Patent Publication Number: US-7899944-B2

Title: Open exchange limiting in an I/O processing system

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present disclosure relates generally to input/output processing, and in particular, to limiting a number of open exchanges in an input/output processing system. 
     2. Description of Background 
     Input/output (I/O) operations are used to transfer data between memory and I/O devices of an I/O processing system. Specifically, data is written from memory to one or more I/O devices, and data is read from one or more I/O devices to memory by executing I/O operations. 
     To facilitate processing of I/O operations, an I/O subsystem of the I/O processing system is employed. The I/O subsystem is coupled to main memory and the I/O devices of the I/O processing system and directs the flow of information between memory and the I/O devices. One example of an I/O subsystem is a channel subsystem. The channel subsystem uses channel paths as communications media. Each channel path includes a channel coupled to a control unit, the control unit being further coupled to one or more I/O devices. Exchanges are opened between the channel subsystem and the control unit to manage the flow of messages over the channel paths. 
     The channel subsystem may employ channel command words (CCWs) to transfer data between the I/O devices and memory. A CCW specifies the command to be executed. For commands initiating certain I/O operations, the CCW designates the memory area associated with the operation, the action to be taken whenever a transfer to or from the area is completed, and other options. 
     During I/O processing, a list of CCWs is fetched from memory by a channel. The channel parses each command from the list of CCWs and forwards a number of the commands, each command in its own entity, to a control unit coupled to the channel. The control unit then processes the commands. The channel tracks the state of each command and controls when the next set of commands are to be sent to the control unit for processing. The channel ensures that each command is sent to the control unit in its own entity. Further, the channel infers certain information associated with processing the response from the control unit for each command. 
     Performing I/O processing on a per CCW basis may involve a large amount of processing overhead for the channel subsystem, as the channels parse CCWs, track state information, and react to responses from the control units. Therefore, it may be beneficial to shift much of the processing burden associated with interpreting and managing CCW and state information from the channel subsystem to the control units. Simplifying the role of channels in communicating between the control units and an operating system in the I/O processing system may increase communication throughput as less handshaking is performed. Since different control units may support a variable number of open communication exchanges between control units and the channel subsystem, it would also be beneficial to provide information indicating the maximum number of open exchanges supported by the control unit to the channel subsystem. Supporting a variable number of open exchanges beyond a static limit for all control units would enable support of numerous control units of varying levels of complexity. Moreover, enforcing limits on the number of open exchanges at the channel subsystem could prevent error conditions at the control units attributable to attempting to open too many exchanges, which may further enhance communication throughput. Accordingly, there is a need in the art for limiting the number of open exchanges in an I/O processing system. 
     BRIEF SUMMARY OF THE INVENTION 
     Embodiments of the invention include a computer program product for limiting a number of open exchanges at a channel subsystem of an input/output (I/O) processing system using data from a control unit. The computer program product includes a tangible storage medium readable by a processing circuit and storing instructions for execution by the processing circuit for performing a method. The method includes sending a command message to the control unit, and receiving a transport response information unit message at the channel subsystem in response to sending the command message to the control unit. The method further includes extracting a maximum control unit exchange parameter from the transport response information unit message as populated by the control unit, and determining a limit value for a maximum number of open exchanges between the channel subsystem and the control unit at the channel subsystem as a function of the extracted maximum control unit exchange parameter. The method additionally includes applying the limit value to prevent opening of a new exchange. 
     Additional embodiments include an apparatus for limiting a number of open exchanges at a channel subsystem of an I/O processing system. The apparatus includes a channel subsystem in communication with a control unit. The channel subsystem performs a method that includes sending a command message to the control unit, receiving a transport response information unit message at the channel subsystem in response to sending the command message to the control unit, and extracting a maximum control unit exchange parameter from the transport response information unit message as populated by the control unit. The method also includes determining a limit value for a maximum number of open exchanges between the channel subsystem and the control unit at the channel subsystem as a function of the extracted maximum control unit exchange parameter, and applying the limit value to prevent opening of a new exchange. 
     Further embodiments include a method for limiting a number of open exchanges at a channel subsystem of an I/O processing system using data from a control unit. The method includes sending a command message to the control unit, receiving a transport response information unit message at the channel subsystem in response to sending the command message to the control unit, and extracting a maximum control unit exchange parameter from the transport response information unit message as populated by the control unit. The method further includes determining a limit value for a maximum number of open exchanges between the channel subsystem and the control unit at the channel subsystem as a function of the extracted maximum control unit exchange parameter, and applying the limit value to prevent opening of a new exchange. 
     An additional embodiment includes a computer program product for limiting a number of open exchanges at a channel subsystem of an I/O processing system using data from a control unit. The computer program product includes a tangible storage medium readable by a processing circuit and storing instructions for execution by the processing circuit for performing a method. The method includes sending a transport command information unit message to the control unit including a transport command control block (TCCB) as part of a transport control word (TCW) channel program, and setting a limit value equal to a default value until a transport response information unit message is received from the control unit. The limit value establishes a maximum number of open exchanges between the channel subsystem and the control unit at the channel subsystem. The method further includes receiving the transport response information unit message at the channel subsystem in response to sending the transport command information unit message to the control unit, and extracting a maximum control unit exchange parameter from the transport response information unit message as populated by the control unit. The method additionally includes determining the limit value as a function of the extracted maximum control unit exchange parameter, where the limit value is unmodified in response to the extracted maximum control unit exchange parameter with a value of zero. The method also includes applying the limit value to prevent opening of a new exchange to block a new I/O command from being output on the new exchange in response to a current number of open exchanges being greater than or equal to the limit value. 
     A further embodiment includes an apparatus for limiting a number of open exchanges at a channel subsystem of an I/O processing system. The apparatus includes a channel subsystem in communication with a control unit. The channel subsystem performs a method that includes sending a transport command information unit message to the control unit including a TCCB as part of a TCW channel program, and setting a limit value equal to a default value until a transport response information unit message is received from the control unit. The limit value establishes a maximum number of open exchanges between the channel subsystem and the control unit at the channel subsystem. The method also includes receiving the transport response information unit message at the channel subsystem in response to sending the transport command information unit message to the control unit, and extracting a maximum control unit exchange parameter from the transport response information unit message as populated by the control unit. The method further includes determining the limit value as a function of the extracted maximum control unit exchange parameter, where the limit value is unmodified in response to the extracted maximum control unit exchange parameter with a value of zero. The method additionally includes applying the limit value to prevent opening of a new exchange to block a new I/O command from being output on the new exchange in response to a current number of open exchanges being greater than or equal to the limit value. 
     Other computer program products, apparatuses, and/or methods according to embodiments will be or become apparent to one with skill in the art upon review of the following drawings and detailed description. It is intended that all such additional computer program products, apparatuses, and/or methods be included within this description, be within the scope of the present invention, and be protected by the accompanying claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The subject matter which is regarded as the invention is particularly pointed out and distinctly claimed in the claims at the conclusion of the specification. The foregoing and other objects, features, and advantages of the invention are apparent from the following detailed description taken in conjunction with the accompanying drawings in which: 
         FIG. 1  depicts one embodiment of an I/O processing system incorporating and using one or more aspects of the present invention; 
         FIG. 2   a  depicts one example of a prior art channel command word; 
         FIG. 2   b  depicts one example of a prior art channel command word channel program; 
         FIG. 3  depicts one embodiment of a prior art link protocol used in communicating between a channel and control unit to execute the channel command word channel program of  FIG. 2   b;    
         FIG. 4  depicts one embodiment of a transport control word channel program, in accordance with an aspect of the present invention; 
         FIG. 5  depicts one embodiment of a link protocol used to communicate between a channel and control unit to execute the transport control word channel program of  FIG. 4 , in accordance with an aspect of the present invention; 
         FIG. 6  depicts one embodiment of a prior art link protocol used to communicate between a channel and control unit in order to execute four read commands of a channel command word channel program; 
         FIG. 7  depicts one embodiment of a link protocol used to communicate between a channel and control unit to process the four read commands of a transport control word channel program, in accordance with an aspect of the present invention; 
         FIG. 8  depicts one embodiment of a control unit and a channel, in accordance with an aspect of the present invention; 
         FIG. 9  depicts one embodiment of a response message communicated from a control unit to a channel, in accordance with an aspect of the present invention; 
         FIG. 10  depicts one embodiment of open exchange limiting logic to limit the number of open exchanges in an I/O processing system; 
         FIG. 11  depicts one embodiment of a process for limiting the number of open exchanges at a channel subsystem of an I/O processing system using data from a control unit; and 
         FIG. 12  depicts one embodiment of a computer program product incorporating one or more aspects of the present invention. 
     
    
    
     The detailed description explains the preferred embodiments of the invention, together with advantages and features, by way of example with reference to the drawings. 
     DETAILED DESCRIPTION OF THE INVENTION 
     In accordance with an aspect of the present invention, input/output (I/O) processing is facilitated. For instance, I/O processing is facilitated by readily enabling access to information, such as status and measurement data, associated with I/O processing. Further, I/O processing is facilitated, in one example, by reducing communications between components of an I/O processing system used to perform the I/O processing. For instance, the number of exchanges and sequences between an I/O communications adapter, such as a channel, and a control unit is reduced. This is accomplished by sending a plurality of commands from the I/O communications adapter to the control unit as a single entity for execution by the control unit, and by the control unit sending the data resulting from the commands, if any, as a single entity. 
     The plurality of commands are included in a block, referred to herein as a transport command control block (TCCB), an address of which is specified in a transport control word (TCW). The TCW is sent from an operating system or other application to the I/O communications adapter, which in turn forwards the TCCB in a command message to the control unit for processing. The control unit processes each of the commands absent a tracking of status relative to those individual commands by the I/O communications adapter. The plurality of commands is also referred to as a channel program, which is parsed and executed on the control unit rather than the I/O communications adapter. 
     In an exemplary embodiment, the control unit generates a response message including status and extended status information in response to executing the channel program. The control unit may also generate a response message without executing the channel program under a limited number of communication scenarios, e.g., to inform the I/O communications adapter that the channel program will not be executed. The control unit may include a number of elements to support communication between the I/O communications adapter and I/O devices, as well as in support of channel program execution. For example, the control unit can include control logic to parse and process messages, in addition to one or more queues, timers, and registers to facilitate communication and status monitoring. The I/O communications adapter parses the response message, extracting the status and extended status information, and performs further operations using the extracted information, such as limiting the number of open exchanges between the I/O communications adapter and the control unit. 
     One example of an I/O processing system incorporating and using one or more aspects of the present invention is described with reference to  FIG. 1 . I/O processing system  100  includes a host system  101 , which further includes for instance, a main memory  102 , one or more central processing units (CPUs)  104 , a storage control element  106 , and a channel subsystem  108 . The host system  101  may be a large scale computing system, such as a mainframe or server. The I/O processing system  100  also includes one or more control units  110  and one or more I/O devices  112 , each of which is described below. 
     Main memory  102  stores data and programs, which can be input from I/O devices  112 . For example, the main memory  102  may include one or more operating systems (OSs)  103  that are executed by one or more of the CPUs  104 . For example, one CPU  104  can execute a Linux® operating system  103  and a z/OS® operating system  103  as different virtual machine instances. The main memory  102  is directly addressable and provides for high-speed processing of data by the CPUs  104  and the channel subsystem  108 . 
     CPU  104  is the controlling center of the I/O processing system  100 . It contains sequencing and processing facilities for instruction execution, interruption action, timing functions, initial program loading, and other machine-related functions. CPU  104  is coupled to the storage control element  106  via a connection  114 , such as a bidirectional or unidirectional bus. 
     Storage control element  106  is coupled to the main memory  102  via a connection  116 , such as a bus; to CPUs  104  via connection  114 ; and to channel subsystem  108  via a connection  118 . Storage control element  106  controls, for example, queuing and execution of requests made by CPU  104  and channel subsystem  108 . 
     In an exemplary embodiment, channel subsystem  108  provides a communication interface between host system  101  and control units  110 . Channel subsystem  108  is coupled to storage control element  106 , as described above, and to each of the control units  110  via a connection  120 , such as a serial link. Connection  120  may be implemented as an optical link, employing single-mode or multi-mode waveguides in a Fibre Channel fabric. Channel subsystem  108  directs the flow of information between I/O devices  112  and main memory  102 . It relieves the CPUs  104  of the task of communicating directly with the I/O devices  112  and permits data processing to proceed concurrently with I/O processing. The channel subsystem  108  uses one or more channel paths  122  as the communication links in managing the flow of information to or from I/O devices  112 . As a part of the I/O processing, channel subsystem  108  also performs the path-management functions of testing for channel path availability, selecting an available channel path  122  and initiating execution of the operation with the I/O devices  112 . 
     Each channel path  122  includes a channel  124  (channels  124  are located within the channel subsystem  108 , in one example, as shown in  FIG. 1 ), one or more control units  110  and one or more connections  120 . In another example, it is also possible to have one or more dynamic switches (not depicted) as part of the channel path  122 . A dynamic switch is coupled to a channel  124  and a control unit  110  and provides the capability of physically interconnecting any two links that are attached to the switch. In another example, it is also possible to have multiple systems, and therefore multiple channel subsystems (not depicted) attached to control unit  110 . 
     Also located within channel subsystem  108  are subchannels (not shown). One subchannel is provided for and dedicated to each I/O device  112  accessible to a program through the channel subsystem  108 . A subchannel (e.g., a data structure, such as a table) provides the logical appearance of a device to the program. Each subchannel provides information concerning the associated I/O device  112  and its attachment to channel subsystem  108 . The subchannel also provides information concerning I/O operations and other functions involving the associated I/O device  112 . The subchannel is the means by which channel subsystem  108  provides information about associated I/O devices  112  to CPUs  104 , which obtain this information by executing I/O instructions. 
     Channel subsystem  108  is coupled to one or more control units  110 . Each control unit  110  provides logic to operate and control one or more I/O devices  112  and adapts, through the use of common facilities, the characteristics of each I/O device  112  to the link interface provided by the channel  124 . The common facilities provide for the execution of I/O operations, indications concerning the status of the I/O device  112  and control unlit  110 , control of the timing of data transfers over the channel path  122  and certain levels of I/O device  112  control. 
     Each control unit  110  is attached via a connection  126  (e.g., a bus) to one or more I/O devices  112 . I/O devices  112  receive information or store information in main memory  102  and/or other memory. Examples of I/O devices  112  include card readers and punches, magnetic tape units, direct access storage devices, displays, keyboards, printers, pointing devices, teleprocessing devices, communication controllers and sensor based equipment, to name a few. 
     One or more of the above components of the I/O processing system  100  are further described in “IBM® z/Architecture Principles of Operation,” Publication No. SA22-7832-05, 6th Edition, April 2007; U.S. Pat. No. 5,461,721 entitled “System For Transferring Data Between I/O Devices And Main Or Expanded Storage Under Dynamic Control Of Independent Indirect Address Words (IDAWS),” Cormier et al., issued Oct. 24, 1995; and U.S. Pat. No. 5,526,484 entitled “Method And System For Pipelining The Processing Of Channel Command Words,” Casper et al., issued Jun. 11, 1996, each of which is hereby incorporated herein by reference in its entirety. IBM is a registered trademark of International Business Machines Corporation, Armonk, N.Y., USA. Other names used herein may be registered trademarks, trademarks or product names of International Business Machines Corporation or other companies. 
     In one embodiment, to transfer data between I/O devices  112  and memory  102 , channel command words (CCWs) are used. A CCW specifies the command to be executed, and includes other fields to control processing. One example of a CCW is described with reference to  FIG. 2   a . A CCW  200  includes, for instance, a command code  202  specifying the command to be executed (e.g., read, read backward, control, sense and write); a plurality of flags  204  used to control the I/O operation; for commands that specify the transfer of data, a count field  206  that specifies the number of bytes in the storage area designated by the CCW to be transferred; and a data address  208  that points to a location in main memory that includes data, when direct addressing is employed, or to a list (e.g., contiguous list) of modified indirect data address words (MIDAWs) to be processed, when modified indirect data addressing is employed. Modified indirect addressing is further described in U.S. application Ser. No. 11/464,613, entitled “Flexibly Controlling The Transfer Of Data Between Input/Output Devices And Memory,” Brice et al., filed Aug. 15, 2006, which is hereby incorporated herein by reference in its entirety. 
     One or more CCWs arranged for sequential execution form a channel program, also referred to herein as a CCW channel program. The CCW channel program is set up by, for instance, an operating system, or other software. The software sets up the CCWs and obtains the addresses of memory assigned to the channel program. An example of a CCW channel program is described with reference to  FIG. 2   b . A CCW channel program  210  includes, for instance, a define extent CCW  212  that has a pointer  214  to a location in memory of define extent data  216  to be used with the define extent command. In this example, a transfer in channel (TIC)  218  follows the define extent command that refers the channel program to another area in memory (e.g., an application area) that includes one or more other CCWs, such as a locate record  217  that has a pointer  219  to locate record data  220 , and one or more read CCWs  221 . Each read CCW  220  has a pointer  222  to a data area  224 . The data area includes an address to directly access the data or a list of data address words (e.g., MIDAWs or IDAWs) to indirectly access the data. Further, CCW channel program  210  includes a predetermined area in the channel subsystem defined by the device address called the subchannel for status  226  resulting from execution of the CCW channel program. 
     The processing of a CCW channel program is described with reference to  FIG. 3 , as well as with reference to  FIG. 2   b . In particular,  FIG. 3  shows an example of the various exchanges and sequences that occur between a channel and a control unit when a CCW channel program is executing. The link protocol used for the communications is FICON (Fibre Connectivity), in this example. Information regarding FICON is described in “Fibre Channel Single Byte Command Code Sets-3 Mapping Protocol (FC-SB-3), T11/Project 1357-D/Rev. 1.6, INCITS (March 2003), which is hereby incorporated herein by reference in its entirety. 
     Referring to  FIG. 3 , a channel  300  opens an exchange with a control unit  302  and sends a define extent command and data associated therewith  304  to control unit  302 . The command is fetched from define extent CCW  212  ( FIG. 2   b ) and the data is obtained from define extent data area  216 . The channel  300  uses TIC  218  to locate the locate record CCW and the read CCW. It fetches the locate record command  305  ( FIG. 3 ) from the locate record CCW  217  ( FIG. 2   b ) and obtains the data from locate record data  220 . The read command  306  ( FIG. 3 ) is fetched from read CCW  221  ( FIG. 2   b ). Each is sent to the control unit  302 . 
     The control unit  302  opens an exchange  308  with the channel  300 , in response to the open exchange of the channel  300 . This can occur before or after locate command  305  and/or read command  306 . Along with the open exchange, a response (CMR) is forwarded to the channel  300 . The CMR provides an indication to the channel  300  that the control unit  302  is active and operating. 
     The control unit  302  sends the requested data  310  to the channel  300 . Additionally, the control unit  302  provides the status to the channel  300  and closes the exchange  312 . In response thereto, the channel  300  stores the data, examines the status and closes the exchange  314 , which indicates to the control unit  302  that the status has been received. 
     The processing of the above CCW channel program to read 4 k of data requires two exchanges to be opened and closed and seven sequences. The total number of exchanges and sequences between the channel and control unit is reduced through collapsing multiple commands of the channel program into a TCCB. The channel, e.g., channel  124  of  FIG. 1 , uses a TCW to identify the location of the TCCB, as well as locations for accessing and storing status and data associated with executing the channel program. The TCW is interpreted by the channel and is not sent or seen by the control unit. 
     One example of a channel program to read 4 k of data, as in  FIG. 2   b , but includes a TCCB, instead of separate individual CCWs, is described with reference to  FIG. 4 . As shown, a channel program  400 , referred to herein as a TCW channel program, includes a TCW  402  specifying a location in memory of a TCCB  404 , as well as a location in memory of a data area  406  or a TIDAL  410  (i.e., a list of transfer mode indirect data address words (TIDAWs), similar to MIDAWs) that points to data area  406 , and a status area  408 . TCWs, TCCBs, and status are described in further detail below. 
     The processing of a TCW channel program is described with reference to  FIG. 5 . The link protocol used for these communications is, for instance, Fibre Channel Protocol (FCP). In particular, three phases of the FCP link protocol are used, allowing host bus adapters to be used that support FCP to perform data transfers controlled by CCWs. FCP and its phases are described further in “Information Technology—Fibre Channel Protocol for SCSI, Third Version (FCP-3),” T10 Project 1560-D, Revision 4, Sep. 13, 2005, which is hereby incorporated herein by reference in its entirety. 
     Referring to  FIG. 5 , a channel  500  opens an exchange with a control unit  502  and sends TCCB  504  to the control unit  502 . In one example, the TCCB  504  and sequence initiative are transferred to the control unit  502  in a FCP command, referred to as FCP_CMND information unit (IU) or a transport command IU. The control unit  502  executes the multiple commands of the TCCB  504  (e.g., define extent command, locate record command, read command as device control words (DCWs)) and forwards data  506  to the channel  500  via, for instance, a FCP_Data IU. It also provides status and closes the exchange  508 . As one example, final status is sent in a FCP status frame that has a bit active in, for instance, byte  10  or  11  of the payload of a FCP_RSP IU, also referred to as a transport response IU. The FCP_RSP IU payload may be used to transport FICON ending status along with additional status information, including parameters that support the calculation of extended measurement words and notify the channel  500  of the maximum number of open exchanges supported by the control unit  502 . 
     In a further example, to write 4 k of customer data, the channel  500  uses the FCP link protocol phases, as follows: 
     1. Transfer a TCCB in the FCP_CMND IU. 
     2. Transfer the IU of data, and sequence initiative to the control unit  502 . 
     3. Final status is sent in a FCP status frame that has a bit active in, for instance, byte  10  or  11  of the FCP_RSP IU Payload. The FCP_RSP_INFO field or sense field is used to transport FICON ending status along with additional status information, including parameters that support the calculation of extended measurement words and notify the channel  500  of the maximum number of open exchanges supported by the control unit  502 . 
     By executing the TCW channel program of  FIG. 4 , there is only one exchange opened and closed (see also  FIG. 5 ), instead of two exchanges for the CCW channel program of  FIG. 2   b  (see also  FIG. 3 ). Further, for the TCW channel program, there are three communication sequences (see  FIGS. 4-5 ), as compared to seven sequences for the CCW channel program (see  FIGS. 2   b - 3 ). 
     The number of exchanges and sequences remain the same for a TCW channel program, even if additional commands are added to the program. Compare, for example, the communications of the CCW channel program of  FIG. 6  with the communications of the TCW channel program of  FIG. 7 . In the CCW channel program of  FIG. 6 , each of the commands (e.g., define extent command  600 , locate record command  601 , read command  602 , read command  604 , read command  606 , locate record command  607  and read command  608 ) are sent in separate sequences from channel  610  to control unit  612 . Further, each 4 k block of data (e.g., data  614 - 620 ) is sent in separate sequences from the control unit  612  to the channel  610 . This CCW channel program requires two exchanges to be opened and closed (e.g., open exchanges  622 ,  624  and close exchanges  626 ,  628 ), and fourteen communications sequences. This is compared to the three sequences and one exchange for the TCW channel program of  FIG. 7 , which accomplishes the same task as the CCW channel program of  FIG. 6 . 
     As depicted in  FIG. 7 , a channel  700  opens an exchange with a control unit  702  and sends a TCCB  704  to the control unit  702 . The TCCB  704  includes the define extent command, the two locate record commands, and the four read commands in DCWs, as described above. In response to receiving the TCCB  704 , the control unit  702  executes the commands and sends, in a single sequence, the 16 k of data  706  to the channel  700 . Additionally, the control unit  702  provides status to the channel  700  and closes the exchange  708 . Thus, the TCW channel program requires much less communications to transfer the same amount of data as the CCW channel program of  FIG. 6 . 
     Turning now to  FIG. 8 , one embodiment of the control unit  110  and the channel  124  of  FIG. 1  that support TCW channel program execution are depicted in greater detail. The control unit  110  includes CU control logic  802  to parse and process command messages containing a TCCB, such as the TCCB  704  of  FIG. 7 , received from the channel  124  via the connection  120 . The CU control logic  802  can extract DCWs and control data from the TCCB received at the control unit  110  to control a device, for instance, I/O device  112  via connection  126 . The CU control logic  802  sends device commands and data to the I/O device  112 , as well as receives status information and other feedback from the I/O device  112 . For example, the I/O device  112  may be busy because of a previous reservation request targeting I/O device  112 . To manage potential device reservation contention issues that can arise when the control unit  110  receives multiple requests to access the same I/O device  112 , the CU control logic  802  keeps track of and stores device busy messages and associated data in a device busy queue  804 . In an exemplary embodiment, an OS  103  of  FIG. 1  reserves I/O device  112  to keep other OSs  103  from accessing the I/O device  112  while the reservation is active. Although device reservation is not required for all I/O operations, device reservation can be used to support operations that necessitate exclusive access for a fixed duration of time, e.g., disk formatting. 
     The CU control logic  802  can access and control other elements within the control unit  110 , such as CU timers  806  and CU registers  808 . The CU timers  806  may include multiple timer functions to track how much time a sequence of I/O operations takes to complete. The CU timers  806  may further include one or more countdown timers to monitor and abort I/O operations and commands that do not complete within a predetermined period. The CU registers  808  can include fixed values that provide configuration and status information, as well as dynamic status information that is updated as commands are executed by the CU control logic  802 . The control unit  110  may further include other buffer or memory elements (not depicted) to store multiple messages or status information associated with communications between the channel  124  and the I/O device  112 . The CU registers  808  may include a maximum control unit exchange parameter that defines the maximum number of open control unit exchanges that the control unit  110  supports. 
     The channel  124  in the channel subsystem  108  includes multiple elements to support communication with the control unit  110 . For example, the channel  124  may include CHN control logic  810  that interfaces with CHN subsystem timers  812  and CHN subsystem registers  814 . In an exemplary embodiment, the CHN control logic  810  controls communication between the channel subsystem  108  and the control unit  110 . The CHN control logic  810  may directly interface to the CU control logic  802  via the connection  120  to send commands and receive responses, such as transport command and response IUs. Alternatively, messaging interfaces and/or buffers (not depicted) can be placed between the CHN control logic  810  and the CU control logic  802 . The CHN subsystem timers  812  may include multiple timer functions to track how much time a sequence of I/O operations takes to complete, in addition to the time tracked by the control unit  110 . The CHN subsystem timers  812  may further include one or more countdown timers to monitor and abort command sequences that do not complete within a predetermined period. The CHN subsystem registers  814  can include fixed values that provide configuration and status information, as well as dynamic status information, updated as commands are transported and responses are received. 
     One example of a response message  900 , e.g., a transport response IU, communicated from the control unit  110  to the channel  124  upon completion of a TCW channel program is depicted in  FIG. 9 . The response message  900  provides status information to the channel  124  and may indicate that an open exchange between the channel  124  and the control unit  110  should be closed. The status information provided when a TCW channel program (e.g., as depicted in  FIGS. 5 and 7 ) is executed includes additional information beyond the status information sent upon completion of a CCW channel program (e.g., as depicted in  FIGS. 3 and 6 ). The response message  900  includes a status section  902  and an extended status section  904 . When the channel  124  receives the response message  900 , it stores parts of status section  902  in the subchannel for the device the TCW was operating with and the extended status section  904  in a memory location defined by the TCW associated with the TCW channel program that triggered the response message  900 . For example, a TCW can designate a section of main memory  102  of  FIG. 1  for storage of the extended status section  904 . 
     The status section  902  of the response message  900  can include multiple fields, such as an address header  906 , status flags one  908 , maximum control unit exchange parameter  910 , response flags  912 , response code  914 , residual count  916 , response length  918 , reserved location  920 , SPC-4 sense type  922 , status flags two  924 , status flags three  926 , device status  928 , and a longitudinal redundancy check (LRC) word  930 . Each field in the status section  902  is assigned to a particular byte address to support parsing of the response message  900 . Although one arrangement of fields within the status section  902  is depicted in  FIG. 9 , it will be understood that the order of fields can be rearranged to alternate ordering within the scope of the disclosure. Moreover, fields in the response message  900  can be omitted or combined within the scope of the invention, e.g., combining status flags two  924  and three  926  into a single field. SPC-4 is further described in “SCSI Primary Commands—4 (SPC-4)”, Project T10/1731-D, Rev 11, INCITS (May 2007), which is hereby incorporated herein by reference in its entirety. 
     In an exemplary embodiment, the address header  906  is set to the same value as the value received by the control unit  110  in the TCCB that initiated the TCW channel program. Although the address header  906  is not required, including the address header  906  may support testing to trace command and response messages on an I/O device  112  while multiple I/O devices  112  are being accessed. 
     Status flags one  908  may indicate information such as the success status of an I/O operation. Multiple bits within the status flags one  908  can provide additional status information. 
     The maximum control unit exchange parameter  910  identifies the maximum number of exchanges that the control unit  110  allows the channel  124  to open to it. A value of zero may inform the channel  124  that the control unit  110  is not altering the current value that the channel  124  is using. In an exemplary embodiment, the channel  124  establishes a default value for the maximum number of open exchanges, e.g. 64, which the control unit  110  can modify via the maximum control unit exchange parameter  910 . The value of the maximum control unit exchange parameter  910  sent in the response message  900  may be the actual value desired or a seed value for an equation. For example, the value in the maximum control unit exchange parameter  910  can be incremented and/or multiplied by the channel  124  to determine the actual maximum number of open exchanges, e.g. a value of “1” interpreted as “32” by the channel  124 . 
     Using a default value for the maximum number of open exchanges gives each control unit  110  and channel  124  a common starting point that can be modified as determined by the control unit  110 . In one embodiment, the channel  124  checks the maximum control unit exchange parameter  910  received in the response message  900  from the control unit  110  to determine if the maximum control unit exchange parameter  910  is lower than the default value or a previously received value. If the new number is smaller than the current number of open exchanges, the channel  124  does not drive new I/O commands to the control unit  110  until the current number of exchanges used is less than the new limit. 
     In an exemplary embodiment, the response flags field  912  uses the standard definition as defined in FCP and can be set to default value, e.g., two. The response code  914  may be equivalent to a Small Computer System Interface (SCSI) status field and can be set to a default value, such as zero. The residual count  916  for read or write commands indicates the difference between how many bytes were commanded to be read or written versus the number of bytes that actually were read or written. The response length  918  is an additional count of bytes of information in the response message  900  after the reserved location  920 . The response length  918  supports variable sized response messages  900 . The SPC-4 sense type  922  can be assigned to a particular value based upon message type, e.g., a transport response IU=7F hexadecimal. In one embodiment, the status flags two  924  is set to a value of 80 hexadecimal to indicate that the I/O operation completed, with a valid value of the residual count  916 . Status flags three  926  is set to a value of one when the I/O operation completed, indicating that extended status  904  is included as part of the response message  900 . The device status  928  relays status information generated by the I/O device  112 . The LRC word  930  is a check word that covers the other fields in the status section  902  of the response message  900  to verify the integrity of the status section  902 . The LRC word  930  can be generated through applying an exclusive-or operation to an initial seed value with each field included in the LRC calculation in succession. 
     The extended status section  904  provides information to the channel subsystem  108  and the OS  103  associated with operating the control unit  110  in a transport mode capable of running a TCW channel program. The extended status section  904  may support configurable definitions with different type status definitions for each type. In an exemplary embodiment, the extended status section  904  includes a transport status header (TSH)  932 , a transport status area (TSA)  934 , and an LRC word  936  of the TSH  932  and the TSA  934 . The TSH  932  may include extended status length  940 , extended status flags  942 , a DCW offset  944 , a DCW residual count  946 , and a reserved location  948 . The TSH  932  is common for the different formats, with the each format defined by a type code in the extended status flags  942 . The TSA  934  may include a total device time parameter  950 , defer time parameter  952 , queue time parameter  954 , device busy time parameter  956 , device active only time parameter  958 , and appended device sense data  960 . Each of these fields is described in greater detail in turn. 
     The extended status length  940  is the size of the extended status section  904 . In an exemplary embodiment, the extended status flags  942  has the following definition: 
     Bit  0 —The DCW offset  944  is valid. 
     Bit  1 —The DCW residual count  946  is valid. 
     Bit  2 —This bit set to a one informs the OS  103  of  FIG. 1  in a definitive manner when the control unit  110  had to access slow media for data, e.g., a cache miss. 
     Bit  3 —Time parameters  950 - 958  are valid. The type code set to a one and this bit set to a one indicates that all or the time parameters  950 - 958  are valid. 
     Bit  4 —Reserved. 
     Bits  5  to  7 —These three bits are the type code that defines the format of the TSA  934  of the extended status section  904 . The names of the encodes are:
         0. Reserved.   1. I/O Status. The extended status section  904  contains valid ending status for the transport-mode I/O operation.   2. I/O Exception. The extended status section  904  contains information regarding termination of the transport-mode I/O operation due to an exception condition.   3. Interrogate Status. The extended status section  904  contains status for an interrogate operation.   4. to 7. Reserved.       

     The DCW offset  944  indicates an offset in the TCCB of a failed DCW. Similarly, the DCW residual count  946  indicates the residual byte count of a failed DCW (i.e., where execution of the DCWs was interrupted). 
     In an exemplary embodiment, the TSA  934  definition when the type code of ES flags  942  indicates a type of I/O Status includes time parameters  950 - 958 , as well as optionally appended device sense data  960 . The time parameters  950 - 958  represent time values and can be scaled to any time units, such as microseconds. The CU timers  806  of  FIG. 8  are used to calculate the time parameters  950 - 958 , and the CU registers  808  can also be employed to capture values of the CU timers  806  on a triggering event. 
     The total device time parameter  950  is the elapsed time from when the control unit  110  received the transport command IU until it sent the transport response IU (i.e., response message  900 ) for the I/O operation. The defer time parameter  952  indicates control unit defer time. This is the time accumulated by the control unit  110  working with the I/O device  112  when no communication with the channel  124  is performed. On CCW channel programs, such as that depicted in  FIG. 3 , the control unit  302  disconnects from the channel  300  during this time. 
     The queue time parameter  954  is the time that an I/O operation is queued at the control unit  110 , but does not include queue time for device busy time where the I/O device  112  is reserved by another channel  124  under control of a different OS  103  on the same system or on another system. The device busy time parameter  956  is the time that a transport command IU is queued at the control unit  110  waiting on a device busy caused by the I/O device  112  being reserved by another channel  124  under control of a different OS  103  on the same system or on another system. 
     The device active only time parameter  958  is the elapsed time between a channel end (CE) and a device end (DE) at the control unit  110 , when the control unit  110  holds the CE until DE is available. The CE may indicate that the portion of an I/O operation involving a transfer of data or control information between the channel  124  and the control unit  110  has been completed. The DE may indicate that the device portion of an I/O operation is completed. The appended device sense data  960  is supplemental status that the control unit  110  provides conditionally in response to an active unit check (UC) bit in the device status  928 . 
     The LRC word  936  is a longitudinal redundancy check word of the TSH  932  and the TSA  934 , calculated in a similar fashion as the LRC word  930  in the status  902  section of the response message  900 . The LRC word  936  can be calculated on a variable number of words, depending upon the number of words included in the appended device sense data  960 . 
       FIG. 10  depicts open exchange limiting logic  1000 , which may be implemented as part of the channel subsystem  108  of  FIG. 8 . In an exemplary embodiment, the functionality depicted in open exchange limiting logic  1000  is repeated for each control unit  110  with which the channel subsystem  108  communicates. A default exchange maximum (DEFAULT_EX_MAX)  1002  and a received maximum control unit exchange value (MAX_CU_EXCHANGE)  1004  are input into limit logic  1006 . The DEFAULT_EX_MAX  1002  is a default value representing the maximum number of open exchanges to a control unit. The DEFAULT_EX_MAX  1002  may be a field or a full register value in the CHN subsystem registers  814 , establishing a default value for all control units  110  connected to the channel subsystem  108 . For example, the DEFAULT_EX_MAX  1002  may default to a value of 64, indicating that the maximum number of open exchanges to each control unit  110  is 64 until control units  110  provide actual values as MAX_CU_EXCHANGE  1004 . The MAX_CU_EXCHANGE  1004  is a local copy of the maximum control unit exchange parameter  910  as extracted from the status section  902  of the response message  900  of  FIG. 9 . 
     In an exemplary embodiment, the limit logic  1006  examines the DEFAULT_EX_MAX  1002  and the MAX_CU_EXCHANGE  1004  to produce a limit value output (LIM_OUT)  1008 . The limit logic  1006  initially outputs the LIM_OUT  1008  as function of the DEFAULT_EX_MAX  1002 , either as a direct or scaled value of the DEFAULT_EX_MAX  1002 , until a value for the MAX_CU_EXCHANGE  1004  is received. When the MAX_CU_EXCHANGE  1004  is received with a value of zero, the limit logic  1006  does not modify LIM_OUT  1008 , retaining the same output value. However, if the MAX_CU_EXCHANGE  1004  is updated in response to receiving a new value of the maximum control unit exchange parameter  910  of  FIG. 9 , then LIM_OUT  1008  is updated correspondingly. The limit logic  1006  may also perform scaling of the DEFAULT_EX_MAX  1002  and/or the MAX_CU_EXCHANGE  1004  prior to outputting the LIM_OUT  1008 . For example, the MAX_CU_EXCHANGE  1004  can be incremented by one and multiplied by sixteen ((MAX_CU_EXCHANGE+1)*16) to produce LIM_OUT  1008 . Such a scaled approach consumes fewer bits during transmission (e.g., maximum control unit exchange parameter  910 ), but results in a wider range of limit values for the LIM_OUT  1008 . 
     The LIM_OUT  1008  is input into a compare block  1010  to perform a comparison relative to the number of exchanges presently open (NUM_OPEN_EXCHANGES)  1012 . The NUM_OPEN_EXCHANGES  1012  may be a field or dedicated register in the CHN subsystem registers  814  indicating the number of exchanges currently open between the channel subsystem  108  and an associated control unit  110  which provided the maximum control unit exchange parameter  910 . The compare block  1010  drives an enable signal  1014  that enables or disables an open exchange function  1016 . The open exchange function  1016  determines whether a new exchange can be opened in response to a new I/O command (NEW_I/O_CMD)  1018 , such as a TCCB in a transport command IU. If the open exchange function  1016  is enabled by the enable signal  1014 , then the NEW_I/O_CMD  1018  is output on a new exchange as a new I/O command output (NEW_I/O_CMD_OUT)  1020 , resulting in a TCCB being sent to the associated control unit  110  on the newly opened exchange. Thus, the compare block  1010  establishes a maximum number of open exchanges by preventing the opening of a new exchange in response to the number of open exchanges being less than the currently established limit (NUM_OPEN_EXCHANGES  1012 &lt;LIM_OUT  1008 ). When the compare block  1010  results in a “false” condition, the enable signal  1014  is disabled, preventing the opening of additional exchanges at the open exchange function  1016  until the NUM_OPEN_EXCHANGES  1012  is reduced to a value less than LIM_OUT  1008 . 
     Turning now to  FIG. 11 , a process  1100  for limiting the number of open exchanges at the channel subsystem  108  of the I/O processing system  100  using data from the control unit  110  will now be described in accordance with exemplary embodiments, and in reference to the I/O processing system  100  of  FIG. 1 . At block  1102 , the channel subsystem  108  sends a command message to the control unit  110 . The command message may be a transport command IU, including a TCCB with multiple DCWs as part of a TCW channel program. The control unit  110  receives the command message, parses it, and initiates I/O operations as commanded in the DCWs to the I/O device  112 . Upon termination of the TCW channel program on the control unit  110 , the control unit  110  reports status information to the channel subsystem  108  in a transport response IU message (e.g., response message  900  of  FIG. 9 , including a status section  902 ). The transport response IU message includes maximum control unit exchange parameter  910  of  FIG. 9 . In an alternate exemplary embodiment, the control unit  110  outputs the transport response IU message when the command message was a message other than a transport command IU including a TCCB with multiple DCWs as part of a TCW channel program. 
     At block  1104 , the channel subsystem  108  receives the transport response IU message in response to sending the command message to the control unit  110 . Communication between the channel subsystem  108  and the control unit  110  may be managed by the CU control logic  802  and the CHN control logic  810  of  FIG. 8  for a specific channel  124  of the channel subsystem  108 . 
     At block  1106 , the channel subsystem  108  extracts the maximum control unit exchange parameter  910  from the transport response information unit message as populated by the control unit  110 . For example, the maximum control unit exchange parameter  910  may be extracted from the status section  902  of the response message  900  of  FIG. 9 , where the response message  900  depicts one embodiment of the transport response IU message. 
     At block  1108 , the channel subsystem  108  determines a limit value for a maximum number of open exchanges between the channel subsystem  108  and the control unit  110  at the channel subsystem  108  as a function of the extracted maximum control unit exchange parameter  910 . The limit value is initially set to a default value, such as sixty-four, and then modified in response to receiving and extracting the maximum control unit exchange parameter  910 . For example, the limit value may be LIM_OUT  1008  of  FIG. 10  as determined by the limit logic  1006 , with a default value of DEFAULT_EX_MAX  1002  and MAX_CU_EXCHANGE  1004  set equal to the extracted maximum control unit exchange parameter  910 . The function used to calculate the limit value may be a combination of multiple mathematical operations, such as incrementing by an offset value and multiplying by a scalar value (e.g., add one, then multiply by sixteen). 
     At block  1110 , the channel subsystem  108  applies the limit value to prevent a new exchange from opening. Through comparing the number of exchanges currently open at the channel subsystem  108  (e.g., NUM_OPEN_EXCHANGES  1012  of  FIG. 10 ) to the limit value (e.g., LIM_OUT  1008  of  FIG. 10 ), the channel subsystem  108  can determine whether a new exchange can be opened to send a new I/O command to the control unit  110 , or if the channel subsystem  108  must wait for one or more open exchanges to close. The determination may be performed using the open exchange function  1016  in conjunction with the enable signal  1014  as output from the compare block  1010  of  FIG. 10 . When the open exchange function  1016  is disabled, transmission of a new I/O command (e.g., transport command IU with a TCCB) on a new exchange is blocked until an exchange is available to open. 
     Technical effects of exemplary embodiments include limiting the number of open exchanges at a channel subsystem of an I/O processing system using data from a control unit. The limit is provided in a response message from the control unit. The channel subsystem receiving the limit can prevent attempts at the channel subsystem from exceeding the capability of the control unit. Advantages include limiting the number of open exchanges without interrupting the execution of a TCW channel program on a control unit. Applying the limit at the channel subsystem may prevent the control unit from experiencing error conditions associated with receiving requests to open a greater number of exchanges than the control unit can handle. Avoiding such error conditions can improve communication and processing throughput for the I/O processing system, as new I/O commands are held off until exchanges are available to communicate the new I/O commands. Further advantages include support for various levels of complexity at different control units rather than applying a static limit for all control units. 
     As described above, embodiments can be embodied in the form of computer-implemented processes and apparatuses for practicing those processes. In exemplary embodiments, the invention is embodied in computer program code executed by one or more network elements. Embodiments include a computer program product  1200  as depicted in  FIG. 12  on a computer usable medium  1202  with computer program code logic  1204  containing instructions embodied in tangible media as an article of manufacture. Exemplary articles of manufacture for computer usable medium  1202  may include floppy diskettes, CD-ROMs, hard drives, universal serial bus (USB) flash drives, or any other computer-readable storage medium, wherein, when the computer program code logic  1204  is loaded into and executed by a computer, the computer becomes an apparatus for practicing the invention. Embodiments include computer program code logic  1204 , for example, whether stored in a storage medium, loaded into and/or executed by a computer, or transmitted over some transmission medium, such as over electrical wiring or cabling, through fiber optics, or via electromagnetic radiation, wherein, when the computer program code logic  1204  is loaded into and executed by a computer, the computer becomes an apparatus for practicing the invention. When implemented on a general-purpose microprocessor, the computer program code logic  1204  segments configure the microprocessor to create specific logic circuits. 
     While the invention has been described with reference to exemplary embodiments, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the scope of the invention. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the invention without departing from the essential scope thereof. Therefore, it is intended that the invention not be limited to the particular embodiment disclosed as the best mode contemplated for carrying out this invention, but that the invention will include all embodiments falling within the scope of the appended claims. Moreover, the use of the terms first, second, etc. do not denote any order or importance, but rather the terms first, second, etc. are used to distinguish one element from another. Furthermore, the use of the terms a, an, etc. do not denote a limitation of quantity, but rather denote the presence of at least one of the referenced item.