application Ser. No. 10/150,580, now U.S. Pat. No. 7,269,131 B2, filed May 17, 2002, which claims priority from U.K. Patent Application Serial No. GB 0119070.1, filed on Aug. 6, 2001. 

   FIELD OF INVENTION 
   This invention relates to a method and apparatus for managing a loop network. In particular, the invention relates to managing Fibre Channel Arbitrated Loops. The invention could equally apply to managing other unidirectional loops, for example, Token Ring networks, FDDI (Fibre Data Distributed Interfaces), etc 
   BACKGROUND OF THE INVENTION 
   Fibre Channel Arbitrated Loop (FC-AL) architecture is a member of the Fibre Channel family of ANSI standard protocols. FC-AL is typically used for connecting together computer peripherals, in particular disk drives. The FC-AL architecture is described in NCITS working draft proposal, American National Standard for Information Technology “Fibre Channel Arbitrated Loop (FC-AL-2) Revision 7.0”, 1 Apr. 1999.
 
Electronic data systems can be interconnected using network communication systems. Area-wide networks and channels are two technologies that have been developed for computer network architectures. Area-wide networks (e.g. LANs and WANs) offer flexibility and relatively large distance capabilities. Channels, such as the Small Computer System Interface (SCSI), have been developed for high performance and reliability. Channels typically use dedicated short-distance connections between computers or between computers and peripherals.
 
   Fibre Channel technology has been developed from optical point-to-point communication of two systems or a system and a subsystem. It has evolved to include electronic (non-optical) implementations and has the ability to connect many devices, including disk drives, in a relatively low-cost manner. This addition to the Fibre Channel specifications is called Fibre Channel Arbitrated Loop (FC-AL). 
   Fibre Channel technology consists of an integrated set of standards that defines new protocols for flexible information transfer using several interconnection topologies. Fibre Channel technology can be used to connect large amounts of disk storage to a server or cluster of servers. Compared to Small Computer Systems Interface (SCSI), Fibre Channel technology supports greater performance, scalability, availability, and distance for attaching storage systems to network servers. 
   Fibre Channel Arbitrated Loop (FC-AL) is a loop architecture as opposed to a bus architecture like SCSI. FC-AL is a serial interface, where data and control signals pass along a single path rather than moving in parallel across multiple conductors as is the case with SCSI. Serial interfaces have many advantages including: increased reliability due to point-to-point use in communications; dual-porting capability, so data can be transferred over two independent data paths, enhancing speed and reliability; and simplified cabling and increased connectivity which are important in multi-drive environments. As a direct disk attachment interface, FC-AL has greatly enhanced I/O performance. 
   Devices are connected to a FC-AL using hardware which is termed a “port”. A device which has connections for two loops has two ports or is “dual-ported”. 
   The operation of FC-AL involves a number of ports connected such that each port&#39;s transmitter is connected to the next port&#39;s receiver, and so on, forming a loop. Each port&#39;s receiver has an elasticity buffer that captures the incoming FC-AL frame or words and is then used to regenerate the FC-AL word as it is re-transmitted. This buffer exists to deal with slight clocking variations that occur. Each port receives a word, and then transmits that word to the next port, unless the port itself is the destination of that word, in which case it is consumed. The nature of FC-AL is therefore such that each intermediate port between the originating port and the destination port gets to ‘see’ each word as it passes around the FC-AL loop. 
   FC-AL architecture may be in the form of a single loop. Often two independent loops are used to connect the same devices in the form of dual loops. The aim of these loops is to provide an alternative path to devices on a loop should one loop fail. A single fault should not cause both loops to fail simultaneously. More than two loops can also be used. 
   FC-AL devices typically have two sets of connections allowing them to be attached to two FC-ALs. Thus, in a typical configuration, two independent loops exist and each device is physically connected to both loops. When the system is working optimally, there are two possible loops that can be used to access any dual-ported device. 
   A FC-AL can incorporate bypass circuits with the aim of making the FC-AL interface sufficiently robust to permit devices to be removed from the loop without interrupting throughput and sacrificing data integrity. If a disk drive fails, port bypass circuits attempt to route around the problem so all disk drives on the loop remain accessible. Without port bypass circuits a fault in any device will break the loop. 
   In dual loops, port bypass circuits are provided for each loop and these provide additional protection against faults. A port can be bypassed on one loop while remaining active on the dual loop. 
   A typical FC-AL may have one or two host bus adapters (HBA) and a set of six or so disk drive enclosures or drawers, each of which may contain a set of ten to sixteen disk drives. There is a physical cable connection between each enclosure and the HBA in the FC-AL. Also, there is a connection internal to the enclosure or drawer, between the cable connector and each disk drive in the enclosure or drawer, as well as other components within the enclosure or drawer, e.g. SES device (SCSI Enclosure Services node) or other enclosure services devices. 
   Components in a loop can be categorised as “initiators” or “targets”, or both depending on their function in the loop. For example, a host bus adapter is an initiator and a disk drive is a target. Initiators can arbitrate for a communication path in the loop and can choose a target. A target can request the transfer of a command, data, status, or other information to or from the initiator. 
   If there is a single initiator in a loop, the initiator will login with all the targets in the loop. Targets may accept or reject this login attempt. At any later stage a target can log out with any logged in initiator. In a multi-initiator environment, an initiator operates as both a sender and recipient login attempts. 
   When target devices such as disk drives are provided on dual loops with a port on each loop, such devices do not necessarily cope with being accessed by the same or by different initiators on both ports. This may even cause data transfer rates to be reduced because of the overhead in switching between ports. It is also possible that there are ordering issues to worry about. 
   There may be other advantages in only accessing target devices via one port, such as being able to bypass redundant ports. Therefore, accessing multi-ported targets via only one port is proposed. 
   Devices may not accept or correctly complete log in procedures and this is a problem if devices do not present the same view to each initiator. 
   Providing multiple initiators in a loop network should increase performance levels and achieve a higher degree of connectivity. Therefore, management of a loop network with more than one initiator is proposed. Accesses to dual-ported devices in a loop network should be balanced to evenly distribute the accesses to devices over both of the dual ports. This needs to be coordinated with the proposal to access devices through only one port for all initiators. 
   It is an aim of the present invention to provide management of a loop network with more than one initiator to provide a consistent view of the devices in the loop network and to balance the accesses to the devices. 
   DISCLOSURE OF THE INVENTION 
   According to a first aspect of the present invention there is provided a method for managing a loop network, the loop network including at least one loop, a plurality of devices connected to the at least one loop via ports, wherein at least two of the devices are initiators, the method including: each initiator sending a frame to all other initiators in the loop network identifying any ports which should not be used; each initiator merging the information from all other initiators with its own information identifying any ports which should not be used; resulting in all the initiators generating a single list of ports to be used which is consistent across all the initiators. 
   The frame may either list all the ports which can be used or all the ports which cannot be used. 
   A port may be identified as not to be used if it does not report a node name and a port name to an initiator during initialisation of a loop. A port may be identified as not to be used if it does not log on to an initiator. 
   Preferably, each initiator applies an algorithm to determine a common set of ports to be used by all the initiators and to balance port accesses across the loop network. The algorithm may select a least utilised route to a device. Accesses to ports may be sorted by a node name as a first key and a port name as a second key. The algorithm may count ports providing the only access to devices on each loop and may determine a balance value of accesses across the loops, the algorithm may seek to reduce the balance value towards zero. 
   The loop network may preferably be a Fibre Channel Arbitrated Loop (FC-AL) network with dual loops and at least some devices having ports on each of the dual loops. 
   According to a second aspect of the present invention there is provided a loop network including at least one loop, a plurality of devices connected to the at least one loop via ports, wherein at least two of the devices are initiators, the loop network also including: means for each initiator to send a frame to all other initiators in the loop network identifying any ports which should not be used; means for merging in each initiator the information from all other initiators with the initiator&#39;s own information identifying any ports which should not be used; resulting in a single list of ports to be used, the list being consistent across all the initiators. 
   At least one of the devices may be a dual ported device with a port on a first loop and a port on a second loop. The initiators may be host bus adapters and the devices may be disk drives. 
   The loop network may preferably be a Fibre Channel Arbitrated Loop (FC-AL) with dual loops. 
   According to a third aspect of the present invention there is provided a computer program product stored on a computer readable storage medium comprising computer readable program code means for managing a loop network, the loop network having at least one loop, a plurality of devices connected to the at least one loop via ports, wherein at least two of the devices are initiators, the program code means performing the steps of: each initiator sending a frame to all other initiators in the loop network identifying any ports which should not be used; each initiator merging the information from all other initiators with its own information identifying any ports which should not be used; resulting in all the initiators generating a single list of ports to be used which is consistent across all the initiators. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Embodiments of the invention are now described, by means of examples only, with reference to the accompanying drawings in which: 
       FIG. 1  is a diagram of a dual loop network in which the teaching of the present invention may be practiced; 
       FIGS. 2A and 2B  are diagrams of a dual loop network in accordance with the present invention; and 
       FIG. 3  is a flow diagram of an algorithm in accordance with the present invention. 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   A loop network system with a plurality of serially connected ports in the form of a Fibre Channel Arbitrated Loop (FC-AL) is described for connecting together computer peripherals, in particular disk drives. The described embodiments are given in the context of FC-AL architecture although the described method and apparatus could be applied to any unidirectional loop network. 
   Referring to  FIG. 1 , an exemplary loop network  100  is shown in the form of a Fibre Channel Arbitrated Loop with two host bus adapters  102 ,  104 .  FIG. 1  shows one form of a loop network on which the present invention may be practiced. However, not all the components of the loop network  100  of  FIG. 1  are essential for the operation of the present invention. 
   The loop network  100  in the shown embodiment has two enclosures  106 ,  108 . Each enclosure in this embodiment has three disk drives  120  although in practice there are usually 10 or more disk drives in an enclosure. Dual loops  116 ,  118  each connect the components in the loop network  100 . A first loop  116  is shown along the top of the loop network  100  in the diagram and a second loop  118  is shown along the bottom of the loop network  100  in the diagram. 
   The adapters  102 ,  104  have external connectors  110  for cables  114  connecting each loop  116 ,  118  from the adapters  102 ,  104  to external connectors  112  of the enclosures  106 ,  108 . Cables  114  also connect the two enclosures  106 ,  108  such that each loop  116 ,  118  passes from one enclosure  106  to the next enclosure  108 . 
   Each loop  116 ,  118  passes from the first adapter  102  via an adapter external connector  110 , a cable  114  and an enclosure external connector  112  to the first enclosure  106 . In the first enclosure  106  of the exemplary loop network  100 , each loop  116 ,  118  passes through its own SES (SCSI Enclosure Services) device or controller  122 ,  124  and then through each of the disk drives  120  in turn. The two loops  116 ,  118  both pass through the same shared disk drives  120 . Each loop  116 ,  118  then leaves the first enclosure via an enclosure external connector  112  and passes through a cable  114  to a second enclosure  108  which it enters via an enclosure external connector  112 . The second enclosure  108  has the same set of components as the first enclosure  106 . Each loop  116 ,  118 , after passing through the second enclosure  108  is connected to the second adapter  104  via enclosure external connectors  112 , cables  114  and adapter external connectors  110 . 
   In each enclosure  106 ,  108 , a loop  116  enters from an external connector  112  and is routed through each of the disk drives  120  and an SES device  122 ,  126 . Each disk drive  120  or SES device  122 ,  126  has a bypass circuit to enable it to be bypassed by the loop, if required. The disk drives  120  are examples of dual port devices in that they are common to both the loops  116 ,  118  of the loop network  100 . 
   An SES device  122 ,  124  is provided on each loop  116 ,  118  in each enclosure and the two SES devices  122 ,  124  are connected together through the enclosure&#39;s backplane. One SES device can be used to control the other SES device. An SES device manages an enclosure and provides a point of control for each enclosure. It can monitor parameters such as power and cooling and obtain information as to which slots for disk drives are occupied. It accepts a limited set of SCSI commands. SES devices can be used to instruct a bypass of a disk drive and to check which disk drives are bypassed. 
   In the embodiment shown in  FIG. 1 , a dual loop network  100  is shown by way of example, with two enclosures  106 ,  108  each with three disk drives  120  and two SES controllers  122 ,  124 , one for each loop. Typical loop networks may have one or two host bus adapters and a set of six or so disk drive enclosures each of which may typically contain a set of ten to sixteen disk drives. 
   All devices in the loop  100 , including host bus adapters  102 ,  104 , disk drives  120  and any enclosure controllers  122 ,  124  have hardware connections to a loop  106 ,  108  referred to as ports. Each port has a receiver and a transmitter. The ports are connected such that each port&#39;s transmitter is connected to the next port&#39;s receiver, and so on, forming the loop  106 ,  108 . Each port&#39;s receiver has an elasticity buffer that captures the incoming FC-AL frame and is then used to regenerate the FC-AL frame as it is re-transmitted. 
   Referring to  FIG. 2A , a dual loop network  200  is shown in a simplified form with two initiators in the form of two host bus adapters A  207  and B  209  and five targets in the form of five devices  210  which are individually referred to as V  201 , W  202 , X  203 , Y  204  and Z  205 . The devices  210  may be disk drives or other loop components. 
   The loop network  200  has two loops  206 ,  208  and each of the devices  210  in the loop network  200  is dual ported in that each device  210  has a port  211 ,  212  on each loop  206 ,  208 . Each port  211  on the first loop  206  will be referred to as port  1  and each port  212  on the second loop  208  will be referred to as port  2 . 
   Devices  210  may also be included which are single ported with a single port on only one of the loops  206 ,  208 . 
   During initialisation of a loop, a Loop Initialisation Procedure (LIP) allows each port  211 ,  212  to obtain an Arbitrated Loop Physical Address (AL_PA) that is unique within the loop  206 ,  208  for that port. This effectively uniquely identifies each port  211 , 212  in a loop  206 ,  208 . 
   The loop initialisation involves one port winning as Loop Initialisation Master (LIM). The LIM port manages the initialisation procedure. Disk drives  210  can indicate that they do not wish to be the LIM. The Arbitrated Loop Physical Addresses (AL_PAs) are then allocated to each of the ports  211 ,  212  in the loop  206 ,  208 . The LIM sends a frame around the loop  206 ,  208  with bits corresponding to AL_PAs. Each port  211 ,  212  finds the relevant bit for its AL_PA and changes the bit from “0” to “1” indicating that the AL_PA is not available for subsequent ports. The AL_PAs can be defined by previous addresses, assigned hardware addresses or software addresses. If there are multiple enclosures, each address indicates the enclosure and the device within the enclosure ensuring that each port  211 ,  212  in a loop  206 ,  208  has a unique address. 
   The initialisation procedure can also send special frames around the loop  206 ,  208  called the Loop Initialisation Report Position (LIRP) frame and the Loop Initialisation Loop Position (LILP) frame which detail the topology of the loop as seen by the Loop Initialisation Master (LIM). This involves each port  211 ,  212  indicating in a frame its AL_PA in the order that it is physically situated in the loop. This frame contains each port&#39;s AL_PA in turn as seen by the LIM for the whole of the loop and is broadcast around the loop. 
   The loop initialisation allows a host bus adapter  207 ,  209  to know where each port  211 ,  212  is in relation to the adapter  207 ,  209 . The host bus adapter  207 ,  209  will identify all the devices in a loop including, for example, SES devices as distinct from disk drives and may also determine from an SES devices details of the ports housed within that SES device&#39;s enclosure. 
   Each port  211 ,  212  in a loop network  200  has a port identifier called a “World Wide Port Name” (WWPN). Each node on a loop  206 ,  208  in the form of devices  210  or host bus adapters  207 ,  209  also has a World Wide Node Name (WWNN). These are referred to as Node Names and Port Names. To ensure that the WWPN and WWNN are unique they may contain, for example, a unique identifier of the manufacturer of the device including the port and the manufacturer&#39;s serial number of the device. The WWPN is too long (usually 64 bits) to be used for source and destination addresses transmitted over the network and therefore the AL_PA is used as a temporary address that is unique to the configuration of the network at any given time. 
   A log in process is instigated by an initiator after the loop initialisation has completed. An initiator issues PDISC or ADISC frames to all observed AL_PAs, to ‘discover’ information about the AL_PAs. This identifies targets that the initiators should log in with. PDISC may be accepted by an ACC frame or rejected with an LS_RJT frame. Next, the initiator will attempt to Port Log In (PLOGI) with all the identified targets, i.e. all targets where there was a successful PDISC or ADISC and the device was identified as being a target. This may be accepted with an ACC frame or rejected with an LS_RJT frame. Once accepted, a port log out (LOGO) may occur at any stage thereafter. After the PLOGI, a Process Log In (PRLI) occurs to establish a SCSI or similar connection. Again there is an ACC frame or an LS_RJT frame. Once processed logged in then a Process Log Out (PRLO) may occur at any time thereafter. In some environments, targets keep track of open exchanges with initiators. In the described environment, after every loop initialisation, targets are required to validate the log-ins and if anything has changed then a LOGO is issued, forcing the initiator to start the log in process again. When there is more than one initiator in a loop, each initiator must send PLOGI frames to each of the targets. A target may, having already logged in, decide to log out with some but not all initiators. 
   In the described embodiment, the initiators in the form of host bus adapters  207 ,  209  communicate with each other to obtain a common set of devices which are available and specify which port is to be used for a device with more than one port. 
   The information sent between initiators can be as follows: Each host bus adapter  207 ,  209  sends a Vendor Unique SCSI command to each other host bus adapter  207 ,  209 . The reply contains that host bus adapter&#39;s relevant data. For example: 
   Mapping via Loop IDs instead of AL_PAs (loop IDs have a one to one mapping with AL_PAs and loop IDs are in the range 0 to 126 inclusive). Table of one entry per loop ID, the first entry being for loop ID  0  and the last being for loop ID  126 . 0 =OK1=NotOK. Each host bus adapter  207 ,  209  sends its own table with its own relevant data. Each host bus adapter then logically ORs each received entry in the table with each entry in its own table (the one it sent), for each loop ID, keeping a separate table as the result. This result is the merged view and is the same on every host bus adapter  207 ,  209 . Therefore, each host bus adapter  207 ,  209  knows which ports are OK and which ports are not OK. 
   In environments with multi-ported host bus adapters, this communication of tables must be carried out for each loop  206 ,  207 . 
   Referring to  FIG. 2A , at initialisation of a first loop  206 , the initialisation procedure obtains and transmits around the loop  206  a Loop Initialisation Loop Position (LILP) frame as previously described which contains the AL_PAs of each of the five ports  211  on loop  206  in the order that they are physically located in the loop  206 . The Node Names and Port Names are determined for all of the AL_PAs on the first loop  206  except, in this example, for port  1  of device Y  204  which has not reported its Node Name and is therefore eliminated. 
   Initialisation is also carried out for the second loop  208  in the loop network  200  and the LILP frame with the AL_PAs of the five ports  212  on the second loop  208  is transmitted around the loop  208 . The Node Names and Port Names are determined for all the AL_PAs of the five ports  212  on the second loop  208 . 
   Any AL_PAs that do not report their Node Name must be eliminated and a record of the elimination kept. This is done by any initiators, i.e. the host bus adapters  207 ,  209 , that wish to share access to the same devices  210  in the loop network  200 . 
   After the loop initialisation procedure has been completed, a log in procedure is carried out. At log in, the host bus adapter A  207  which is an initiator sends a PLOGI frame to each of the targets in the form of the five devices  210 . The second host bus adapter B  209  also sends a PLOGI frame to each of the five devices  210 . It is possible that the response frame sent by one device, for example device W  202 , is different in response to each of the two PLOGI frames sent by the two host bus adapters  207 ,  209 . An ACC frame may be sent to host bus adapter A  207  and a LS_RJT frame may be sent to host bus adapter B  209 . This results in an inconsistent picture being obtained by the host bus adapters  207 ,  209  of the availability of the device W  202 . 
   The record of AL_PAs to be ignored due to their elimination or failure to log in is then communicated to all other initiators. The records are then merged at each initiator to result in a common set of AL_PAs to be used by the initiators with each AL_PA having a Node Name and a Port Name. 
   Host bus adapter A sends a frame to host bus adapter B indicating that device Y  204  is to be ignored as it did not report its Node Name. Host bus adapter B sends a frame to host bus adapter A indicating that port  1  of device W  202  is not logged on and is therefore to be ignored. The host bus adapters  207 ,  209  combine the information from the other host bus adapter  207 ,  209  into a single list of AL_PAs which can be used by the host bus adapters  207 ,  209  merging any devices that have the same Node Name into one entry, but keeping both sets of data. 
   A host bus adapter  207 ,  209  which is aware of an on-going problem with a particular device  210  can elect not to use that device and this is communicated to the other host bus adapters  207 ,  209  and the ports of the particular device will not be present in the single list of AL_PAs which is then used by all host bus adapters  207 ,  209 . 
     FIG. 2B  shows the arrangement of active ports  211 ,  212  in the resultant single list of AL_PAs as determined by the communication between the host bus adapters  207 ,  209  in the loop network  200  of  FIG. 2A . Devices V  201 , X  203  and Z  205  have both ports  1   211  and  2   212  active on the first and second loops  206 ,  208 . Device W  202  only has port  2  on the second loop  208  active. Device Y  204  has neither ports  1  nor  2  active. 
   In addition, the described embodiment provides an algorithm which is applied by both host bus adapters  207 ,  209  to determine which port  211 ,  212  to use for devices  210  which have two available ports  211 ,  212 . The result is a set of devices  210  which are seen by both host bus adapters  207 ,  209  in the same order with the same port of a device  210  defined for use by both host bus adapters  207 ,  209 . Both host bus adapters  207 ,  209  will communicate with a device  210  via the same port and therefore the same loop  206 ,  208  is used by both host bus adapters  207 ,  209  for that device  210 . Different loops  206 ,  208  can be used for other devices  210 . The second port of a dual-ported device  210  which is not defined as the communicating port, is still available as a port should the communicating port be bypassed. 
   At the end of the algorithm, the choice of which port to use will be the same on each host port adapter  207 ,  209  and the use of ports will be spread as evenly as possible across the ports  211 ,  212  of the dual ported devices  210 . 
   The algorithm includes the following steps: 
   
       
       
         
           Count the number of single port access devices only accessible by a port on the first loop. 
           Count the number of single port access devices only accessible by a port on the second loop. 
           Determine a starting balance of device access. 
           Take each Node Name in order with the lowest Node Name first that has both AL_PAs and therefore both ports active and choose the AL_PA for the port that moves the balance towards zero. In other words, use the least utilised loop to the device where the choice exists. 
           If the balance is already zero choose the AL_PA for the port with the lowest Port Name. 
         
       
     
  
     FIG. 3  is a flow diagram illustrating the algorithm  300  of port selection. The first steps  302 ,  304  are to count the number of single port access devices on each of the first and second loops. 
   The next step  306  is to determine the balance of single port access devices across the loops. Each device with two active ports is then taken in turn  308  starting with the lowest Node Name. A decision  310  is then taken as to whether or not the balance determined at step  306  is zero. If the balance is zero, the port is chosen  312  with the lowest Port Name. If the balance is not zero, the port is chosen  314  which moves the balance towards zero. 
   A decision  316  is then taken as to whether or not there are more devices with two active ports. If there are more devices with two active ports, a loop  318  returns to step  308  and takes the next lowest Node Name. If there are no more devices with two active ports the algorithm is finished  320 . 
   If there are more than two loops, this can also be accommodated by counting the number of single port access devices on any additional loops and attempting to balance the port access between all the loops. 
   Examples are now given of the described method of balancing port accesses. 
   In Table 1, there are five devices  210  as shown in  FIG. 2A , each device  210  having a port  211  on a first loop  206  and a port  212  on a second loop  208 . All devices  210  have both ports  211 ,  212  active. The algorithm will determine that there is no bias between the loops  206 ,  208  and as a result will evenly distribute the ports chosen to be used by the host bus adapters  207 ,  209  for each device  210 . The chosen ports are shown underlined in Table 1. 
                                               TABLE 1                           PORT ON LOOP1     1     1     1     1     1             DEVICE   V   W   X   Y   Z           PORT ON LOOP2   2     2     2     2     2                        
Table 2 illustrates the example shown in  FIG. 2B  in which device W  202  has only one port (port  2 ) active and device Y  204  is not used. The algorithm determines that device W  202  is a single port access device and there is a bias to loop  2 . The algorithm then determines that device V  201  should be accessed by port  1  as this brings the balance of the loops to zero. The port of device X  203  is chosen by the lowest Port Name which is port  1  in the example. The balance is then biased to loop  1  which has two ports whereas loop  2  has only one port. Therefore, the algorithm chooses port  2  for device Z  205  which again brings the balance to zero.
 
                                               TABLE 2                           PORT ON LOOP1     1           1           1             DEVICE   V   W   X   Y   Z           PORT ON LOOP2   2     2     2         2                          
Another example is shown in Table 3 in which device X  203  has only one port access which is port  1  and device Z  205  has only one port access which is port  2 . Therefore, the balance is zero. The remaining devices have the access ports determined to distribute the accesses as evenly as possible across loops  1  and  2 .
 
                                               TABLE 3                           PORT ON LOOP1     1     1     1       1                 DEVICE   V   W   X   Y   Z           PORT ON LOOP2   2     2         2     2                          
Devices may be included on loops within a loop network which are only single ported and these devices are counted in the algorithm as devices with single port access. No distinction is made as to whether there is a port which is not being used as it did not respond to a log in command or whether there is only one port. The balance of use of ports between loops in a loop network results in performance improvement by making use of the full bandwidth of the loops. The method described herein is typically implemented as a computer program product, comprising a set of program instructions for controlling a computer or similar device. These instructions can be supplied preloaded into a system or recorded on a storage medium such as a CD-ROM, or made available for downloading over a network such as the Internet or a mobile telephone network.
 
Improvements and modifications can be made to the foregoing without departing from the scope of the present invention.