Abstract:
A system including a storage processing device with an input/output module. The input/output module has port processors to receive and transmit network traffic. The input/output module also has a switch connecting the port processors. Each port processor categorizes the network traffic as fast path network traffic or control path network traffic. The switch routes fast path network traffic from an ingress port processor to a specified egress port processor. The storage processing device also includes a control module to process the control path network traffic received from the ingress port processor. The control module routes processed control path network traffic to the switch for routing to a defined egress port processor. The control module is connected to the input/output module. The input/output module and the control module are configured to interactively support data virtualization, data migration, data replication, and snapshotting. The distributed control and data path processors achieve scaling of storage network software. The storage processors provide line-speed processing of storage data using a rich set of storage-optimized hardware acceleration engines. The multi-protocol switching fabric provides a low-latency, protocol-neutral interconnect that integrally links all components with any-to-any non-blocking throughput.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]    This application claims the benefit under 35 U.S.C. § 119(e) of U.S. Provisional Patent Application Serial No. 60/393,017 entitled “Apparatus and Method for Storage Processing with Split Data and Control Paths” by Venkat Rangan, Ed McClanahan,Guru Pangal, filed Jun. 28, 2002; Serial. No. 60/392,816 entitled “Apparatus and Method for Storage Processing Through Scalable Port Processors” by Curt Beckman, Ed McClanahan, Guru Pangal, filed Jun. 28, 2002; Serial No. 60/392,873 entitled “Apparatus and Method for Fibre Channel Data Processing in a Storage Processing Device” by Curt Beckmann, Ed McClanahan filed Jun. 28, 2002; Serial No. 60/392,398 entitled “Apparatus and Method for Internet Protocol Processing in a Storage Processing Device” by Venkat Rangan, Curt Beckmann, filed Jun. 28, 2002; Serial No. 60/392,410 entitled “Apparatus and Method for Managing a Storage Processing Device” by Venkat Rangan, Curt Beckmann, Ed McClanahan, filed Jun. 28, 2002; Serial No. 60/393,000 entitled “Apparatus and Method for Data Snapshot Processing in a Storage Processing Device” by Venkat Rangan, Anil Goyal, Ed McClanahan filed Jun. 28, 2002; Serial No. 60/392,454 entitled “Apparatus and Method for Data Replication in a Storage Processing Device” by Venkat Rangan, Ed McClanahan, Michael Schmitz filed Jun. 28, 2002; Serial No. 60/392,408 entitled “Apparatus and Method for Data Migration in a Storage Processing Device” by Venkat Rangan, Ed McClanahan, Michael Schmitz filed Jun. 28, 2002; Serial No. 60/393,046 entitled “Apparatus and Method for Data Virtualization in a Storage Processing Device” by Guru Pangal, Michael Schmitz, Vinodh Ravindran and Ed McClanahan filed Jun. 28, 2002 which are hereby incorporated by reference. 
     
    
     
       BRIEF DESCRIPTION OF THE INVENTION  
         [0002]    This invention relates generally to the storage of data. More particularly, this invention relates to a storage application platform for use in storage area networks.  
         BACKGROUND OF THE INVENTION  
         [0003]    The amount of data in data networks continues to grow at an unwieldy rate. This data growth is producing complex storage-management issues that need to be addressed with special purpose hardware and software.  
           [0004]    Data storage can be broken into two general approaches: direct-attached storage (DAS) and pooled storage. Direct-attached storage utilizes a storage source on a tightly coupled system bus. Pooled storage includes network-attached storage (NAS) and storage area networks (SANs). A NAS product is typically a network file server that provides pre-configured disk capacity along with integrated systems and storage management software. The NAS approach addresses the need for file sharing among users of a network (e.g., Ethernet) infrastructure.  
           [0005]    The SAN approach differs from NAS in that it is based on the ability to directly address storage in low-level blocks of data. SAN technology has historically been associated with the Fibre Channel topology. Fibre Channel technology blends gigabit-networking technology with I/O channel technology in a single integrated technology family. Fibre Channel is designed to run on fiber optic cables and copper cabling. SAN technology is optimized for I/O intensive applications, while NAS is optimized for applications that require file serving and file sharing at potentially lower I/O rates.  
           [0006]    In view of these different approaches, a new network storage solution, Internet Small Computer System Interface (iSCSI), has been introduced. ISCSI features the same Internet Protocol infrastructure as NAS, but features the block I/O protocol inherent in SANs. ISCSI technology facilitates the deployment of storage area networking over an Internet Protocol (IP) network, rather than a Fibre Channel based SAN.  
           [0007]    ISCSI is an open standard approach in which SCSI information is encapsulated for transport over IP networks. The storage is attached to a TCP/IP network, but is accessed by the same I/O commands as DAS and SAN storage, rather than the specialized file-access protocols of NAS and NAS gateways.  
           [0008]    An emerging architecture for deploying storage applications moves storage resource and data management software functionality directly into the SAN, allowing a single or few application instances to span an unbounded mix of SAN-connected host and storage systems. This consolidated deployment model reduces management costs and extends application functionality and flexibility. Existing approaches for deploying application functionality within a storage network present various technical tradeoffs and cost-of-ownership issues, and have had limited success.  
           [0009]    In-band appliances using standard compute platforms do not scale effectively, as they require a general-purpose server to process every storage data stream “in-band”. Common scaling limits include PCI I/O buses limited to a single 2 Gb/sec data stream and contention for centralized processor and memory systems that are inefficient at data movement and transport operations.  
           [0010]    Out-of-band appliances distribute basic storage virtualization functions to agent software on custom host bus adapters (HBAs) or host OS drivers in order to avoid a single data path bottleneck. However, high value functions, such as multi-host storage volume sharing, data replication, and migration must be performed on an off-host appliance platform with similar limitations as in-band appliances. In addition, the installation and maintenance of customer drivers or HBAs on every host introduces a new layer of host management and performance impact.  
           [0011]    Appliance blades within modular SAN switches are effectively a special case of in-band appliances. These centralized blade processors handle all of the intelligent data path storage operations within a switch and face the same in-band data movement and processing inefficiencies as standalone appliances.  
           [0012]    In view of the foregoing, it would be highly desirable to provide a storage application platform to facilitate increased management and resource efficiency for larger numbers of servers and storage systems. The storage application platform should provide increased site-wide data replication and movement across a hierarchy of storage systems that enable significant improvements in data protection, information management, and disaster recovery. The storage application platform would, ideally, also provide linear scalability for simple and complex processing of storage I/O operations, and compact and cost-effective deployment footprints, line-rate data processing with the throughput and latency required to avoid incremental performance or administrative impact to existing hosts and data storage systems. In addition, the storage application should provide transport-neutrality across Fibre Channel, IP, and other protocols, while providing investment protection via interoperability with existing equipment.  
         SUMMARY OF THE INVENTION  
         [0013]    Systems according to the invention include a storage processing device with an input/output module. The input/output module has port processors to receive and transmit network traffic. The input/output module also has a switch connecting the port processors. Each port processor categorizes the network traffic as fast path network traffic or control path network traffic. The switch routes fast path network traffic from an ingress port processor to a specified egress port processor. The storage processing device also includes a control module to process the control path network traffic received from the ingress port processor. The control module routes processed control path network traffic to the switch for routing to a defined egress port processor. The control module is connected to the input/output module. The input/output module and the control module are configured to interactively support data virtualization, data migration, data replication, and snapshotting.  
           [0014]    Advantageously, the invention provides performance, scalability, flexibility and management efficiency. The distributed control and data path processors of the invention achieve scaling of storage network software. The storage processors of the invention provide line-speed processing of storage data using a rich set of storage-optimized hardware acceleration engines. The multi-protocol switching fabric utilized in accordance with an embodiment of the invention provides a low-latency, protocol-neutral interconnect that integrally links all components with any-to-any non-blocking throughput. 
       
    
    
     BRIEF DESCRIPTION OF THE FIGURES  
       [0015]    The invention is more fully appreciated in connection with the following detailed description taken in conjunction with the accompanying drawings, in which:  
         [0016]    [0016]FIG. 1 illustrates a networked environment incorporating the storage application platforms of the invention.  
         [0017]    [0017]FIG. 2 illustrates an input/output (I/O) module and a control module utilized to perform processing in accordance with an embodiment of the invention.  
         [0018]    [0018]FIG. 3 illustrates a hierarchy of software, firmware, and semiconductor hardware utilized to implement various functions of the invention.  
         [0019]    [0019]FIG. 4 illustrates an I/O module configured in accordance with an embodiment of the invention.  
         [0020]    [0020]FIG. 5 illustrates an embodiment of a port processor utilized in connection with the I/O module of the invention.  
         [0021]    [0021]FIG. 6 illustrates a control module configured in accordance with an embodiment of the invention.  
         [0022]    [0022]FIG. 7 illustrates a Fibre Channel connectivity module configured in accordance with an embodiment of the invention.  
         [0023]    [0023]FIG. 8 illustrates an IP connectivity module configured in accordance with an embodiment of the invention.  
         [0024]    [0024]FIG. 9 illustrates a management module configured in accordance with an embodiment of the invention.  
         [0025]    [0025]FIG. 10 illustrates a snapshot processor configured in accordance with an embodiment of the invention.  
         [0026]    FIGS.  11 - 13  illustrate snapshot processing performed in accordance with an embodiment of the invention.  
         [0027]    [0027]FIG. 13A illustrates mirroring performed in accordance with an embodiment of the invention.  
         [0028]    [0028]FIG. 14 illustrates replication processing performed in accordance with an embodiment of the invention.  
         [0029]    [0029]FIG. 15 illustrates migration processing performed in accordance with an embodiment of the invention.  
         [0030]    [0030]FIG. 16 illustrates a virtualization operation performed in accordance with an embodiment of the invention.  
         [0031]    [0031]FIG. 17 illustrates virtualization operations performed on port processors and a control module in accordance with an embodiment of the invention.  
         [0032]    [0032]FIG. 18 illustrates port processor virtualization processing performed in accordance with an embodiment of the invention. 
     
    
       [0033]    Like reference numerals refer to corresponding parts throughout the several views of the drawings.  
       DETAILED DESCRIPTION OF THE INVENTION  
       [0034]    The invention is directed toward a storage application platform and various methods of operating the storage application platform. FIG. 1 illustrates various instances of a storage application platform  100  of the invention positioned within a network  101 . The network  101  includes various instances of a Fibre Channel host  102 . Fibre Channel protocol sessions between the storage application platform and the Fibre Channel host, as represented by arrow  104 , are supported in accordance with the invention. Fibre Channel protocol sessions  104  are also supported between Fibre Channel storage devices or targets  106  and the storage application platform  100 .  
         [0035]    The network  101  also includes various instances of an iSCSI host  108 . ISCSI sessions, as shown with arrow  110 , are supported between the iSCSI hosts  108  and the storage application platforms  100 . Each storage application platform  100  also supports iSCSI sessions  110  with iSCSI targets  112 . As shown in FIG. 1, the iSCSI sessions  110  cross an Internet Protocol (IP) network  114 .  
         [0036]    The storage application platform  100  of the invention provides a gateway between iSCSI and the Fibre Channel Protocol (FCP). That is, the storage application platform  100  provides seamless communications between iSCSI hosts  102  and FCP targets  106 , FCP initiators  102  and iSCSI targets  112 , and FCP initiators  102  to remote FCP targets  106  across IP networks  114 . Combining the iSCSI protocol stack with the Fibre Channel protocol stack and translating between the two achieves iSCSI-FC gateway functionality in accordance with the invention.  
         [0037]    In some situations, for example sessions with multiple switch hops, iSCSI session traffic will not terminate at the storage application platform  100 , but will only pass through on its way to the final destination. The storage application platform  100  supports IP forwarding in this case, simply switching the traffic from an ingress port to an egress port based on its destination address.  
         [0038]    The storage application platform  100  supports any combination of iSCSI initiator, iSCSI target, Fibre Channel initiator and Fibre Channel target interactions. Virtualized volumes include both iSCSI and Fibre Channel targets. Additionally, the storage application platforms  100  may also communicate through a Fibre Channel fabric, with FC hosts  102  and FC targets  106  connected to the fabric and iSCSI hosts  108  and iSCSI targets  112  connected to the storage application platforms  100  for gateway operations. Further, the storage application platforms  100  could be connected by both an IP network  114  and a Fibre Channel fabric, with hosts and targets connected as appropriate and the storage application platforms  100  acting as needed as gateways.  
         [0039]    In accordance with the invention, IP, iSCSI, and iSCSI-FCP processing in the storage application platform  100  is divided into fast path and control path processing. In this document, the fast path processing is sometimes referred to as XPath™ processing and the control path processing is sometimes referred to as slow path processing. The bulk of the processed traffic is expedited through the fast path, resulting in large performance gains. Selective operations are processed through the control path when their performance is less critical to overall system performance.  
         [0040]    [0040]FIG. 2 illustrates an input/output (I/O) module  200  and a control module  202  to implement fast path and control path processing, respectively. In one direction of processing, an I/O stream  204  is received from a host  206 . A mapping operation  208  is used to divide the I/O stream between fast path and control path processing. For example, in the event of a SCSI input stream the following standards defined operations would be deemed fast path operations: Read(6), Read(10), Read(12), Write(6), Write(10), and Write(12). IP forwarding for known routes is another example of a fast path operation. As will be discussed further below, fast path processing is executed on the port processors according to the invention. In the event of a fast path operation, traffic is passed from an ingress port processor to an egress port processor via a crossbar. After routing by a crossbar (not shown in FIG. 2), the fast path traffic is directed as mapped input/output streams  210  to targets  212 .  
         [0041]    The mapping operation sends control traffic to the control module  202 . Control path functions, such as iSCSI and Fibre Channel login and logout and routing protocol updates are forwarded for control task processing  214  within the control module  202 .  
         [0042]    Split control and data path processing exploits the general nature of networked storage applications to greatly increase their scalability and performance. Control path components handle configuration, control, and management plane activities. Data path processing components handle the delivery, transformation, and movement of data through SAN elements.  
         [0043]    This split processing isolates the most frequent and performance sensitive functions and physically distributes them to a set of replicated, hardware-assisted data path processors, leaving more complex configuration coordination functions to a smaller number of centralized control processors. Control path operations have low frequency and performance sensitivity, while having generally high functional complexity.  
         [0044]    Fast path and control path operations are implemented through a hierarchy of software, firmware, and physical circuits. FIG. 3 illustrates how different functions are mapped in a processing hierarchy. Certain industry standard applications, such as industry application program interfaces, topology and discovery routines, and network management are implemented in software. Various custom applications can also be implemented in software, such as a Fibre Channel connectivity processor, an IP connectivity processor, and a management processor, which are discussed below.  
         [0045]    Various functions are preferably implemented in firmware, such as the I/O processor and port processors according to the invention, which are described in detail below. Custom application segments and a virtualization engine are also implemented in firmware. Other functions, such as the crossbar switch and custom application segments, are implemented in silicon or some other semiconductor medium for maximum speed.  
         [0046]    Many of the functions performed by the storage application platform of the invention are distributed across the I/O module  200  and the control module  202 . FIG. 4 illustrates an embodiment of the I/O module  200 . The I/O module  200  includes a set of port processors  400 . Each port processor  400  can operate as both an ingress port and an egress port. A crossbar switch  402  links the port processors  400 . A control circuit  404  also connects to the crossbar switch  402  to both control the crossbar switch  402  and provide a link to the port processors  400  for control path operations. The control circuit  404  may be a microprocessor, a dedicated processor, an Application Specific Integrated Circuit (ASIC), a Programmable Logic Device, or combinations thereof. The control circuit  404  is also attached to a memory  406 , which stores a set of executable programs.  
         [0047]    In particular, the memory  406  stores a Fibre Channel connectivity processor  410 , an IP connectivity processor  412 , and a management processor  414 . The memory  406  also stores a snapshot processor  416 , a replication processor  418 , a migration processor  420 , a virtualization processor  422 , and a mirroring processor  424 . Each of these processors is discussed below. The memory  406  may also stores a set of industry standard applications  426 .  
         [0048]    The executable programs shown in FIG. 4 are disclosed in this manner for the purpose of simplification. As will be discussed below, the functions associated with these executable programs may also be implemented in silicon and/or firmware. In addition, as will be discussed below, the functions associated with these executable programs are partially performed on the port processors  400 .  
         [0049]    [0049]FIG. 5 is a simplified illustration of a port processor  400 . Each port processor  400  includes Fibre Channel and Gigabit Ethernet receive nodes  430  to receive either Fibre Channel or IP traffic. The receive node  430  is connected to a frame classifier  432 . The frame classifier  432  provides the entire frame to frame buffers  434 , preferably DRAM, along with a message header specifying internal information such as destination port processor and a particular queue in that destination port processor. This information is developed by a series of lookups performed by the frame classifier  432 .  
         [0050]    Different operations are performed for IP frames and Fibre Channel frames. For Fibre Channel frames the SID and DID values in the frame header are used to determine the destination port, any zoning information, a code and a lookup address. The F_CTL, R_CTL, OXID and RXID values, FCP CMD value and certain other values in the frame are used to determine a protocol code. This protocol code and the DID-based lookup address are used to determine initial values for the local and destination queues and whether the frame is to by processed by an ingress port, an egress port or none. The SID and DID-based codes are used to determine if the initial values are to be overridden, if the frame is to be dropped for an access violation, if further checking is needed or if the frame is allowed to proceed. If the frame is allowed, then the ingress, egress or no port processing result is used to place the frame location information or value in the embedded processor queue  436  for ingress cases, an output queue  438  for egress cases or a zero touch queue  439  for no processing cases. Generally control frames would be sent to the output queue  438  with a destination port specifying the control circuit  404  or would be initially processed at the ingress port. Fast path operations could use any of the three queues, depending on the particular frame.  
         [0051]    IP frames are handled in a somewhat similar fashion, except that there are no zero touch cases. Information in the IP and iSCSI frame headers are used to drive combinatorial logic to provide coarse frame type and subtype values. These type and subtype values are used in a table to determine initial values for local and destination queues. The destination IP address is then used in a table search to determine if the destination address is known. If so, the relevant table entry provides local and destination queue values to replace the initial values and provides the destination port value. If the address is not known, the initial values are used and the destination port value must be determined. The frame location information is then placed in either the output queue  438  or embedded processor queue  436 , as appropriate.  
         [0052]    Frame information in the embedded processor queue  436  is retrieved by feeder logic  440  which performs certain operations such as DMA transfer of relevant message and frame information from the frame buffers  434  to the embedded processors  442 . This improves the operation of the embedded processors  442 . The embedded processors  442  include firmware, which has functions to correspond to some of the executable programs illustrated in memory  406  of FIG. 4. In various embodiments this includes firmware for determining and re-initiating SCSI I/Os; implementing data movement from one target to another; managing multiple, simultaneous I/O streams; maintaining data integrity and consistency by acting as a gate keeper when multiple I/O streams compete to access the same storage blocks; and handling updates to configurations while maintaining data consistency of the in-progress operations.  
         [0053]    When the embedded processor  442  has completed ingress operations, the frame location value is placed in the output queue  438 . A cell builder  444  gathers frame location values from the zero touch queue  439  and output queue  438 . The cell builder  444  then retrieves the message and frame from the frame buffers  434 . The cell builder  444  then sends the message and frame to the crossbar  402  for routing.  
         [0054]    When a message and frame are received from the crossbar  402 , they are provided to a cell receive module  446 . The cell receive module  446  provides the message and frame to frame buffers  448  and the frame location values to either a receive queue  450  or an output queue  452 . Egress port processing cases go to the receive queue  450  for retrieval by the feeder logic  440  and embedded processor  442 . No egress port processing cases go directly to the output queue  452 . After the embedded processor  442  has finished processing the frame, the frame location value is provided to the output queue  452 . A frame builder  454  retrieves frame location values from the output queue  452  and changes any frame header information based on table entry values provided by an embedded processor  442 . The message header is removed and the frame is sent to Fibre Channel and Gigabit Ethernet transmit nodes  456 , with the frame then leaving the port processor  400 .  
         [0055]    [0055]FIG. 6 illustrates an embodiment of the control module  202 . The control module  202  includes an input/output interface  500  for exchanging data with the input/output module  200 . A control circuit  502  (e.g., a microprocessor, a dedicated processor, an Application Specific Integrated Circuit (ASIC), a Programmable Logic Device, or combinations thereof) communicates with the I/O interface  500  via a bus  504 . Also connected to the bus  504  is a memory  506 . The memory stores control module portions of the executable programs described in connection with FIG. 4. In particular, the memory  506  stores: a Fibre Channel connectivity processor  410 , an IP connectivity processor  412 , a management processor  414 , a snapshot processor  416 , a replication processor  418 , a migration processor  420 , a virtualization processor  422 , and a mirroring processor  424 . In addition to these custom applications, industry standard applications  426  may also be stored in memory  506 . The executable programs of FIG. 6 are presented for the purpose of simplification. It should be appreciated that the functions implemented by the executable programs may be realized in silicon and/or firmware.  
         [0056]    As previously indicated, various functions associated with the invention are distributed between the input/output module  200  and the control module  202 . Within the input/output module  200 , each port processor  400  implements many of the required functions. This distributed architecture is more fully appreciated with reference to FIG. 7. FIG. 7 illustrates the implementation of the Fibre Channel connectivity processor  410 . As shown in FIG. 7, the control module  202  implements various functions of the Fibre Channel connectivity processor  410  along with the port processor  400 .  
         [0057]    In one embodiment according to the invention, the Fibre Channel connectivity processor  410  conforms to the following standards: FC-SW-2 fabric interconnect standards, FC-GS-3 Fibre Channel generic services, and FC-PH (now FC-FS and FC-PI) Fibre Channel FC-0 and FC-1 layers. Fibre Channel connectivity is provided to devices using the following: (1) F_Port for direct attachment of N_port capable hosts and targets, (2) FL_Port for public loop device attachments, and (3) E_Port for switch-to-switch interconnections.  
         [0058]    In order to implement these connectivity options, the apparatus implements a distributed processing architecture using several software tasks and execution threads. FIG. 7 illustrates tasks and threads deployed on the control module and port processors. The data flow shows a general flow of messages.  
         [0059]    FcFramelngress  500  is a thread that is deployed on a port processor  400  and is in the datapath, i.e., it is in the path of both control and data frames. Because it is in the datapath, this task is engineered for very high performance. It is a combination of port processor core, feeder queue (with automatic lookups), and hardware-specific buffer queues. It corresponds in function to a port driver in a traditional operating system. Its functions include: (1) serialize the incoming fiber channel frames on the port, (2) perform any hardware-assisted auto-lookups, and (3) queue the incoming frame.  
         [0060]    Most frames received by the FcFramelngress are placed in the embedded processor queue  436  for the FcFlowLtWt task  502 . However, if a frame qualifies for “zero-touch” option, that frame is placed on the zero touch queue  439  for the crossbar interface  504 . The FcFlowLtWt task  502  is deployed on each port processor in the datapath. The primary responsibilities of this task include:  
         [0061]    1. Dispatch the incoming Fibre Channel frame from the Fibre Channel interface (FcFramelngress) to an appropriate task/thread either in the embedded processor  442  or to the control module  202 . If the port is configured for GigE frames, this module receives frames from the iSCSI thread.  
         [0062]    2. Dispatch any incoming Fibre Channel frame from other tasks (such as iSCSI, FcpNonRw) to the FcXbar thread  508  for sending across the crossbar interface  504 .  
         [0063]    3. Allocate and de-allocate any exchange related contexts.  
         [0064]    4. Perform any Fibre Channel frame translations.  
         [0065]    5. Recognize error conditions and report “sense” data to the FcNonRw task.  
         [0066]    6. Update usage and related counters.  
         [0067]    The FcFlowHwyWt thread  506  is deployed on the port processor  400  in the datapath. The primary responsibilities of this task include:  
         [0068]    1. Forward a virtualized frame to multiple targets (such as a Virtual  
         [0069]    Target LUN that spans or mirrors across multiple Physical Target LUNs).  
         [0070]    2. Create and manage any new exchange-related contexts.  
         [0071]    3. Recognize error conditions and report “sense” data to the FcNonRw task in the Control Module.  
         [0072]    4. Updating usage and related counters.  
         [0073]    The FcXbar thread  508  is responsible for sending frames on the crossbar interface  504 . In order to minimize data copies, this thread preferably uses scatter-gather and frame header translation services of hardware.  
         [0074]    The FcpNonRw thread  510  is deployed on the control module  202 . The primary responsibilities of this task include:  
         [0075]    1. Analyze FC frames that are not Read or Write (basic link service and extended link service commands). In general, many of these frames would be forwarded to the GenericScsi Task.  
         [0076]    2. Keep track of error processing, including analyzing AutoSense data reported by the FcFlowLtWt and FcFlowHwyWt threads.  
         [0077]    3. Invoke NameServer tasks to add any newly discovered Initiators and Targets to the NameServer database.  
         [0078]    The Fabric Controller task  512  is deployed on the control module  202 . It implements the FC-SW-2 and FC-AL-2 based Fibre Channel services for frames addressed to the fabric controller of the switch (D_ID 0×FFFFFD as well as Class F frames with PortID set to the DomainId of the switch). The task performs the following operations:  
         [0079]    1. Selects the principal switch and principal inter-switch link (ISL).  
         [0080]    2. Assigns the domain id for the switches.  
         [0081]    3. Assigns an address for each port.  
         [0082]    4. Forwards any SW_ILS frames (Switch FSPF frames) to the FSPF task.  
         [0083]    The Fabric Shortest Path First (FSPF) task  514  is deployed on the control module  202 . This task receives Switch ILS messages from FabricController  512 . The FSPF task  514  implements the FSPF protocol and route selection algorithm. It also distributes the results of the resultant route tables to all exit ports of the switch. An implementation of the FSPF task  514  is described in the co-pending patent application entitled, “Apparatus and Method for Routing Traffic in a Multi-Link Switch”, U.S. Ser. No. ______, filed Jun. 30, 2003; this application is commonly assigned and its contents are incorporated herein.  
         [0084]    The generic SCSI task  516  is also deployed on the control module  202 . This task receives SCSI commands enclosed in FCP frames and generates SCSI responses (as FCP frames) based on the following criteria:  
         [0085]    1. For Virtual Targets, this task maintains the state of the target. It then constructs responses based on the state.  
         [0086]    2. The state of a Virtual Target is derived from the state of the underlying components of the physical target. This state is maintained by a combination of initial discovery-based inquiry of physical targets as well as ongoing updates based on current data.  
         [0087]    3. In some cases, an enquiry of the Virtual Target may trigger a request to the underlying physical target.  
         [0088]    The FcNameServer task  518  is also deployed on the control module  202 . This task implements the basic Directory Server module as per FC-GS- 3  specifications. The task receives Fibre Channel frames addressed to 0×FFFFFC and services these requests using the internal name server database. This database is populated with Initiators and Targets as they perform a Fabric Login. Additionally, the Name Server task  518  implements the Distributed Name Server capability as specified in the FC-SW-2 standard. The Name Server task  518  uses the Fibre Channel Common Transport (FC-CT) frames as the protocol for providing directory services to requesters. The Name Server task  518  also implements the FC-GS-3 specified mechanism to query and filter for results such that client applications can control the amount of data that is returned.  
         [0089]    The management server task  520  implements the object model describing components of the switch. It handles FC Frames addressed to the Fibre Channel address 0×FFFFFA. The task  520  also provides in-band management capability. The module generates Fibre Channel frames using the FC-CT Common Transport protocol.  
         [0090]    The zone server  522  implements the FC Zoning model as specified in FC-GS-3. Additionally, the zone server  522  provides merging of fabric zones as described in FC-SW-2. The zone server  522  implements the “Soft Zoning” mechanism defined in the specification. It uses FC-CT Common Transport protocol service to provide in-band management of zones.  
         [0091]    The VCMConfig task  524  performs the following operations:  
         [0092]    1. Maintain a consistent view of the switch configuration in its internal database.  
         [0093]    2. Update ports in I/O modules to reflect consistent configuration.  
         [0094]    3. Update any state held in the I/O module.  
         [0095]    4. Update the standby control module to reflect the same state as the one present in the active control module.  
         [0096]    As shown in FIG. 7, the VCMConfig task  524  updates the VMMConfig task  526 . The VMMConfig task  526  is a thread deployed on the port processor  400 . The task  524  performs the following operations:  
         [0097]    1. Update of any configuration tables used by other tasks in the port processor, such as FC frame forwarding tables. This update shall be atomic with respect to other ports.  
         [0098]    2. Ensure that any in-progress I/Os reach a quiescent state.  
         [0099]    The VMMConfig task  526  also updates the following: FC frame forwarding tables, IP frame forwarding tables, frame classification tables, access control tables, snapshot bit, and virtualization bit.  
         [0100]    [0100]FIG. 8 illustrates an implementation of the IP connectivity processor  412  of the invention. The IP connectivity processor  412  implements IP and iSCSI connectivity tasks. As in the case of the Fibre Channel connectivity processor  410 , the IP connectivity processor  412  is implemented on both the port processors  400  of the I/O module  200  and on the control module  202 .  
         [0101]    The IP connectivity processor  412  facilitates seamless protocol conversion between Fibre Channel and IP networks, allowing Fibre Channel SANs to be interconnected using IP technologies. ISCSI and IP Connectivity is realized using tasks and threads that are deployed on the port processors  400  and control module  202 .  
         [0102]    The iSCSI thread  550  is deployed on the port processor  400  and implements iSCSI protocol. The iSCSI thread  550  is only deployed at the ports where the Gigabit Ethernet (GigE) interface exists. The thread  550  has two portions, originator and responder. The two portions perform the following tasks:  
         [0103]    1. Interact with the RnTCP task  552  to send and receive iSCSI PDUs. It also responds to TCP/IP error conditions, as generated by the RnTCP task.  
         [0104]    2. Generate FC Frames across the crossbar interface  504  for frames that need to be converted into FC frames.  
         [0105]    3. Interact with the FcNameServer task  518  to map the WWN of an FC target and obtain its DAP address.  
         [0106]    4. Resolve IP end-point and switch port information from the iSNS task  558 .  
         [0107]    5. Manage the context space associated with currently active I/Os.  
         [0108]    6. Optimize FC frame generation using scatter-gather techniques.  
         [0109]    The ISCSI thread  550  also implements multiple connections per iSCSI session. Another capability that is most useful for increasing available bandwidth and availability is through load balancing among multiple available IP paths.  
         [0110]    The RnTCP thread  552  is deployed on each port processor  400  and also has two portions, send and receive. This thread is responsible for processing TCP streams and provides PDUs to the iSCSI module  550 . The interface to this task is through standard messaging services. The responsibilities of this task include:  
         [0111]    1. Listening for and handling incoming TCP connection requests.  
         [0112]    2. Managing TCP sequence space using TCP ACK and Window updates.  
         [0113]    3. Recognizing iSCSI PDU boundaries.  
         [0114]    4Constructing an iSCSI PDU that minimizes data copies, using a scatter-gather paradigm.  
         [0115]    5. Managing TCP connection pools by actively monitoring and terminating idle TCP connections.  
         [0116]    6. Identifying TCP connection errors and reporting them to upper levels.  
         [0117]    The Ethernet Frame Ingress thread  554  is responsible for performing the MAC functionality of the GigE interface, and delivering IP packets to the IP layer. In addition, this thread  554  dispatches the IP packet to the following tasks/threads.  
         [0118]    1. If the frame is destined for a different IP address (other than the IP address of the port) it consults the IP forwarding tables and forwards the frame to the appropriate switch port. It uses forwarding tables set up through ARP, RIP/OSPF and/or static routing.  
         [0119]    2. If the frame is destined for this port (based on its IP address) and the protocol is ARP, ICMP, RIP etc. (anything other than iSCSI), it forwards the frame to a corresponding task in the control module.  
         [0120]    3. If the frame is an iSCSI packet, it invokes the RnTCP task  552 , which is responsible for constructing the PDU and delivering it to the appropriate task.  
         [0121]    4. Update performance and related counters.  
         [0122]    The Ethernet Frame Egress thread  556  is responsible for constructing Ethernet frames and sending them over the Gigabit Ethernet node  432 . The Ethernet Frame Egress thread  556  performs the following operations:  
         [0123]    1. If the frame is locally generated, it uses scatter-gather lists to construct the frame.  
         [0124]    2. If the frame is generated at the control module, it adds the appropriate MAC header and routes the frame to the Ethernet transmit node  456 .  
         [0125]    3. If the frame is forwarded from another port (as part of the IP Forwarding), it generates a MAC header and forwards the frame to the Ethernet node.  
         [0126]    4. Update performance and related counters.  
         [0127]    The VMMConfig thread  526  is responsible for updating IP forwarding tables. It uses internal messages and a three-phase commit protocol to update all ports. The VCMConfig task  524  is responsible for updating IP forwarding tables to each of the port processors. It uses internal messages and a three-phase commit protocol to update all ports.  
         [0128]    The iSNS task  558  is responsible for updating IP Forwarding tables to the port processors  400 . This task uses internal messages and a three-phase commit protocol to update all ports.  
         [0129]    The FcFlow module  560  is used for Fibre Channel connectivity services. This module includes modules  502  and  506 , which were discussed in connection with FIG. 7. Frames arriving at the Ethernet receive node  430  are routed to the Ethernet Frame Ingress module  554 . As discussed above, TCP processing is performed at the RnTCP module  552 , and the iSCSI module  550  generates FC Frames and sends them to the FcFlow thread  560  for transmission to appropriate modules. Note that this flow of messages allows both virtual and physical targets to be accessible using the iSCSI connections.  
         [0130]    The ARP task  570  implements an ARP cache and responds to ARP broadcasts, allowing the GigE MAC layer to receive frames for both the IP address configured at that MAC interface as well as for other IP addresses reachable through that MAC layer. Since the ARP task is deployed centrally, its cache reflects all MAC to IP mappings seen on all switch interfaces.  
         [0131]    The ICMP task  572  implements ICMP processing for all ports. The RIP/OSPF task  574  implements IP routing protocols and distributes route tables to all ports of the switch. Finally, the MPLS module  576  performs MPLS processing.  
         [0132]    [0132]FIG. 9 illustrates an implementation of the management processor  414  of the invention. The operations of the management processor  414  are distributed between the control module  202  and the I/O module  200 . FIG. 9 illustrates a port processor  400  of the I/O module  200  as a separate block simply to underscore that the port processor  400  performs certain operations, while other operations are performed by other components of the I/O processor  200 . It should be appreciated that the port processor  400  forms a portion of the I/O module  200 .  
         [0133]    The management processor  414  implements the following tasks:  
         [0134]    1. Basic switch configuration.  
         [0135]    2. Persistent repository of objects and related configuration information in a relational database.  
         [0136]    3. Performance counters, exported as raw data as well as through SNMP.  
         [0137]    4. In-band management using Fibre Channel services, such as management services.  
         [0138]    5. Configuring storage services, such as virtualization and snapshot.  
         [0139]    6. In-band management using Fibre Channel services.  
         [0140]    7. Support topology discovery.  
         [0141]    8. Provide an external API to switch services.  
         [0142]    Communication between tasks may be implemented through the following techniques.  
         [0143]    1. Messages sent using standard messaging services.  
         [0144]    2. XML messages from an external network management system to the switch.  
         [0145]    3. SNMP PDUs.  
         [0146]    4. In-band Fibre Channel (FC-CT) based messages.  
         [0147]    The Network Management System (NMS) Interface task  600  is responsible for processing incoming XML requests from an external NMS  602  and dispatching messages to other switch tasks. The Chassis Task  604  implements the object model of the switch and collects performance and operational status data on each object within the switch.  
         [0148]    The Discovery Task  606  aids in discovery of physical and virtual targets. This task issues FC-CT frames to the FcNameServer task  608  with appropriate queries to generate a list of targets. It then communicates with the FcpNonRW task  610 , issuing a FCP SCSI Report LUNs command, which is then serviced by the GenericScsi module  612 . The Discovery Task  606  also collects and reports this data as XML responses.  
         [0149]    The SNMP Agent  614  interfaces with the Chassis Task  604  on the control module  202  and a Statistics Collection task  620  on the I/O module  200 . The SNMP Agent  614  services SNMP requests. FIG. 9 also illustrates hardware and software counters  618  on the port processor  400 . The remaining modules of FIG. 9 have been previously described.  
         [0150]    Returning to FIG. 4, the I/O module  200  includes a snapshot processor  416 . The snapshot processor  416  also forms a portion of the control module  202  of FIG.  6 . The difficulties associated with backing up data in a multi-user, high-availability server system with many users is known. If updates are made to files or databases during a backup operation, it is likely that the backup copy will have parts that were copied before the data was updated, and parts that were copied after the data was updated. Thus, the copied data is inconsistent and unreliable.  
         [0151]    There are two ways to deal with this problem. One approach is called cold backup, which makes backup copies of data while the server is not accepting new updates from end users or applications. The problem with this approach is that the server is unavailable for updates while the backup process is running.  
         [0152]    The other backup approach is called hot backup. With hot backup, the system can be backed up while users and applications are updating data. There are two integrity issues that arise in hot backups. First, each file or database entity needs to be backed up as a complete, consistent version. Second, related groups of files or database entities that have correlated data versions must be backed up as a consistent linked group.  
         [0153]    One approach to hot backup is referred to as copy-on-write. The idea of copy-on-write is to copy old data blocks on disk to a temporary disk location when updates are made to a file or database object that is being backed up. The old block locations and their corresponding locations in temporary storage are held in a special bitmap index, which the backup system uses to determine if the blocks to be read next need to be read from the temporary location. If so, the backup process is redirected to access the old data blocks from the temporary disk location. When the file or database object is done being backed up, the bitmap index is cleared and the blocks in temporary storage are released.  
         [0154]    A technology similar to copy-on-write is referred to as snapshot. There are two kinds of snapshots. One is to make a copy of data as a snapshot mirror. The other way is to implement software that provides a point-in-time image of the data on a system&#39;s disk storage, which can be used to obtain a complete copy of data for backup purposes.  
         [0155]    Software snapshots work by maintaining historical copies of the file system&#39;s data structures on disk storage. At any point in time, the version of a file or database is determined from the block addresses where it is stored. Therefore, to keep snapshots of a file at any point in time, it is necessary to write updates to the file to a different data structure and provide a way to access the complete set of blocks that define the previous version.  
         [0156]    Software snapshots retain historical point-in-time block assignments for a file system. Backup systems can use a snapshot to read blocks during backup. Software snapshots require free blocks in storage that are not being used by the file system for another purpose. It follows that software snapshots require sufficient free space on disk to hold all the new data as well as the old data.  
         [0157]    Software snapshots delay the freeing of blocks back into a free space pool by continuing to associate deleted or updated data as historical parts of the filing system. Thus, filing systems with software snapshots maintain access to data that normal filing systems discard.  
         [0158]    Snapshot functionality provides point-in-time snapshots of volumes. The volume that is snapshot is called the Source LUN. The implementation is based on a copy-on-write scheme, whereby any write I/Os to a Source LUN copies a block of data into the Snapshot Buffer. The size of the block copied is referred to as the Snapshot Line Size. Access to the Snapshot Volume resolves the location of a Snapshot Line between the Snapshot Buffer and the Source LUN and retrieves the appropriate block.  
         [0159]    Snapshot is implemented using the snapshot processor  416 , which includes the tasks illustrated in FIG. 10. FIG. 10 illustrates that the snapshot processor  416  is implemented on the I/O module  200 , including a host ingress port  400 A and a snapshot buffer port  400 D. The snapshot processor  416  is also implemented on the control module  202 . The snapshot processor  416  implements:  
         [0160]    1. Processing both in-band and out-of-band requests for Snapshot Configuration, such as Snapshot Creation, Deletion and Snapshot Buffer Allocation.  
         [0161]    2. Generating messages to VCMConfig  524  in order to deliver new configurations automatically to other tasks involved in the snapshot. Configurations are distributed on the I/O module  200  and port processors  400  of the Snapshot Buffer as well as to update tables on ports where WRITE I/Os to the Source LUN enter the switch.  
         [0162]    3. Managing policies, security, and the like.  
         [0163]    4. Error logging, error recovery, and the like.  
         [0164]    5. Status and information reporting.  
         [0165]    A snapshot meta-data manager  700  is also deployed on the I/O module  200  and implements:  
         [0166]    1. Snapshot meta-data lookup.  
         [0167]    2. Keeping an up-to-date map of the block list corresponding to Snapshot Line size.  
         [0168]    3. Recreating and re-building meta-data during initialization from the Snapshot Buffer.  
         [0169]    A snapshot engine  702  is deployed on the port processors  400  where the snapshot buffer is attached. The snapshot engine  702  implements:  
         [0170]    1. Receipt of Copy-On-Write requests from the Snapshot Meta-Data Manager  700 .  
         [0171]    2. Frame forwarding to FcFlow  560 , which then forwards a READ I/O of the old data for Copy-On-Write to the port where the snapshot buffer is attached.  
         [0172]    3. Sending the new WRITE I/O to the Source LUN port after the READ I/O is complete.  
         [0173]    4. Monitoring for errors and invoking appropriate error-handling activities in the snapshot manager.  
         [0174]    [0174] 3  The operation of the snapshot processor  416  is more fully appreciated in connection with FIGS.  11 - 13 . The following example uses the terms fault on read (FOR) and fault on write (FOW). If FOR=1, the read operation sends a fault condition to the control path. If FOR=0, the read operation is allowed. There is a similar definition for fault on write (FOW).  
         [0175]    In this example, the VT/LUN used is called the primary VT/LUN. Its point-in-time image is called a snapshot VT/LUN. Assume that the primary VT/LUN has an extent list  710  that contains a single extent. The extent references slot  0  in a legend table  712 . This slot has FOR=0 and FOW=0. FIG. 11 illustrates this configuration before setting up a snapshot. In particular, the figure illustrates an extent list  710 , a legend table  712 , a virtual map (VMAP)  714 , and physical storage  716 .  
         [0176]    To prepare the VT/LUN for a snapshot, a snapshot extent list  710 A, legend table  712 A, and VMAP  714 A are developed. The VMAP  714 A can be initially empty or fully populated. FIG. 12 illustrates duplicate versions of the extent list  710 , legend table  712 , and VMAP  714  after setting up the snapshot. Some of the legend table slots reference the same VMAPs. In both cases, legend slot  1  is allocated but not used because there are no extents that map to legend slot  1 .  
         [0177]    [0177]FIG. 13 illustrates after a write operation where the write operation occurs to the source or primary VT/LUN. A write operation attempt occurs and sends a fault condition to the control path. The control path uses a COPY command to copy the original data from the primary storage  716  to the snapshot buffer  716 A. If the snapshot buffer  716 A is not previously allocated, it is allocated at this point. The extent lists  710  and  710 A are adjusted and a new extent is created corresponding to the data range copied. Future access to this extent through the extent list  710 A leads to legend slot  1  that references the new storage copied. Now the legend map entry for 0 is changed to FOR=1 so that any requests to read data not yet in the snapshot buffer  716 A are faulted and transferred for operation from the source storage  716 . This assumes an entry in the list for each extent in the primary VT/LUN. Alternatively, the 0 entry could remain FOR=0 and any read operation to the snapshot buffer would fault if the data had not been copied. The extent list  710  on the primary VT/LUN is adjusted and a new extent is created corresponding to the data range copied. The referenced legend action is now 1, with the FOR and the FOW both now zero (0). The original write operation is allowed to continue. In the future, write operations to the same extent do not cause a FOW. Thus, any reads or writes to the primary VT/LUN occur normally, after copying of the data on the initial write. Writes to the snapshot VT/LUN occur normally to the snapshot buffer  716 A, though this is an unusual operation. Reads to the snapshot VT/LUN occur from the snapshot buffer  716 A if the data has been copied or occur from the source  716  if the data has not been copied.  
         [0178]    Observe that in accordance with the invention, a snapshot operation is implemented based upon the setting of a few bits (e.g., the FOR and FOW bits). Thus, the snapshot operation is compactly and efficiently executed on a port basis, as opposed to a system wide basis, which results in delay and central control issues.  
         [0179]    Returning to FIG. 4, the I/O processor  200  also includes a mirroring processor  424 . Mirroring is an operation where duplicate copies of all data are kept. Reads are sourced from one location but write operations are copied to each volume in the mirror. The phrase “mirroring” is normally used when the multiple write operations occur synchronously, as opposed to replication described below.  
         [0180]    [0180]FIG. 13A illustrates mirroring. A legend map entry  720  is provided for each extent that is mirrored. This map entry  720  indicates FOR=0 and FOW=1. This is done so that on a write a fault occurs and reference is made to the VMAP  722 . The VMAP  722  has two entries, one for storage  724  and one for storage  724 A, the two storage units in the exemplary mirror, though more units could be used if desired. On processing the VMAP  722 , a copy of the write operation is sent to each of the listed devices. However, a read does not fault and so is sourced only from storage  724 . Thus, as with snapshotting, mirroring can be implemented by setting a few bits in a table.  
         [0181]    Returning to FIG. 4, the I/O processor  200  also includes a replication processor  418 . The replication processor  418  is also implemented on the control module  202 , as shown in FIG. 6. Replication is closely related to disk mirroring. As its name implies, disk mirroring provides a duplicated data image of a set of information. As described above, disk mirroring is implemented at the block layer of the I/O stack, and done synchronously. Replication provides similar functionality to disk mirroring, but works at the data structure layer of the I/O stack. Data replication typically uses data networks for transferring data from one system to another and is not as fast as disk mirroring, but it offers some management advantages.  
         [0182]    Asynchronous replication is implemented using write splitting and write journaling primitives. In write splitting, a write operation from a host is duplicated and sent to more than one physical destination. Write splitting is a part of normal mirroring. In write journaling, one of the mirrors described by the storage descriptor is a write journal. When a write operation is performed on the storage descriptor, it splits the write into two or more write operations. One write operation is sent to the journal, and the other write operations are sent to the other mirrors.  
         [0183]    The write journal provides append-only privileges for write operations initiated by the host. Data is formatted in the journal with a header describing the virtual device, LBA start and length, and a time stamp. When the journal file fills, it sends a fault condition to the control path (similar to a permission violation) and the journal is exchanged for an empty one. The control path asynchronously copies the contents of the journal to the remote image with the help of an asynchronous copy agent. Data from the journal is moved through the control path.  
         [0184]    [0184]FIG. 14 shows a sequence of operations performed in accordance with an embodiment of the replication processor  418 . First, the write request is delivered to the virtual device, as shown with arrow  1  of FIG. 14. The write request is sent natively to normal storage as shown with arrow  2 . Further a header for a journaling write request is formatted. The header includes LBA offset and length, a timestamp, and a sequence number as shown by arrow  3 . The header and the data are either written to the journal in a write operation, or the data is written first followed by the header, as shown with arrow  4 . The status of the write operation is collected at the storage descriptor level as shown by arrow  5 . Finally, the SCSI status for the host&#39;s write operation is then returned as shown by arrow  6 .  
         [0185]    If the formatted write reaches the end of the write journal, it sends a fault condition to the control path as if it were writing to a read-only extent. The control path waits for the write operations to the segment in progress to complete. After the write operations complete, the control path swaps out the old journal and swaps in a new journal so that the fast path can resume journaling. The control path sends the old journal to an asynchronous copy agent to be delivered to a remote site, where journals can be reassembled.  
         [0186]    Each segment of a virtual device has its own write journal. This design works well if there are only a few segments (no more than 16), and the segments are at least 50 Gigabytes in size. These numbers ensure that a large number of tiny journals are not created.  
         [0187]    When replication takes place among several virtual devices, write operations across all the replica drivers must be serial. An example of this condition is a database with table space on one virtual device and a log on a different virtual device. If the database sends a write operation to a device and receives successful completion status, it then sends a write operation to a second device. If some components crash or are temporarily inaccessible, the write operation sent to second device may not return a completed status. When all components are back in service, the database must never see that the write operation to the second device is completed and that the write operation to the first device did not complete. This behavior is free on local devices. If there is a disaster at the source site and the stream of journal write operations received by the remote copy agent abruptly stops, the remote copy agent finishes replaying the journal write operations it has received. After it finishes, the condition that the write operation sent to the second device completed, but the write operation sent to the first device was not completed must be true.  
         [0188]    Returning to FIG. 4, the I/O processor  200  also includes a migration processor  420 . The migration processor  420  is also implemented on the control module  202  of FIG. 6.  
         [0189]    [0189]FIG. 15 illustrates the concept of online data migration.  
         [0190]    Online migration uses the following three legend slots. Slot  0  represents data that has not been copied. It points to the old physical storage and has read/write privileges. Slot  1  represents the data that is being migrated (at the granularity of the copy agent). It points to the old physical storage and has read-only privileges. Slot  2  represents the data that has already been copied to the new physical storage. It points to the new physical storage and has read-write privileges.  
         [0191]    The Extent List  710  determines which state (legend entry) applies to the extents in the segment. During the migration process, the legend table does not change, but the extent list  710  entries change as the copy barrier progresses. The no access symbol on the write path in FIG. 15 indicates the copy barrier extent. Write operations to the copy barrier must be held until released by the copy agent. To avoid the risk of a host machine time out, the copy agent must not hold writes for a long time. The write barrier granularity must be small.  
         [0192]    In this example, the data is moved from the storage (described by the source storage descriptor) to the storage described by the destination storage descriptor. In FIG. 15, source and destination correspond to part of physical volumes P 1  and P 2 .  
         [0193]    The copy agent moves the data and establishes the copy barrier range by setting the corresponding disk extent to its legend slot  1 , copies the data in the copy barrier extent range from P 1  to P 2 , and advances the copy barrier range by setting the corresponding disk extent to legend slot  2 . Data that is successfully migrated to P 2  is accessed through slot  2 . Data that has not been migrated to P 2  is accessed through slot  0 . Data that is in the process of being migrated is accessed through slot  1 .  
         [0194]    Accesses before or after the copy barrier range and read operations to the copy barrier range itself are accomplished without involving the control path. Only a write operation to the copy barrier range itself is sent to the control path, and retried when the copy barrier range moves to the next extent of the map. The migration is complete when the entire MAP references legend slot  2 . After this, legend slot  0  and  1  are no longer needed.  
         [0195]    Returning again to FIG. 4, the I/O module also includes a virtualization processor  422 . As shown in FIG. 6, the virtualization processor  422  is also resident on the control module  202 . Storage virtualization provides to computer systems a separate, independent view of storage from the actual physical storage. A computer system or host sees a virtual disk. As far as the host is concerned, this virtual disk appears to be an ordinary SCSI disk logical unit. However, this virtual disk does not exist in any physical sense as a real disk drive or as a logical unit presented by an array controller. Instead, the storage for the virtual disk is taken from portions of one or more logical units available for virtualization (the storage pool).  
         [0196]    This separation of the hosts&#39; view of disks from the physical storage allows the hosts&#39; view and the physical storage components to be managed independently from each other. For example, from the host perspective, a virtual disk&#39;s size can be changed (assuming the host supports this change), its redundancy (RAID) attributes can be changed, and the physical logical units that store the virtual disk&#39;s data can be changed, without the need to manage any physical components. These changes can be made while the virtual disk is online and available to hosts. Similarly, physical storage components can be added, removed, and managed without any need to manage the hosts&#39; view of virtual disks and without taking any data offline.  
         [0197]    [0197]FIG. 16 provides a conceptual view of the virtualization processor  422 . The virtualization processor  422  includes a virtual target  800  and virtual initiator  801 . A host  802  communicates with the virtual target  800 . A volume manager  804  is positioned between the virtual target  800  and a first virtual logical unit  806  and a second virtual logical unit  808 . The first virtual logical unit  806  maps to a first physical target  810 , while the second virtual logical unit  808  maps to a second physical target  812 .  
         [0198]    The virtual target  800  is a virtualized FCP target. The logical units of a virtual target correspond to volumes as defined by the volume manager. The virtual target  800  appears as a normal FCP device to the host  802 . The host  802  discovers the virtual target  800  through a fabric directory service.  
         [0199]    Once a host request to a virtual device is translated, requests must be issued to physical target devices. The entity that provides the interface to initiate I/O requests from within the switch to physical targets is the virtual initiator  801 . Apart from virtual target implementation, the virtual initiator interface is used by other internal switch tasks, such as the snapshot processor  416 . The virtual initiator  801  is the endpoint of all exchanges between the switch and physical targets. The virtual initiator  801  does not have any knowledge of volume manager mappings.  
         [0200]    [0200]FIG. 17 illustrates that the virtualization processor is implemented on the port processors  400  of the I/O module  200  and on the control module  202 . Host  802  constitutes a physical initiator  820 , which accesses a frame classification module  822  of the ingress port processor  400 . The ingress port processor  400 -I includes a virtual target  800  and a virtual initiator  801 . The egress port  400 -E includes a frame classifier  838  to receive traffic from physical targets  810  and  812 .  
         [0201]    The control module  202  includes a virtual target task  824 , with a virtual target proxy  826 . A virtual initiator task  828  includes a virtual initiator proxy  830  and a virtual initiator local task  832 , which interfaces with a snapshot task  834  and a discovery task  836 .  
         [0202]    Fibre Channel frames are classified by hardware and appropriate software modules are invoked. The virtual target module  800  is invoked to process all frames classified as virtual target read/write frames. Frames classified as slow path frames are forwarded by the ingress port  400 -I to the virtual target proxy  826 . The virtual target proxy  826  is the slow path counterpart of the virtual target  800  instance running on the port processor  400 -I. While the virtual target instance  800  handles all read and write requests, the proxy virtual target  826  handles all login/logout requests, non-read/write SCSI commands and FCP task management commands.  
         [0203]    The processing of a host request by a virtual target  800  instance at the port processor  400 -I and a proxy virtual target instance  824  at the control module  202  involves initiating new exchanges to the physical targets  810 ,  812 . The virtual target  800  invokes virtual initiator  801  interfaces to initiate new exchanges. There is a single virtual initiator instance associated with each port processor. The port number within the switch identifies the virtual instance. The port number is encoded into the Fibre Channel address of the virtual initiator and therefore frames destined for the virtual initiator can be routed within the switch. The proxy virtual initiator  826  establishes the required login nexus between the port processor virtual instance  801  and a physical target.  
         [0204]    Fibre Channel frames from the physical targets  810 ,  812  destined for virtual initiators are forwarded over the crossbar switch  402  to virtual initiator instances. The virtual initiator module  801  processes fast path virtual initiator frames and the virtual initiator module  830  processes slow path virtual initiator frames. Different exchange ID ranges are used to distinguish virtual initiator frames as slow path and fast path. The virtual initiator module  801  processes frames and then notifies the virtual target module  800 . On the port processor  400 -I, this notification is through virtual target function invocation. On the control module  202 , the virtual target task  824  is notified using callbacks. The common messaging interface is used for communication between the virtual initiator task  828  and other local tasks.  
         [0205]    Virtualization at the port processor  400 -I happens on a frame-by-frame basis. Both the port processor hardware and firmware running on the embedded processors  442  play a part in this virtualization. Port processor hardware helps with frame classifications, as discussed above, and automatic lookups of virtualization data structures. The frame builder  454  utilizes information provided by the embedded processor  442  in conjunction with translation tables to change necessary fields in the frame header, and frame payload if appropriate, to allow the actual header translations to be done in hardware. The port processor also provides firmware with specific hardware accelerated functions for table lookup and memory access. Port processor firmware  440  is responsible for implementing the frame translations using mapping tables, maintaining mapping tables and error handling.  
         [0206]    A received frame is classified by the port processor hardware and is queued for firmware processing. Different firmware functions are invoked to process the queued-up frames. Module functions are invoked to process frames destined for virtual targets. Other module functions are invoked to process frames destined for virtual initiators. Frames classified for slow path processing are forwarded to the crossbar switch  404 .  
         [0207]    Frames received from the crossbar switch  404  are queued and processed by firmware according to classification. No frame classification is done for frames received from the crossbar  402 . Classification is done before frames are sent on the crossbar  402 .  
         [0208]    [0208]FIG. 18 is a state machine representation of the virtualization processor operations performed on a port processor  400 . A virtual target frame received from a physical host or physical target is routed to the frame classifier  822 , which selectively routes the frame to either the embedded processor or feeder queue  840  or to the crossbar switch  402 . The virtual target module  800  and the virtual initiator module  801  process fast path frames provided to the queue  840 . The virtual target module  800  accesses virtual message maps  844  to determine which frame values are to be changed. Slow path frames are provided to the crossbar switch  402  via the crossbar transmit queue  846  for slow path forwarding  842  to the control module.  
         [0209]    The virtualization functions performed on the port processor include initialization and setup of the port processor hardware for virtualization, handling fast path read/write operations, forwarding of slow path frames to the control module, handling of I/O abort requests from hosts, and timing I/O requests to ensure recovery of resources in case of errors. The port processor virtualization functions also include interfacing with the control module for handling login requests, interacting with the control module to support volume manager configuration updates, supporting FCP task management commands and SCSI reserve/release commands, enforcing virtual device access restrictions on hosts, and supporting counter collection and other miscellaneous activities at a port.  
         [0210]    The foregoing description, for purposes of explanation, used specific nomenclature to provide a thorough understanding of the invention. However, it will be apparent to one skilled in the art that specific details are not required in order to practice the invention. Thus, the foregoing descriptions of specific embodiments of the invention are presented for purposes of illustration and description. They are not intended to be exhaustive or to limit the invention to the precise forms disclosed; obviously, many modifications and variations are possible in view of the above teachings. The embodiments were chosen and described in order to best explain the principles of the invention and its practical applications, they thereby enable others skilled in the art to best utilize the invention and various embodiments with various modifications as are suited to the particular use contemplated. It is intended that the following claims and their equivalents define the scope of the invention.