Abstract:
An initiator node for a storage area network used with storage nodes having command queue capability maintains a current queue depth, and a maximum queue depth associated with each storage node. It initializing the maximum queue dependent on the type of the storage nodes it sees, and limits the number of commands queued to each storage node to the current queue depth associated with the storage node. It dynamically adjusts the current queue depth for each storage node based upon queue refusals generated by the storage nodes and the maximum queue depth associated with the storage nodes.

Description:
RELATED APPLICATIONS 
     This application is related to a simultaneously filed application entitled “Data Structure and Method for Tracking Network Topology in a Fibre Channel Port Driver”, identified as Ser. No. 09/642,800. This application is also related to a simultaneously filed application entitled “Method for Recovery of Paths Between Storage Area Network Nodes with Probationary Period and Desperation Repair”, identified as Ser. No. 09/642,801. 
    
    
     FIELD OF THE INVENTION 
     The invention relates to the field of computer networking. In particular, the invention relates to the field of nodes and switches for Fibre Channel storage-area networks. With more particularity, the invention relates to data structures for use by nodes in dynamically tracking network resources and configuration so as to properly encode and route packets over the network. 
     Nature of the Problem 
     Most modern computer networks, including switched and arbitrated-loop fibre-channel networks, are packet oriented. In these networks, data transmitted between machines is divided into chunks of size no greater than a predetermined maximum. Each chunk is typically packaged with a header and a trailer into a packet for transmission. In Fibre-Channel networks, packets are known as Frames. 
     A Fibre-Channel network having at least one switch is a switched Fibre-Channel fabric. A Fibre-Channel switch is a routing device generally capable of receiving frames, storing them, decoding destination information from headers, and forwarding them to their destination or another switch further along a path toward their destination. A network interface for connection of a machine to a Fibre Channel fabric is known as an N_port, and a machine attached to a Fibre-Channel network is known as a node. Nodes may be computers, or may be storage devices such as RAID systems. An NL_port is an N_port that supports additional arbitration required so that it may be connected either to a Fibre Channel Fabric or to a Fibre Channel Arbitrated Loop, and an L_port is a network interface for connecting a node to a Fibre Channel Arbitrated Loop. 
     A device including an N_port, L_port, or an NL_port together with hardware for high-speed connection to a machine is a fibre channel host bus adapter (physical HBA). For example, a physical HBA may comprise a printed circuit card having one or more NL_ports communicating through a PCI bus interface to an edge connector for connection to a PCI bus of a machine. A physical HBA may, but need not, also incorporate a processor for controlling its ports and its connection to the machine. 
     A Fibre Channel Switched Fabric may connect one or more Fibre Channel Arbitrated Loops. 
     In a switched fibre channel fabric, there may be more than one possible path, or sequence of links, loops, switches, routers, etc. that may be traversed by a frame, between two nodes. Multiple paths may be intentional, providing extra capacity or redundancy to protect against switch, node, or line failures, or may be unintentional consequences of network topology. 
     Multiple paths between two nodes may also be provided through multiple fibre channel arbitrated loops. For example, an initiator node may have two NL_ports, one connected to each of two fibre channel arbitrated loops. If each of these loops connects to an NL_port of a target node, then multiple paths from the initiator to the target node exist. This could provide redundancy should a failure occur on one of the arbitrated loops. 
     Fibre Channel storage-area-network (SAN) nodes and switches, especially network nodes having multiple ports, must keep track of a variety of information about the network and resources available over the network. This information is used by each node to format and properly route frames onto and over the network. 
     Typically, a program references files in storage by passing a command block to an operating system. At this level, the command block may reference files by name and device name, or by handle for files already opened. The operating system passes block I/O requests derived from the command block to an appropriate driver. The command block includes a command field, a file name field, and a drive name or number field in addition to other fields. The block I/O requests derived from the command block typically include fields for device identification, command, and a count of logical blocks to be operated upon, and may include pointers to data. 
     The device driver typically converts the block I/O requests into a sequence of one or more device level commands. Once the driver attempts to execute each command, it returns to the operating system a response having status information, and other information possibly including data read from storage. Information from the response may then be transferred to the program. 
     When a command block references storage accessible over a Fibre Channel network, the driver must encapsulate the device level commands into one or more command frames, and for write operations one or more data frames. The driver may use the network information to determine header information and routing for the one or more fibre channel network frames, or packets, that implement the command. 
     Typically, the operating system passes block I/O requests to the device driver with a device tag identifying the specific device intended to perform the desired operation. This tag may comprise a referenced device name, handle, or SCSI nexus, where a SCSI nexus includes bus identity, target device number, and logical unit number. 
     In particular, the driver must translate device tags, into a multilevel address as required to reach the indicated device. In a Fibre Channel context, that multilevel address field must include valid Destination Identification (D_ID) fields and routing information field for each frame. A command frame may specify a specific logical unit. There may also be additional destination address fields such as those in association headers; association headers permit addressing multiple devices or processes through a single fibre channel port. 
     The driver, especially a multiport driver servicing multiple ports, must therefore determine an appropriate destination and routing for each frame required to implement a command, and transmit those frame over a port appropriate for that routing. It is desirable that these translations and assignments be done quickly and accurately using network topology information maintained in a local topology database. 
     The command and data frames transmitted by a node in implementing an I/O command, together with any frames transmitted by another node in response to those command and data frames, is known as an exchange. 
     As nodes, switches, and links are added to or removed from the network, any local topology database must be updated to reflect valid devices on the network, and valid paths through the network to those devices. Nodes also may determine one or more paths of the valid paths to a given device to be an “active” path. An active path is a path that may be used for exchanges. 
     The Fibre Channel specifications define Class  1  and Class  4  services to be virtual-circuit, or connection, based services between pairs of nodes. Packets of a given sequence in these services must arrive in-order with respect to other packets of the same sequence. The specifications presume that frames transiting between nodes of each pair follow a virtual circuit between the nodes—all following the same path through the network from node to node and arriving in-order. 
     Links, loops, and switches of a network may fail. 
     Fibre channel networks may provide more than one path between a pair of nodes. Multiple, redundant, paths provide redundancy to allow continued communications between a pair of nodes should one or sometimes more, but not all, links, loops, or switches, and therefore paths through the network, fail. 
     Some existing fibre-channel systems can recognize failure of a path, switching traffic between a pair of nodes to an alternate path if one exists. This is known as failover of a path. Unfortunately, switching Class  1  and Class  4  fibre channel connections, and associated network traffic, to alternate paths is known to cause considerable delay to ongoing exchanges—on the order of seconds. These delays are caused in part by the need to ensure that Class  1  and Class  4  frames arrive at their destination in-order; time is allowed for flushing in-transit frames before frames are transmitted on the alternate path. 
     Further delay may occur if redundant controllers having access to the target device must transfer ownership of a logical unit as part of a path failover. 
     While a link or switch of a fibre channel network may fail, it may also be repaired. It is desirable that paths associated with repaired links, loops, or switches be dynamically returned to use to restore network redundancy and capacity. 
     Loops, links or switches may develop intermittent problems, where they may alternate between repaired and failed conditions. Considerable delay of network traffic can be encountered if paths through an intermittent device are used, since the delays associated with switching traffic to repaired paths, thence back to alternate paths upon the next failure may be cumulative. Paths may also be reported as repaired but actually have higher error rates. High error rates cause delay as frames are retransmitted as errors are detected; high error rates may also cause a path to be considered to have failed. It is therefore desirable to control use of repaired paths such that extended delay does not occur. 
     It is known that many nodes, including RAID storage subsystems, have the ability to queue multiple commands in a command queue for subsequent execution. For example, a RAID system may queue several read or write commands, received from one or more machines. Once queued, these commands are executed from the queue to or from cache, or to or from disk, in an order depending on availability of data in cache, disk availability and disk rotation. With proper interlocks, execution may often be in an order different from that in which the commands were received. Commands placed in a command queue of a node by an initiator node and not yet completed are pending commands. For this purpose, commands that are aborted, including those aborted for timeouts, are considered completed. 
     Commands that may be queued in these devices may include commands from multiple processes, or threads, running on a single node having one or more processors. For example, a transaction-processing system may have several processes running, each process requiring access to a different record of a database on a RAID system, all requesting access to the database at about the same time. Each process may create read, write, lock, or unlock commands for the database. Commands may also be queued from multiple nodes, where each node is running one or more processes or threads that require access to the device. Queuing and execution of each of these commands requires an exchange of frames between the machine and the device. 
     The maximum number, or queue depth, of commands that may be pending in any one device at any one time is finite and characteristic of the device. Each node may, but need not, know the queue depth of the device nodes it is attempting to use, and may throttle its commands appropriately. Typically, each node originating commands and transmitting them to the command queue of a device is unaware of the commands originated by other nodes and assumes it has the full queue depth of the device available to it. 
     If a node assumes that the full queue depth of a device is available to it, the queue depth of the device may be exceeded by commands from multiple nodes. When this happens, the excess commands are refused by the device and must be retransmitted later; the need for delay and retransmission degrades overall system performance. Conversely, if a node assumes that a device has only a small queue depth available to it, commands may be throttled unduly, such that overall system performance is impaired. It is desirable to adjust the maximum number of commands from each node transmitted to a device command queue in light of the command traffic to that device from other nodes of the network. 
     Manually tuned allocations of queue depth available to individual initiator nodes of a storage area network is possible, but is laborious and error prone. Further, it is known that the load on a system may vary from time to time throughout a day; it is desirable to change queue depth allocations as load changes for optimum performance. 
     Solution to the Problem 
     A driver for one or more Fibre Channel N_ports, L_ports, or NL_ports maintains a local network topology database. This database includes linked lists of host bus adapter (HBA) port records, target node records, and device records. 
     HBA port records correspond to individual physical N_ports, L_ports or NL_ports and are linked to node records through linked lists of node links. Similarly, node records are linked to device records through linked lists of device links, and devices are linked to node links through linked lists of path links. Additional HBA port records may be added as ports are discovered or as hot-pluggable ports are added to the system. 
     Each node record has information pertinent to a-particular port of a target node of the fibre channel network. A target node may have more than one port, and may therefore have more than one node record. Similarly, each device record has information pertinent to a particular device that can be accessed from one or more nodes. Each path link has information, including status information, pertinent to a particular path through a fibre channel fabric or arbitrated loop between a physical HBA port and a physical node or device of the network. 
     In order to reduce command queue overflow errors, a command queue depth variable is maintained for each node record. Initially, this is set to match a maximum queue depth determined from the type of node represented by the record. This queue depth variable is adjusted downwardly when queue overflow errors occur. Queue depth is also adjusted downwardly when other initiators are first detected on the fibre channel network, so as to allow those initiators to access the node. The queue depth is adjusted upwardly when a predetermined period elapses without queue overflow errors. 
     Nodes that have successfully accessed a target node without queue overflow errors automatically try successively higher values of queue depth until they find a depth for optimum performance. Further, queue depth is automatically reallocated to active nodes as other nodes become idle. 
     In order to limit dispatch of frames over intermittent or problem paths, with attendant errors and retries, freshly repaired and problem links and their associated paths are given probationary status. Frames are preferentially dispatched over non-probationary paths if any exist, if no non-probationary paths exist a probationary path may, in desperation, be returned to normal status. If no probationary paths exist, failed links are tested to determine if they have been repaired; a repaired path may then be activated bypassing the probationary state. 
     Probationary paths that function without error for a preset period of time are returned to normal status, where they may once again be subjected to normal use. Probationary paths that record excessive error counts are returned to failed status, unless these errors occur within a few seconds of another device logging into a fibre channel fabric; this prevents marking paths failed solely because of transient errors induced by nodes bringing their laser diodes online. 
     The foregoing and other features, utilities and advantages of the invention will be apparent from the following more particular description of a preferred embodiment of the invention as illustrated in the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is an illustration of a Switched Fibre Channel Fabric having several nodes with multiple paths between, and showing drivers and topology databases; 
         FIG. 2 , a block diagram of a machine that may run a driver and topology database of the present invention and serve as a network node; 
         FIG. 3 , a block diagram illustrating where a local network information database resides in a system running on a network node; 
         FIG. 4 , an illustration of a data structure for a local network information database as maintained on a node of the present invention; 
         FIG. 5 , an illustration of a fibre channel frame, or packet, header, as is known in the art; 
         FIG. 6 , a diagram of a node record of the topology database; 
         FIG. 7 , a diagram of a device record of the topology database; 
         FIG. 8 , a diagram of a path link of the topology database; 
         FIG. 9 , a diagram of a node link of the topology database; 
         FIG. 9A , a diagram of an HBA-port record of the topology database; 
         FIG. 10 , a flowchart of dynamic queue depth throttling operations performed upon completion of an exchange; 
         FIG. 11 , a flowchart of dynamic queue depth throttling performed periodically; 
         FIG. 12 , a flowchart of dynamic queue depth throttling operations performed when another initiator node logs into the storage area network; 
         FIG. 13 , a flowchart illustrating some steps of probationary repair of unstable or failed links 
         FIG. 14 , a flowchart illustrating actions taken when a path is detected repaired; and 
         FIG. 15 , a flowchart illustrating actions taken when the first frame of an exchange is transmitted. 
     
    
    
     DETAILED DESCRIPTION 
     Context in Which the Invention Operates 
     A Fibre-Channel network ( FIG. 1 ) has a first node  100  that comprises a computer  102 , a software or firmware driver  104  operating in either the computer or in a dedicated processor of a fiber channel interface card, and a fibre channel port  106 . The network also has a fibre channel fabric  108  comprising one or more switches  110  and  112 , a plurality of links  114 ,  116 ,  118 ,  120 ,  122 , and  124 , and at least one storage node  126  and  128 . The network may also have one or more additional computer nodes, such as node  130 . 
     The driver  104  maintains a network topology database  132 . This database  132  contains information regarding the topology of the network as is useful for routing packets and determining destinations for valid packets. 
     Typically, driver  104  and topology database  132  reside in memory of the computer system of a node. 
     For example, portions of the driver  104  and topology database  132  may reside in main memory devices  200  (FIG.  2 ), caches  202  and  204 , or disk memory  206  of the computer system. The driver  104  may execute in a first  208  or an additional  210  processor of the node. Typically, one or more fibre channel N_ports (node ports) are located on a host bus adapter (HBA)  212  attached to the computer system. Portions of the driver  104  and topology database  132  may also reside in local memory (not shown) of the HBA  212 , for execution by an embedded processor (not shown) of the HBA. There may be an additional HBA  214 , or an additional port of the HBA  212 . The one or more levels of cache memory  204 , main memory  200 , local disk  206  of the node, and local memory of the HBA, together form a memory system of the node, which stores the driver and the network topology database. 
     Whenever a process  300  ( FIG. 3 ) executing on the first  208  ( FIG. 2 ) or additional  210  processor of the node generates an I/O request  302 , the request is processed by the operating system  303  to determine which driver should handle the transaction. The request may also be translated into block I/O requests  304  for accessing specific blocks of storage, including blocks of directories as well as blocks of data files. Those block I/O requests accessing fibre-channel-accessible devices are passed to the driver  104 , which breaks these transactions into packets  305  for the indicated transaction. Packets  305  are processed by the driver  104  to include a valid header, and dispatched to one of one or more ports  306  for dispatch into the network fabric  308  or arbitrated loop. 
     Network Topology Database Structure 
     HBA Port List 
     Dispatch of packets to the one of one or more ports is controlled by information from the network topology database  132 . Within this database as detailed in  FIG. 4 , is maintained an HBA port list  400 . Within this list is a pair of head and tail pointers  402  and an HBA Port Record ( 404  and  406 ) for each N_Port of an HBA on the system. 
     Each HBA Port Record ( 404  and  406 ) incorporates port status information. The HBA Port Record also incorporates the timers and failure counters necessary for implementing probationary paths. 
     Additional HBA port records may be added as ports are discovered or as hot-pluggable ports are added to the system. 
     Node Links 
     Each HBA port record, such as HBA port record  404 , contains head and tail pointers to a linked list of Node Links  408  and  410 . Each Node Link  408  contains a pointer to a node record, such as node records  412  and  414 . The node record contains routing information required for dispatch of packets over the network from an associated hardware HBA port  212  or  214  to a specific target port of a target node of the storage area network. Target nodes of the storage area network may be storage nodes, such as storage nodes  126  and  128  (FIG.  1 ), or may be processor nodes such as node  130 . Each target node has one or more NL_ports, L_ports, or N_ports  136 ,  138  and  140 , through which it attaches to the fabric  108 . 
     Mode Records 
     Each node record  412  and  414  has information relevant to a target node of the network. Each node record reachable through pointers in one or more node links, node record  414  is accessible through node link  410  and  416 . The node records, including node records  412  and  414 , are linked together as a linked target node list  417  accessible through target node list head and tail pointers  418 . 
     Device Links 
     Each node record also contains pointers to a list of device links, such as device links  420 ,  422 ,  424 , and  426 , associated with that node record. As illustrated, node record  412  has pointers to a device link list containing device links  420  and  422 , while node record  414  has pointers to a device link list containing device links  424  and  426 . Each device link points to a device record, such as device records  430 ,  432 , and  434 . 
     Device Records 
     Device records, such as device records  430 ,  432 , and  434 , include information relevant to individual devices. Each device may be accessible through more than one node or port. For example, a RAID disk array  142  (FIG.  1 ), that may be accessed through two RAID controllers having ports  138  and  140 , would have a corresponding device record accessible through multiple node records and device links. 
     The device records  430 ,  432 , and  434  are linked together with a set of head and tail pointers  436  into a searchable device list  438 . 
     Path Links 
     Each device record, such as device records  430 ,  432 , and  434 , also has pointers to associated lists of path links  440 ,  442 ,  444 ,  446 , and  448 . Each path link points to a node link, such as node links  408 ,  410 , or  416 , of the topology database; and corresponds to a physical path through the network from an HBA port to the target node and device. Each path link also has status information about the corresponding path. 
     The network topology database of  FIG. 4  as drawn does not reflect the specific structure of the example network of FIG.  1 . Each figure is by way of example, showing the types of records and their linkages, or nodes and devices; the topology database is configured automatically to represent any of a large variety of possible networks. 
     Detail of Network Topology Database Structure 
     Node Record Detail 
     A node record, such a node record  412 , is shown in more detail in FIG.  6 . Referring to  FIGS. 6 and 4 , included in the record are forward and reverse list pointers  600  for linking the record to the target node list pointers  418  and to other node records such as node record  414 . There are also device-link list pointers  602  for accessing the list of device links, such as device links  420  and  422 . The device-link list pointers  602  are therefore pointers through which associated device records may be located. 
     Each node record also has node information  604  about the corresponding node of the network, including node identity information. The node identity information includes the destination ID (D_ID)  606  required for routing frames to the node, and node identifying information including a globally unique identifier  608  for the node and port. This identifier is unique among all the nodes of the storage area network. The node identity information may incorporate a node name  610 . The globally unique identifier  608  may comprise the node name  610  and a port name associated with the HBA port record through which the node record is reachable since the combination of port name and node name is unique in the network. The node identity information permits determination of whether nodes logging in through a port have already been seen through another port. There may also be information for use in higher levels of protocol, such as a SCSI address  612  for use with SCSI-over-Fibre-Channel protocols. 
     The node information also has a number of fields used to manage command queues. These include a maximum queue depth  620  for the node, a current queue depth  622 , a number of pending operations  624 , a timer  626 , and a queue depth decrement field  628  for logging the time of the most recent queue depth reduction due to a queue depth refusal. 
     Device Record Detail 
     A device record such as device record  430  has forward and reverse pointers  700  ( FIG. 7 ) through which it is linked to the device list pointers  436  and other device records. There are also path link list pointers  702  to any associated list of path links, and device information  703 . Included in the device information  703  is device identification  704 , including a device name for user access to the device. The device identification  704  may also include unique device identification, such as device serial numbers. The device information  703  also includes any logical unit number  706  needed to reference the device, device type, status  707 , and other device specific information  708  of interest to the driver. 
     Path Link Detail 
     Each path link, such as path link  440 , has list pointers  800  ( FIG. 8 ) for linking it with the associated path link list pointers  702  in the associated device record and to any other path links associated with the device. The path link also has path information  804 , including path status  806 , and redundancy mode information  814 . There is also a pointer  816  to an associated node link and a reverse pointer  818  pointing back to the associated device. 
     Path status  806  includes information indicating which path, of possibly multiple paths, is an active path on which frames should be sent, and which paths are standby paths. It may also include path failure flags. 
     Node Link Detail 
     Each node link, such as node link  408 , has pointers  900  for linking with pointers of an HBA port record and other node links into a node link list. Each node link list has at least one link for each and every network node accessible through a particular HBA port. Each node link also has an HBA pointer  902  to the associated HBA Port record, and a node pointer  904  to a node record of the target node list. Each node link also has route information, including a login delay timer  906  for suppressing traffic while node login is in progress. 
     The node link may also have routing information  908  for controlling routing of frames through the network fabric. 
     HBA Port Record Detail 
     Each HBA port record, such as HBA port record  404 , has forward and reverse pointers  950  ( FIG. 9A ) for linking it to other HBA port records, such as HBA port record  406 . The HBA port record also has error counters  952 , probation timers  954 , status flags  956 , warning counters  958  and failure counters  960  for implementing the unstable link detection, link failure, and probationary repair algorithm herein described. The HBA port record also has node link pointers  962  for pointing to node links. 
     Network Topology Database Operation 
     Topology Maintenance 
     Fibre channel networks are dynamic; nodes may appear or disappear as the network operates, paths may fail or be repaired, and devices may appear or disappear. Host bus adapters may be enabled, added, disabled, removed, or replaced while the system is running, as may individual links. Consider the case when a target node logs into the fabric, and appears to an HBA port  406  (FIG.  4 ). In this event, the target node list  417  is searched from head and tail pointers  418  for a node record having the same node identity. If a node record, such as record  414 , is found, node link  416  is added to the database to indicate that the node can be reached through the relevant port  406  as well as through any other port it is reachable through, such as port  404 . If no matching node record is found, a new node record is created and linked with a new node link. An inquiry is made of the new node to determine what devices it supports—if any. 
     Similarly, when a node reports the devices it supports, the device list  438  can be searched for a device record corresponding to each device for the same device appearing through any other node. If a matching device record is found, new device links are created to link the node record to the device record and a new path link is created to indicate the new path through the node; if no matching device is found new device links are created and linked to a new device record that is linked into the device list. 
     Each node record has forward and reverse list pointers to facilitate dynamic addition and deletion of node structures to the list. 
     The device and node lists are empty of device structures when the driver is started, device structures are added to the device list as the driver logs into the network and discovers its topology. Each device record also has forward and reverse list pointers to facilitate dynamic addition and deletion of device structures to the list. 
     The HBA port list is also empty at driver initialization. The driver discovers any physical N, L, or NL ports and adds HBA port records as required. 
     Frame Dispatch with the Topology Database 
     Fibre channel frames, or packets, are known to have a structure as illustrated in FIG.  5 . Each frame has a header  500  that includes a destination identification field (D_ID)  502 , a source identification field (S_ID)  504 , a routing control field (R_CTL)  505 , and a frame type  506 . There are also sequence count (SEQ_CNT)  508 , indicating where the frame is in a sequence, originator exchange identity (OX_ID)  510 , sequence in transaction identity (Seq_ID)  512  and receiver exchange identity (RX_ID)  514  fields for identifying a frame of an exchange. In most fibre channel networks, the D_ID field  502 , type  506 , and routing control fields  505  influence the path followed by the packet from source to destination. The S_ID field  504  influences the path followed by any reply frame. 
     Consider the case where a process desires access to a specific device by name rather than with a tag. The device may be located by searching from the pointers  436  ( FIG. 4 ) of the device list  438  of the network topology database for a device record having the desired device name, such as device record  432 . A pointer to this record may be returned as a handle or tag to the calling routine to expedite later access. The list of path links  442  and  444  associated with this device record  432  is then searched for a path link having active status; the associated node link  416  may then be located. Alternatively, an active path pointer  710 , pointing to a currently active path link, may be stored in the device record. If no path link has active status, a path link having standby or probationary status may be made active and used instead. 
     Following the node link pointer  816  of the path link  416 , through the node record pointer  904  of the node link, the associated node record  414  is found. From the node record is read the outbound D_ID, and other relevant information, for packets intended for the specific device. 
     From the node link  416 , pointer  902  is followed to the associated HBA port record  406  to determine the port on which the packets intended for the device should be transmitted, and thereby identify the port that should appear in the S_ID field of the frame. 
     Once the D_ID and S_ID fields of the frame are determined, along with any relevant routing control information, the frame is dispatched into the fabric or onto the arbitrated loop. 
     Dynamic Control of Command Queue Depth 
     Target nodes often have a command queue associated with them. These nodes may receive and buffer a finite number of queuable commands, which are then executed. As known in the art, these commands need not be executed in order. When an attempt is made to queue more commands than the command queue can hold, excess command exchanges are terminated with queue overflow errors—these commands generally must be re-queued by the initiator node. 
     Command queue overflow errors are limited through a dynamically-adjusted command queue depth  622  ( FIG. 6 ) and a number of pending operations  624  maintained in each node record. The maximum queue depth  620  is initialized to a maximum queue depth suitable for the type of the target node when the node logs into the target node and the node record is created; a minimum queue depth (not shown) is also established for each node. The current queue depth  622  is set to the maximum of the minimum queue depth or the maximum queue depth  620  divided by the number of initiators visible to the HBA port. The number of initiators  962  seen logged into the network and visible to the HBA port then maintained in the HBA port record. 
     Each time a new I/O operation is requested, the number of pending operations  624  is compared to the current queue depth  622  for the target node of the operation. If the new operation is of a queueable type and will cause the current queue depth  622  to be exceeded, the new operation is linked to a delayed operation queue. These operations will be performed when currently executing operations complete or abort. 
     If the new operation will not cause the current queue depth  622  to be exceeded, the number of pending operations  624  is incremented, and frames are transmitted as required to initiate the operation. 
     When I/O operation exchanges terminate  1000  (FIG.  10 ), frames returned by the target node are tested  1002  for queue overflow errors. If a queue overflow error occurs, its time is compared to the last queue decrement time  628 , the prior contents of the last queue decrement time being used to permit downward adjustment of queue depth at a maximum rate of once every five seconds. If downward adjustment of queue depth is done, the last queue decrement time  628  is updated. The current queue depth  622  is checked  1004  to determine if it is already at the minimum value. If a queue overflow error occurs with the queue depth at other than the minimum value, the current queue depth  622  is adjusted downwardly. The downward adjustment is either by five  1010 , or to the minimum value  1006  or  1008 , depending on the value of the queue depth and the prior contents of the last queue refusal variable. 
     A timer triggers periodic reexamination of current queue depth. Upon expiration of the timer, the last queue decrement time  628  is checked  1100  ( FIG. 11 ) to determine if the current queue depth was decreased since the timer last expired. If not, and the current queue depth  622  is less than maximum  1110 , the current queue depth  622  is adjusted upwardly by five  1120  or to maximum  1125 , and the timer is restarted. 
     In an embodiment, the timer expiration interval is also dynamically adjusted between a minimum and maximum value during operation. The timer interval is decreased  1135  when it is greater than minimum and current queue depth  622  is adjusted upwardly; the timer interval is increased  1140  when it is less than maximum and no adjustment to the queue depth  622  has been made since the most recent previous timer expiration. Timer expiration interval decreases serve to allow for rapid convergence upon optimal performance, timer expiration interval increases serve to avoid excess processing time when the queue depth is likely already at the maximum usable queue depth. 
     In order to provide queue capacity to compute or initiator nodes that have just logged into the storage area network, a list (not shown) of known initiators is maintained in the network topology database  132 . The node on which the network topology database  132  resides can see some logins by other initiators to the storage area network. 
     Each time a login by a new initiator node to the fabric is seen, where the new initiator node has access to a target node, the target node list  418  of the network topology database  132  is searched  1200  ( FIG. 12 ) to find any node record, such as node record  412 , that is likely to be addressed by that newly logged-in initiator node. 
     If a node record is found that may be a target node of the newly logged-in initiator node, the current queue depth  622  of the node record is reduced  1202  to the greater of the minimum queue depth for that node and the maximum queue depth divided by the number of initiators  962  now known to be logged into the associated HBA port. 
     Since the current queue depth  622  is maintained in the node record, there are one or more separate current queue depth variables for each target node of the network. These variables are adjusted independently; different target nodes may therefore have different current queue depth  622  associated with them. Similarly, there are separate numbers (or counts) of pending operations  624  maintained in the node records, each corresponding to different pairings of N_port, L_port, or NL_port to a target node of the network. 
     Recovery and Probation of Failed Paths 
     Each physical path set up through the network from an initiator node to a target node is associated with an HBA port record and a path link, as heretofore described. The path link has a status flag  806  (FIG.  8 ), which may have the conditions Active, or Normal, among other possible status conditions. The HBA port record has further status information regarding the path and particularly relevant to status of the physical link between the physical HBA port corresponding to the HBA port record and the fabric. 
     In order to limit dispatch of frames over intermittent or problem links and paths, with attendant errors and retries, freshly repaired and problem paths are given probationary status. Frames are preferentially dispatched over non-probationary paths if any exist, if no non-probationary paths exist a probationary path may, in desperation, be returned to active status when they become available for frames. 
     Each time an error is detected  1300 , a routine ( FIG. 13 ) is invoked for logging that error against the link and path used for the exchange. Errors are filtered  1302  to exclude errors not potentially related to a failed or unstable link, and evaluated for error severity. Severe errors  1304  may cause the link to be marked failed  1306  immediately in the HBA port record and the HBA port&#39;s laser diodes taken off-line. Less serious errors are counted  1308 , with the link marked failed or marked unstable only if high numbers of these errors are encountered. 
     Errors on non-probationary links are identified  1310 , and binned into 10-second intervals. Records are maintained  1312  for four consecutive 10-second intervals. Only if errors are found in the current 10-second interval and three immediately previous 10-second intervals is a link considered unstable  1314 . If the link is not unstable, retransmission may be attempted or error reported, and the error routine returns  1316 . 
     If the link is determined unstable  1314 , the HBA port record is marked unstable  1318  and a search  1320  is made in the topology database for alternate paths to all destinations reached over that link. Failover is done for those paths for which alternate paths exist. If any path does not have an alternate, the error routine returns  1316  without failing the link  1306  or taking the laser diodes off-line. 
     Whenever the laser diodes for a link are taken off-line, a 5-minute delay  1322  timer is started. When this timer expires, or all other paths to a node previously reachable over that link fail, a repair detection attempt  1324  is made on that link. Repair detection requires that the laser diodes be brought back on line and link testing be performed. If the link test  1326  fails, the laser diodes are taken back offline  1328  and the 5-minute delay  1322  restarted. 
     If the link test passes, the link is marked  1330  probationary in the HBA port record (FIG.  9 ). The link error and warning counters  952  are cleared and a one minute restartable and cancellable probation timer  956  and  1332  ( FIG. 13 ) is started. Login attempts are begun to reestablish paths from this port to all accessible target nodes reachable over this port, including those accessible target nodes that were formerly reachable over this port. 
     Upon expiration of the one minute probation timer  1332 , the logged error count  952  on the link is checked  1334  to verify that it is zero. If no errors have occurred on the link, the link and associated paths are marked  1336  normal status. If errors occurred without their having been associated with logins, as discussed below, the link is treated as still unstable  1318 , and may be marked failed  1306  and the laser diodes once again taken off-line. 
     Should errors occur on a probationary path  1310 , a five-second login window delay  1340  is started. Should any login by another node be observed during this five-second delay, the errors are considered to be related to that login. The error counter  952  is cleared  1342  and the one-minute probation delay  1332  is restarted  1344 . 
     Each time a path is detected repaired (FIG.  14 ), a firmware or software timer associated with that link is cleared  1400  in the HBA port record. The link is also mark ed probationary  1402 . 
     When an attempt to transmit an exchange is made (FIG.  15 ), the active path pointer  710  ( FIG. 7 ) is followed to see if it points to a valid active path. If that pointer does not point to a currently valid and active path, or it points to a failed path, a search  1500  is made among the path links for an active path. If an active path is found, the associated frames are transmitted on that path  1502 , this may be an active path over an unstable link if no alternate path exists. If no path is marked active, a search  1504  is made for one or more paths marked normal. If one or more paths marked normal exist, one of those paths is selected, marked active  1506 , and used to transmit  1508  the frame. If no path marked Normal exists, a search  1510  is made for one or more paths associated with a link marked probationary. If one or more paths marked Probationary exist, one of those paths is selected, marked active  1512 , and used to transmit  1514  the frame. Any associated one-minute probation timer or five-second error login window timer is cancelled. In this way, a Probationary path may be made active in a desperation recovery. 
     If no active, normal, or probationary path is found, a search is made  1516  for a path taken off-line as unstable or failed. If none is found, the exchange attempt is rejected  1518  since there is no path on which it can be sent. If such an offline path is found, a repair detect attempt  1520  is made; this requires bringing the laser diodes back online and testing the associated link. If the link tests good, any required logins  1522  are done, the path is marked active  1524  in the path link, any other paths through the link are marked normal, and the path is used  1526  to transmit the frame. In this way, an offline link may be made active in a desperation recovery. If the link tests bad, the laser diodes are taken back offline  1530  and the frame is rejected  1518  since there is no path on which it can be sent. 
     CONCLUSION 
     While the invention has been particularly shown and described with reference to one or more embodiments thereof, it will be understood by those skilled in the art that various other changes in the form and details may be made without departing from the spirit and scope of the invention. 
     For example, the order of fields within the node record, path link, and device record may be readily altered for a particular embodiment of the invention. Similarly, the reverse pointers of the node record and the device record are present for convenience in inserting and removing records from the associated lists, while these confer advantages it is possible to construct a functional system with only forward links. If the number of devices addressed through a single node is limited, device links and the associated device link list pointers of the node records may be replaced by a plurality of device record pointers in the node records. 
     Node records have been described as accessed through node link records, device records through device link records, and paths through path link records. Each link record has pointers for linking into a linked list and pointers to associated node and device records. It is also possible to use a long fixed-length or a variable-length HBA-port record, node record, or device record; the longer or variable length record containing multiple pointers to other node, device, or node records of the database. Using long fixed-length or variable length records may, however, limit the number of nodes, devices and paths trackable by the database. Further, excessive delays and memory fragmentation may be encountered as variable-length record sizes are adjusted while adding nodes and devices to the database. 
     While the above description has focused on dispatch of frames from a node, switches of a switched fabric also have processors and must maintain a network topology database. Packets received by a switch may be routed on preferred or alternate paths by such a switch according to information in a network topology database of the present invention. Each such switch may also implement the probationary period of the present invention should paths through that switch be repaired. Switches implementing probationary periods on repaired paths may also promote probationary paths to normal operation in desperation when no alternative, normal status, paths remain available. 
     A computer program product is a computer readable memory device or media having recorded upon it computer readable instructions. 
     It should be understood that this description has been made by way of example, and that the invention is defined by the scope of the following claims.