Patent Publication Number: US-8127128-B2

Title: Synchronization of swappable module in modular system

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates in general to computers, and more particularly to a method, system, and computer program product for synchronization of swappable modules in modular systems, such as a computer storage or processor system. 
     2. Description of the Related Art 
     In existing modular systems, many components may be replaceable or swappable. However in many modular systems, component modules are often initialized with data that links the component modules to the given system. Thus when the component is moved to a different modular system, the component retains the identity of the previous system. As an example, in the IBM® BladeCenter® blade server system, the Advanced Management Module (AMM) will write data in a history log persistently stored on a swappable component such as a switch or blade server. This write by the AMM identifies which chassis the switch or blade has been associated with in past operation. 
     SUMMARY OF THE INVENTION 
     While the ability to retain identification information for swappable components between modular systems may be acceptable in most cases, there are systems where for proper operation of the swappable module cannot retain a past identity. For instance, returning to the BladeCenter® system, in a RAIDed SAS Switch Module (RSSM), persistent data is initialized at a genesis startup phase that is required for the switch to operate. However, this data links the switch with the BladeCenter® chassis the switch first starts in. If the switch is moved with this persistent data and initialized into another chassis, the switch will not operate properly. 
     In view of the foregoing, a need exists for a mechanism by which synchronization of persistent data is achieved, so that components such as a RSSM may be interchangeable between modular systems, and yet operate consistently. Accordingly, in one embodiment, by way of example only, a method for synchronizing a swappable module between storage systems is provided. The storage systems have dual controllers and distributed copies of states. Upon an insertion of the swappable module in a storage system, a plurality of storage components are queried, including a partner swappable module, to determine if a quorum of identification information is present. If the quorum is present, at least one of the plurality of storage components having non-matching identification information is overwritten with the identification information of the quorum. If the quorum is not present, a reset to default procedure is performed. The reset to default procedure designates at least as many storage components of the plurality of storage components with the identification information sufficient to constitute the quorum. 
     In another embodiment, again by way of example only, an additional exemplary embodiment for synchronizing a swappable module between storage systems is provided. Again, the storage systems have dual controllers and distributed copies of states. Upon an insertion of the swappable module in a storage system, a plurality of storage components, including a partner swappable module, is queried to determine if a quorum of identification information is present. If the quorum is present, at least one of the plurality of storage components having non-matching identification information is overwritten with the identification information of the quorum. If the quorum is not present, a reset to default procedure is performed. The reset to default procedure is performed pursuant to a selection of bays in the storage system by a user. If the user selects the reset to default procedure pursuant to a first bay, identification information of one of the plurality of the storage components is used to designate the identification information of the quorum for a remainder of the plurality of the storage components. If the user selects the reset to default procedure pursuant to a second bay, persistent identification information stored in the storage system to is used designate the identification information of the quorum for each of the plurality of the storage components. 
     Related system and computer program product embodiments are also disclosed and provide additional advantages. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       In order that the advantages of the invention will be readily understood, a more particular description of the invention briefly described above will be rendered by reference to specific embodiments that are illustrated in the appended drawings. Understanding that these drawings depict only exemplary embodiments of the invention and are not therefore to be considered to be limiting of its scope, the invention will be described and explained with additional specificity and detail through the use of the accompanying drawings, in which: 
         FIG. 1  is an exemplary multi-blade server chassis in which various aspects of the following description and claimed subject matter may be implemented; and 
         FIG. 2  is a flow chart diagram of an exemplary method for synchronizing a swappable module in a modular system; 
         FIG. 3A  is a flow chart diagram of an additional exemplary method for synchronizing a swappable module in a modular system; and 
         FIG. 3B  is a flow chart diagram of an additional exemplary method for synchronizing a swappable module in a modular system. 
     
    
    
     DETAILED DESCRIPTION OF THE DRAWINGS 
     The illustrated embodiments below provide mechanisms for synchronizing swappable module components, such as the RSSM switch previously described, between modular systems such as computer processing and computer storage systems. These mechanisms may leverage other components of the modular system to determine if a quorum of identification information if present using various techniques as will be described. In this way, the swappable modular component can take on the identity that the modular system defines for that component. 
       FIG. 1  is an exemplary block diagram of an exemplary modular system in which various aspects of the present invention may be implemented. Computing environment  100  has several hosts with access to a multi-blade server chassis. For the sake of clarity, three hosts  110   a,b,n  are depicted. However, additional hosts may be associated with the chassis as one skilled in the art will appreciate. Hosts  110   a,b,n  are connected through a network fabric  120  to a multi-blade server blade chassis  200   a . Again, for the sake of clarity, only three server blades  204   a,b,n  are depicted. However, in one embodiment, server blade chassis  200   a  has a midplane  206  capable of connecting fourteen or more server blades  204 . 
     Server blade chassis  200   a  has one or more advanced management modules (AMM)  202 . In the depicted embodiment, server blade chassis  200   a  has a primary advanced management module  202   a  and a back-up advanced management module  202   b . Each management module  202  is capable of managing multiple server blades  204 . During normal operations, one of the local management modules  202   a  or  202   b  are coupled to server blades  204   a - n  via a Local Area Network (LAN)  240   a , a midplane  206 , and a plurality of Baseboard Management Controllers (BMCs)  208  (each server blade  204  having a BMC  208 ) to form an in-band management pathway. LAN  240  and BMC  208  are discussed in further detail below. 
     Midplane  206  is a backplane, mounted in the middle of server blade chassis  200   a , that contains circuitry and sockets  222  into which additional electronic devices or cards, including server blades  204  may be inserted. Midplane  206  contains at least one bus for secure in-band internal communication between management module  202  and server blades  204   a - n , as well as between and among server blades  204   a - n  themselves, via respective BMCs  208   a - n.    
     When a server blade  204  is inserted into a specific socket  222 , a physical address is established for that server blade  204 . For example, consider server blade  204   a  being inserted into socket  222   a . A control logic  224   a  detects the presence of server blade  204   a  in socket  222   a . Logic  224   a  may comport with the Electronics Industry Association (EIA) RS485 Standard for data communication. In other embodiments, Logic  224   a  may be compliant with the Phillips&#39; Inter-IC (Inter-Integrated Circuit) standard (incorporated by reference in its entirety herein and commonly referred to as “I 2 C”), or with an Ethernet network standard. Logic  224   a , operating in conjunction with management module  202 , assigns a physical address on a bus in midplane  206  to server blade  204   a  when server blade  204   a  is inserted into socket  222   a . Each server blade  204  may be associated with a unique logic  224  that is connected to midplane  206  as depicted in  FIG. 2   a . Alternatively, all server blades  204  may use a single logic  224 . 
     Each server blade  204  may have a unique Internet Protocol (IP) address on midplane  206 . That is, midplane  206  may support intercommunication using IP addressing protocol, in which each device connected or coupled to midplane  206  contains an IP address assigned by logic (not shown) that is either within or outside server blade chassis  200 . For example, a Dynamic Host Configuration Protocol (DHCP) server may be used to assign an IP address to server blade  204   a . Communication with server blade  204   a  is thereafter via a Network Interface Card (NIC)  226   a  that is associated with server blade  204   a.    
     In accordance with the illustrated embodiment, an I/O module  242   a  is connected to NIC  226   a . Module  242   a  may be used in pairs (e.g., module  242   b ) to provide redundancy. I/O module  242   a  includes an integrated switch module  244   a , such as a serial attached SCSI (SAS) switch module. Switch modules  242   a ,  242   b  provide connectivity to Ethernet or SAS, for example. RAID controllers  246   a  and  246   b  are incorporated into the I/O modules  242   a  and  242   b . The RAID controllers  246   a ,  246   b  do not take up a blade slot. RAID controller  246   a  is interconnected to RAID devices, such as storage devices in a RAID configuration. The RAID devices located within one or more blades  204 . The RAID controllers  246   a ,  246   b  and attached RAID devices may collectively be thought of as a RAID subsystem of the server blade chassis. 
     A baseboard management controller (BMC)  248   a  is also integrated into the I/O module  242   a . BMC  248   a  may be adapted to store IP addresses of various components of chassis  200   a  in several locations. A copy may be stored in a persistent storage location of each switch module  244   a . A copy may be stored in a persistent storage location of RAID controller  246   a . A copy may be stored in persistent storage of a media tray (not shown). Similarly, a copy may be stored in switch module  244   b  and RAID controller  246   b . The BMC  248   a , in cooperation with the I/O module  242   a , controls the process of synchronizing the various copies. In addition to controlling synchronization of component addresses, the BMC  248   a , and I/O module  242   a  may be adapted to perform additional functionality as will be described, following. 
     Each server blade  204  may have at least one central processing unit (CPU)  212 , and a non-volatile memory (NVM)  214 . NVM  214  is a Flash Read Only Memory (“Flash ROM” or “Flash Memory”) that can be erased and reprogrammed in units of memory referred to as “blocks.” NVM  214  may also include non-volatile Electrically Erasable Programmable Read Only Memory (EEPROM) that is similar to Flash Memory, except that EEPROM is erased and rewritten at the byte level and is usually smaller in capacity. The server blade  204  may be oriented as a storage blade (with a number of integrated storage devices such as disk drives) or a processor blade (with one or more processing devices) for performing computing processing. 
     When a server blade  204  is shipped from a manufacturer, the NVM  214  may be pre-burned with firmware, including a BIOS as well as software for monitoring the server blade  204 . Such monitoring may include controlling Direct Access Storage Devices (DASD&#39;s), monitoring and controlling voltages throughout the system, determining the power-on status of the server blade  204 , requesting access to a shared keyboard, video, mouse, Compact Disk-Read Only Memory (CD-ROM) and/or floppy disk drives, as well as monitoring the Operating System (OS) running on the server blade  204 . 
     Advanced management modules  202  are capable of detecting the presence, quantity, type and revision level of each server blade  204 , power module  210 , and midplane  206  in the system. Management modules  202  may also directly control the operation of each server blade  204  and the power module  210 , and may directly (without using the BIOS in the server blades  204 ) or indirectly (using the BIOS) control the operation of cooling fans  215  and other chassis  200   a  components. 
     Each server blade  204  has a BMC  208  that provides local supervisory control of the server blade  204  to which the BMC  208  is associated. Each BMC  208  is able to communicate with a local management module  202  by either using communication path  240   a  (in-band network) or alternatively by using switches  242   a  and NICs  226  (out-of-band network). The local management modules  202   a ,  202   b  may utilize a variety of communications paths  240   a , such as an RS485 path  240   a , a LAN path  240   a  and an I 2 C path  240   a  to communicate with each blade  204 . 
     LAN  240  is an in-band network also comporting with the Electronics Industry Association (EIA) RS485 Standard for data communication. Management modules  202  (either primary management module  202   a  or back-up management module  202   b  if management module  202   a  is down) communicate via LAN  240  with BMC  208 , which includes logic for coordinating communication with server blades  204  via sockets  222 . 
     LAN  240   a  may be configured to allow communications between server blades  204   a - n  and the management modules  202   a ,  202   b  relating to the remote BIOS settings and BIOS management. The blades  204   a - n  may leverage BMCs  208   a - n  as proxies to communicate with the management modules  202   a ,  202   b  through the RS485 protocol. Similarly, the management modules may leverage BMCs  208   a - n  as proxies to communicate with the blades  204   a - n  through the RS485 protocol. In an alternative embodiment, an RS485 connection may be separately made between each blade  204   a - n  and the management modules  202   a ,  202   b . Additionally, other communications protocols and paths may be utilized, such as the aforementioned I 2 C channel or the aforementioned TCP/IP and/or Ethernet channel over switches  244 . 
     Chassis  200   a  may follow a predetermined policy upon setup in which addresses of various components in the chassis  200   a  are propagated throughout. For example, in one embodiment, the AMM  202  notifies the switch module  244  of any IP address changes. The switch module  244  in turn notifies the BMC  248 . Once the BMC  248  is notified of an IP address change, the BMC  248  queries the IP address information from the switch persistent storage, and updates the component IP address list in another area of the switch persistent storage. The BMC  248  also updates the component IP address list in an area of persistent storage resident in the RAID controller  246 . The BMC  248  then notifies the partner I/O module BMC  248  to update its component IP address list. The partner BMC  248  then updates the persistent storage in its associated switch module  244  and RAID controller  246 . 
     In chassis  200   a , both switch modules  244   a  and  244   b  have access to the management modules  202   a  and  202   b , including information stored within the modules  202   a  and  202   b . The RAID subsystem, including RAID controllers  246   a  and  246   b , does not have this access. Since the RAID subsystem is managed via a different interface, the subsystem has no way of knowing which chassis the subsystem belongs to, which switch  244   a  or  244   b  it is packaged with, or what other switches are in the same chassis  200   a . The relationship among switches  244   a  and  244   b  and their respective RAID subsystems is necessary in order to configure host access to the RAID subsystem, and to perform service and maintenance operations. 
     To simplify the user experience in managing chassis  200   a , an API may be implemented as previously described, that allows switches  244  to access and persist all network information, such as port IP addresses of the devices. For example, in one embodiment, the API may be operational as software, firmware, hardware, or a combination thereof operable on a particular blade  204 . In this way, CPU  212  and NVM  214  may be utilized to execute and store processing instructions relating to the operation of the API. The API may be configured to maintain a relationship between the management modules  202  and switches  244 . As a result, the API may be adapted to determine information such as port IP addresses from the switches  244  and provide the information as an intermediary to other chassis  200   a  components, such as the BMCs  248 . Exemplary functionality of the API will be later described in the context of executing an exemplary reset to default operation. 
     The API may be adapted to query the switch(es)  244  for the address or other information, based on initial information provided by a user, such as an initial address. For example, in one embodiment, based on an IP address provided by a user, the API may then query switch(es)  244  for additional addresses of additional components (e.g., remaining 3 IP addresses). The switches  244  may obtain the information from their persistent storage, this storage having been populated by BMCs  248 . The API may then validate the information, perhaps notifying the user the additional addresses and that the addresses are valid. The API may then persistently store the addresses in locations accessible by the BMCs  248 , the switches  244 , and the RAID controllers  246 . 
     In the depicted embodiment, switches  244  may be considered swappable modular components that may be moved from one chassis  200   a  (modular system) to another. In a genesis phase of operation, in which all the elements of the chassis  200   a  are newly initialized, one of the BMCs  248 , such as BMC  248   a  depicted in module  242   a  is the master of the initialization process. In server blade environments, the BMCs  248  may be associated with a particular bay. For example, BMC  248   a  may be associated with bay  3  of a particular BladeCenter® chassis, while BMC  248   b  may be associated with bay  4  of the chassis. During the genesis phase in such an implementation, BMC  248  copies factory persistent vital product data (VPD) (herein also referred to generically as “identification information”) to the media tray (not shown). This VPD data includes key fields that distinctly identify the system, such as a license key, machine signature, and worldwide name (WWN) information. Again, this information is referred to herein as identification information. 
     In one embodiment, after the master BMC has initialized the media tray and set its component state to installed, the BMC then copies the machine identity fields to its persistent memory location previously described, such as an electrically-erasable programmable read-only memory device (EEPROM) in local communication with the BMC. The BMC then sets its local component state to installed, and instructs a partner BMC associated with a differing bay, such as bay  4 , to begin the synchronization process for the remaining components in the chassis. The BMC in bay  4  then copies from the media tray the machine identity to its own local memory, and sets its state to installed. At this point, the genesis synchronization phase is complete. 
     To provide synchronization functionality to swappable components such as switches  244 , exemplary methodologies are now described. These methodologies apply when the swappable component is placed into a new modular environment, such as a new chassis  200   a . While the following methodologies continue the exemplary implementation in BladeCenter® server blade environments, the skilled artisan will appreciate that the same methodologies may be applied in, and tailored to, a variety of situations involving swappable modular components in a variety of modular systems. In one of the exemplary methodologies described, following, the switches (RSSM modules) serve as the swappable modular component in a modular system including a chassis having dual controllers and shared copies of states. However, the skilled artisan will appreciate that additional modular components in modular server blade environments may serve as swappable modular components in other implementations. 
     As a first step in the exemplary methodology described above, when an RSSM is inserted into a new chassis, the RSSM will query the media tray to determine if the media tray&#39;s component state is installed. If the BMC determines that it is installed, it will validate that the machine signature of the media tray matches the local BMC. If a mismatch is determined (which should occur if the RSSM is moving between chassis), the RSSM will then query a partner RSSM (in the opposing I/O module) if the partner RSSM matches the media tray. If a match is found, the RSSM determines that it (the local) is not in sync with the system identity. It will then overwrite its local machine identity with the system identity from the media tray. With this operation, an RSSM can move to any chassis with another RSSM already installed and will take on the identity of the system. 
     If the RSSM determines that the partner RSSM and the media tray do not match signatures, then the RSSM will query if the local and the partner match signatures. If the local and partner match, the local RSSM will overwrite the media tray machine identity with the factory installed persistent data of the local RSSM. In this case, the two RSSMs override the system identity that was previously there. Thus if two RSSMs in sync are moved from one system to another chassis and inserted at virtually the same time, the identity of the new system will be overwritten. This is by design and is a way that enables moving the identity of one system to a new system. 
     In one embodiment, a purpose of the foregoing identity transfer mechanism is to support when a media tray is replaced. To the RSSM BMC, replacement of the media tray looks the same as if both RSSM switches are moved to a new chassis. Thus a mechanism is included in the synchronization process that supports the replacement of a media tray. 
     Turning to  FIG. 2 , following, an exemplary method  200  for synchronizing a swappable module in a modular system (again in this case, an RSSM switch between storage systems) is illustrated. Method  200  employs various techniques to determine if storage components match a quorum of identification information in the system. If a match is not determined, for example, a component may then overwrite non-matching identification information with that of the quorum of identification information. These techniques will be further described, below. 
     As one skilled in the art will appreciate, various steps in the method  200  (as well as in the following exemplary methods later described) may be implemented in differing ways to suit a particular application. In addition, the described methods may be implemented by various means, such as hardware, software, firmware, or a combination thereof operational on or otherwise associated with the storage environment. For example, a method may be implemented, partially or wholly, as a computer program product including a computer-readable storage medium having computer-readable program code portions stored therein. The computer-readable storage medium may include disk drives, flash memory, digital versatile disks (DVDs), compact disks (CDs), and other types of storage mediums. 
     Method  200  begins (step  202 ) with an RSSM BMC exiting a reset, indicating a switch insertion. The RSSM queries the media tray component state (step  204 ). If the media tray is determined not to be initialized (step  206 ), the master BMC (such as in bay  3 ) overwrites the media tray with the local machine identification information (step  208 ). If the media tray is determined to be initialized, the RSSM queries the BMC to determine if the local setting is initialized (step  210 ). If no, the RSSM receives the identification from the media tray and updates the local machine identity, resulting in a signature match in the next step (step  212 ). 
     In the next step, the RSSM queries the media tray to determine if the tray&#39;s identification signature matches the local RSSM (step  214 ). If the local RSSM and the media tray match (step  216 ), then the method  200  ends (step  230 ). If the local RSSM and media tray do not match (again, step  216 ), then the RSSM queries a partner RSSM (in the opposing I/O module) to determine if the partner RSSM and the media tray match (step  218 ). If a match is found (step  220 ), then the local RSSM is out of sync with the system. The RSSM then overwrites the local machine identity with that of the media tray (step  222 ) to match the quorum of identification information of the system, and the method  200  ends (again, step  230 ). 
     If the identification information of the partner RSSM and the media tray do not match (again, step  220 ), the RSSM queries if it and the partner RSSM match identity (step  224 ). If the local RSSM and the partner RSSM match (step  226 ), then the identification information of the media tray is out of sync, implying that a new media tray has been inserted. In this case, the RSSM in bay  3  overwrites the identification of the media tray to match the quorum of identification information in the system (step  228 ). 
     Returning to step  226 , if the identification information of the local RSSM and partner RSSM do not match, then a situation is presented in which three components (local RSSM, partner RSSM, and media tray) have a differing view of what the quorum of identification information should be. Since there is no majority quorum in this case, it is not possible to proceed with synchronization as previously described. In this case, the components in the modular system are not usable since they will not start up properly. A mechanism is necessary that enables the user to move past a state where the identification is not common among at least a quorum of the modular components. In this case, a reset to default procedure is performed (step  232 ), and the method  200  ends (again, step  230 ). The reset to default procedure of step  232  allows for component synchronization in situations is more fully detailed in the following exemplary methods. 
     In one embodiment, and in accordance with the RSSM example described in  FIG. 2 , the reset to default has two paths that can be executed by the user. The user may choose to reset the switches only such that the media tray machine identity takes precedence, or the user can choose to reset all three elements. In this case, the system takes on the characteristics of a genesis system where all elements are new and the RSSM in bay  3  will propagate its local system machine identity to the rest of the system. 
     In one embodiment, the advanced management module (AMM) along with the API previously described is used to initiate a reset to default. The AMM provides an interface where a user can select an item such as a switch and reset it to default. For the RSSMs, the user is instructed that a reset to default to bay  3  will reset each component, whereas a reset to bay  4  will only reset the switches as related to the machine identity. Because the reset will potentially change the machine identity the switch currently has locally, both switches are required to be powered off; otherwise the reset is ignored and no change in machine identity will take place. 
     If the user elects to reset bay  4 , the reset signal is only seen by bay  4 . The BMC on bay  4  will inform bay  3  that the reset signal was received. Bay  3  will then ensure that both switches are powered off before proceeding with the reset to default. Bay  3  will inform bay  4  it is safe to proceed after this check (if the check clears) and both BMCs will then erase their local machine identity. They will then copy the machine identity of the media tray locally and this will enable the system to begin operating normally. 
     If the user elects to reset bay  3 , bay  3  sees the reset only. The BMC on bay  3  will query to see that both switches are powered off and then proceed (if the check clears). Bay  3  will then erase the media tray data. It will then write the new media tray machine identity using its local system persistent data. It then instructs bay  4  to proceed and both  3  and  4  proceed as described in the case where only the switches are reset. 
     Notably, if both switches receive the reset to default signal, the method will execute as if only bay  3  identified the signal where everything will get reset. After the reset to default has taken place (either type), the system is now synchronized and ready for normal operation. VPD is consistent across all elements. An alert mechanism may accompany a reset to default procedure to inform the user. The user may be instructed, such as by documentation, that when this alert is received a reset to default is required. 
     Methods  300  and  350  in  FIGS. 3A and 3B , following, describe exemplary procedures that restore factory defaults in modular components such that the modular components may attain a common VPD state as previously described. Once the modular components are restored to default, they may then proceed to synchronize as necessary such that the modular system will properly operate. 
     Turning first to  FIG. 3A , method  300  begins (step  302 ) with the user initiating a reset to default via the AMM (step  304 ) as a follow up to step  232  ( FIG. 2 ). The AMM then sends a reset signal to the RSSM the user selected for reset (based on bay selection) (step  306 ). If the switch on bay  3  is not reset (step  308 ), then bay  4  sends the reset request to bay  3 , and the process of reset continues as depicted in  FIG. 3B . If the switch on bay  3  is reset (again, step  308 ), then bay  3  queries if both bay  3  and  4  are powered off as previously described (step  312 ). If both switches are not off, then the method  300  ends (step  320 ) and must be reinitiated when the switches have been powered off. 
     If the switches are determined to be both off (again, step  314 ), then bay  3  determines the type of reset to perform based on the switch that received the reset signal (step  316 ). Bay  3  then performs the respective reset type and informs bay  4  then the reset is complete (step  318 ). 
     Turning now to  FIG. 3B , method  350  continues the process first described in  FIG. 3A  where the user selects a reset to default pursuant to bay  3 . Method  350  begins (step  352 ), with a query as to whether the switch in bay  3  was reset (step  354 ). If not, the method skips to step  360 , where bay  3  informs bay  4  it may proceed with the reset. If bay  3  did get reset (again, step  354 ), the reset type is a complete reset, with bay  3  overwriting the media tray with factory default identification information (step  356 ). Bay  3  then writes its system persistent machine identify to the media tray (step  358 ), and informs bay  4  that it may proceed (again, step  360 ). 
     As a following step, the switches in both bays  3  and  4  reset the local EEPROM machine identity with blank identification data (step  362 ). Both switches then obtain machine identity information from the media tray, and update their local identification information (step  364 ). The method  350  then ends (step  366 ). 
     The reset to default procedures described in  FIGS. 3A and 3B  may also be operable in a similar fashion in situations where a RSSM module is not present. In the event that one switch is present, and the information in the switch and media tray are mismatching, a quorum is not identified. In such a scenario, the user may be allowed to perform a reset to default. In this situation, if the single identified switch is associated with bay  4 , the local VPD is overwritten in favor of the media tray. Conversely, if the single identified switch is associated with bay  3 , both media tray and local switch VPD is replaced by the factory installed VPD of the switch associated with bay  3 . 
     Some of the functional units described in this specification have been labeled as modules in order to more particularly emphasize their implementation independence. For example, a module may be implemented as a hardware circuit comprising custom VLSI circuits or gate arrays, off-the-shelf semiconductors such as logic chips, transistors, or other discrete components. A module may also be implemented in programmable hardware devices such as field programmable gate arrays, programmable array logic, programmable logic devices, or the like. 
     Modules may also be implemented in software for execution by various types of processors. An identified module of executable code may, for instance, comprise one or more physical or logical blocks of computer instructions which may, for instance, be organized as an object, procedure, or function. Nevertheless, the executables of an identified module need not be physically located together, but may comprise disparate instructions stored in different locations which, when joined logically together, comprise the module and achieve the stated purpose for the module. 
     Indeed, a module of executable code may be a single instruction, or many instructions, and may even be distributed over several different code segments, among different programs, and across several memory devices. Similarly, operational data may be identified and illustrated herein within modules, and may be embodied in any suitable form and organized within any suitable type of data structure. The operational data may be collected as a single data set, or may be distributed over different locations including over different storage devices, and may exist, at least partially, as electronic signals on a system or network. 
     While one or more embodiments of the present invention have been illustrated in detail, the skilled artisan will appreciate that modifications and adaptations to those embodiments may be made without departing from the scope of the present invention as set forth in the following claims.