Patent Publication Number: US-7590747-B2

Title: Distributed storage cluster architecture

Description:
CROSS-REFERENCES TO RELATED APPLICATIONS 
   This application is a continuation of U.S. patent application No. 09/753,221, filed Dec. 29, 2000, entitled “Method and Apparatus for Accessing Remote Storage in a Distributed Storage Cluster Architecture”, now U.S. Pat. No. 6, 952,737 which is a continuation-in-part of U.S. patent application No. 09/695,499, filed Oct. 23, 2000, entitled “A Network Storage System”, and claims the benefit of U.S. Provisional Applications Nos. 60/186,693 and 60/186,774, filed Mar. 3, 2000, entitled “Method and Apparatus for Implementing A Network-Based Storage Service” and “Method and Apparatus for Establishing Control and Data Lines To A Storage Facility, And API for Supporting Such Lines”, respectively. 

   BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention is directed toward the field of storage, and more particularly toward accessing remote storage through use of a local device. 
   2. Art Background 
   With the rapid digitization of music, film and photographs, customer demand is driving the Internet to become the most preferred transport mechanism for all forms of digital media. Using the Internet, users have instantaneous worldwide access to their favorite movies, songs, or personal memorabilia. As the producers and owners of media content increasingly use the Internet as a primary method for worldwide distribution, the aggregate amount of rich media content available over the Internet is increasing at an extremely rapid rate. 
   Not only is the number of rich media objects available over the Internet growing exponentially, but the size of the media, generally referred to herein as objects, is also dramatically increasing. A median Web object is 5 kilobytes (KB) in size, while the size of a rich media object may be 100 to 1 million times larger. For example, high-resolution digital photographs average 500 KB per picture. Digital music runs 3 to 5 megabytes (“MB”) per song, and digital movies may reach up to 4 gigabytes (“GB”) in size. 
   As the number of personal computers, digital camcorders, digital cameras, and personal digital audio players grow, demand for Internet bandwidth to store, share and retrieve media files across the Internet also will grow. As the use of high-bandwidth digital subscriber lines (“DSL”), cable modems, and digital broadcast satellite networks gain in popularity, which supports the growth of the Internet backbone, the demand for using the Internet as a primary delivery channel for rich media objects also gains in popularity. This development causes a virtuous cycle, where the installation of broadband networks drives the use of rich media devices, which in turn, creates demand for further improvements in network bandwidth, and so on. 
   The distribution of rich media objects across the Internet creates the need for increased storage capacity to store these rich media objects. As the number of personal media devices grows, and the network bandwidth expands, the amount of storage media required to store the various MP3 files, photographs, films, and video clips will also grow. Also, as more storage becomes readily available, more people will use the Internet to catalog, store, and access their rich media objects (e.g., digital photographs of family members). 
   To date, only traditional storage solutions from established enterprise vendors have been available to a Web site developer implementing rich media repositories. One challenge with adopting today&#39;s existing storage technology for use with the Internet is meeting current and future scalability requirements. Today, large scale storage systems only scale to a few dozen terabytes. This amount of storage space is inadequate for storing substantial amounts of rich media objects. For example, if just 10 percent of America on line (“AOL”) users place two 15 minute videos on a personal home page, then one petabyte (i.e., 1000 terabytes) of storage would be required. Today&#39;s enterprise storage system architectures cannot support this level of storage capacity. 
   In the Internet world, in addition to providing mass storage, it is also critically important to provide universal access to that storage across the wide area network. The content provider, regardless of the location of their content servers, cache servers, or stream servers, would ideally like to provide ubiquitous access to an entire store of rich media objects. Current technology, including storage area networks and network attached storage technologies, do not provide direct access to the wide area network. Only servers located within the same metropolitan area can directly access these types of storage systems. 
   Since Internet users are measured in the tens of thousands or even millions of users, instead of hundreds of users, another challenge in mass storage is the ability to scale delivery of media as the demand increases. A true Internet based storage system must be able to handle peak loads of millions of simultaneous requests from all around the world. Traditional storage architectures are designed to support a few hundred simultaneous requests from the fastest possible response time to match the speed of the server CPU. For the Internet, storage systems must be able to manage literally millions of simultaneous downloads at the speed of the wide area network. Thus, these traditional storage architectures are not “impedance matched” with the wide area network because the storage devices handle far too few simultaneous transactions that far exceed the latency requirements of the wide area network. In addition, these traditional storage architectures are typically implemented with expensive disks and expensive connection technologies. 
   Another issue regarding storage of rich media objects is the time to market. The time to market is often a crucial requirement for new rich media Web sites. Growth rates are measured in terabytes per month. Quickly bringing new capacity online becomes a strategic advantage in fast-moving markets. Typically, with traditional storage solutions, it takes a customer two to six months to integrate a fully operational multi-terabytes storage unit with the content providers site. This start-up time is to slow to meet rapidly increasing business demands. Pre-building large amounts of excess capacity in anticipation of this demand is one tactic to deal with unpredictable demand spikes, but this approach is prohibitively expensive. 
   Traditional storage architectures have been optimized for database and file server applications. The Internet introduces a whole new set of demands on storage devices, including scalability, global access, user accounts, and rapid deployment. With the explosive growth in rich media served over the Internet over the next several years, this is coming to a head. The coming title wave of rich content will surpass the capabilities of even the most robust enterprise storage architectures. Accordingly, there is a demand to develop new paradigms in new ways of designing Internet ready rich media storage systems. 
   SUMMARY OF THE INVENTION 
   A storage system, accessed over a network by a client, includes one or more storage centers. A storage center contains a control nodes (e.g., distributed object storage managers) and a plurality of intelligent storage nodes. The intelligent storage nodes store the object files in one or more disk drives. The client, such as an end-user computer or a content server, issues requests over a network (e.g., Internet) to retrieve object files. In response to a client request, a control node is selected to manage the file retrieval process. In one embodiment, a load balancing fabric, such as an L4 switch, receives the request from the network, and selects one of the control nodes in manner so as to load balance client requests in the storage system. The control node determines the location of the object file in one of the intelligent storage nodes (“destination storage node”). The control node receives the object file from the destination storage node, and transfers the object file to the client. In one embodiment, the control nodes include a data cache to store object files previously retrieved from an intelligent storage node. 
   The client request contains a unique file identifier (e.g. object fingerprint) that uniquely identifies an object file for the client request. The storage system identifies the object file in the storage system through the object fingerprint. In one embodiment, the storage system authenticates client requests. The client request includes, in addition to a unique file identifier, a client identification and a certificate. The storage system stores a client “secret”, such as a client key. The control node retrieves a client key, which corresponds to the client identification, and validates the certificate using the client key. If the client request is valid, then the control node retrieves the object file. If the client request is not valid, then an error message is sent to the client. 
   In one embodiment, the control node stores a reference (e.g., look-up table) that cross references object files to intelligent storage node locations. In response to a client request, the control node determines whether the object file is identified on the reference. If so, the control node, through an interconnect fabric, retrieves the object file from the identified intelligent storage node. If the object file is not identified in the reference, then the control node, using a multi cast protocol (e.g., DOSP), broadcasts a request for the object file to the intelligent storage nodes. Each intelligent storage node determines whether it stores the object file. If it does, the intelligent storage node broadcasts, to all of the control nodes, the intelligent storage node location for the object file. The control nodes update their reference with the intelligent storage node location. If the control node is unsuccessful using the multi-cast broadcast, then the control node individually queries each intelligent storage node for the object file using a point-to-point protocol. 
   In one embodiment, the storage system includes more than one storage center (i.e., each storage center contains control nodes and intelligent storage nodes). The storage centers may be located at geographically disparate locations. The storage system stores an object file at two different storage centers. For this embodiment, if a failure occurs in an intelligent storage node, then the DOSM searches for the object file in a different storage center. Also, in other embodiments, a storage center stores an object file in more than one intelligent storage node. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a block diagram illustrating one embodiment for the storage system of the present invention. 
       FIG. 2  illustrates one embodiment for use of the network storage system as a media storage service. 
       FIG. 3  is a block diagram illustrating one embodiment for the storage cluster. 
       FIG. 4  is a flow diagram illustrating one embodiment for the download operation in the storage cluster. 
       FIG. 5  is a flowchart illustrating one embodiment for authentication in the network storage system. 
       FIG. 6  illustrates one embodiment of a distributed object storage manager (“DOSM”). 
       FIG. 7  is a block diagram illustrating one embodiment for an intelligent storage node. 
       FIG. 8  is a flow diagram illustrating one embodiment for processing upload requests in the storage cluster. 
       FIG. 9  is a flow diagram illustrating one embodiment for generating unique fingerprints of object files. 
       FIG. 10  is a block diagram illustrating one embodiment for caching data in the storage cluster. 
       FIG. 11  is a block diagram illustrating one embodiment for implementing a VFS for use with a network storage system. 
       FIG. 12  illustrates example database tables for implementing the file system with a database. 
       FIGS. 13A and 13B  are flow diagrams illustrating one embodiment for performing directory operations in the VFS. 
       FIG. 14  is a flow diagram illustrating one embodiment for the delete file operation for the network storage system. 
       FIG. 15  illustrates geographical replications of storage centers. 
       FIG. 16  is a block diagram illustrating one embodiment for replicating the storage centers. 
       FIG. 17  illustrates one embodiment for use of the storage center in a content delivery network. 
       FIG. 18  is a flow diagram illustrating one embodiment for use of the storage center with a content delivery network. 
       FIG. 19  illustrates one embodiment for use of the storage port in the network storage system. 
       FIG. 20  is a flow diagram illustrating one embodiment for use of a storage port to deliver content. 
       FIG. 21   a  illustrates one hardware configuration for a storage port device. 
       FIG. 21   b  illustrates embodiments for implementing the storage port in software. 
       FIG. 22  is a block diagram illustrating one embodiment for a storage port. 
       FIG. 23  is a block diagram illustrating one embodiment for file system translation in the storage port. 
       FIG. 24  is a flow diagram illustrating one embodiment for translating a file system operation from a local file system to the network storage file system. 
       FIG. 25  is a block diagram illustrating one embodiment for using the storage port to directly download object files to the end-user. 
       FIG. 26  is a flow diagram illustrating one embodiment for directly downloading object files to an end-user. 
       FIG. 27  is a block diagram illustrating one embodiment to interface a storage center to a client&#39;s private file directory system. 
       FIG. 28  is a flow diagram illustrating one embodiment for accessing object files in a storage center using a client&#39;s private file system. 
       FIG. 29  is a block diagram illustrating one embodiment for a storage port fail over configuration. 
       FIG. 30  is a flow diagram illustrating one embodiment for a storage port fail over process. 
       FIG. 31  is a flow diagram illustrating one embodiment for using the multicast protocol after a storage node fail over condition. 
   

   DETAILED DESCRIPTION 
   The disclosure of U.S. Provisional Patent Applications Nos. 60/186,693 and 60/186,774, filed Mar. 3, 2000, entitled “Method and Apparatus for Implementing A Network-Based Storage Service” and “Method and Apparatus for Establishing Control and Data Lines To A Storage Facility, And API For Supporting Such Lines”, respectively, are hereby incorporated by reference. 
   Network Storage System Overview: 
   The network storage system is designed to meet the storage requirements of rich media content owners. Rich media objects typically represent up to 90 percent of the storage required for a film, music or a photo album associated with a web site. The network storage system uses distributed systems technology to provide scalability to support petabytes of storage and to support millions of users. Users only gain access to their media objects, within the network storage system, using a highly secured “shared secret” authentication certificate technology. The network storage system also provides immediate expandability for any user that desires to increase their storage capacity. Also, the network storage system is extremely cost-effective because, in one embodiment, it consists of standard off the shelf CPUs with the latest high-density disk technology. 
   For purposes of nomenclature, the term “client”, as used herein, refers to an entity that uses the storage system to store object files. For example, a client may consist of a web site owner that desires to deliver, outside their web server, rich media objects associated with content on their web site. Also, for purposes of nomenclature, the term “end-user”, as used herein, refers to a recipient of the object. For example, the end-user may consist of a computer user that downloads objects from a web site across the Internet using a web browser. Also, under this definition, the end-user may also be a client. 
     FIG. 1  is a block diagram illustrating one embodiment for the storage system of the present invention. For the embodiment of  FIG. 1 , the storage system consists of a control path and a data path. The control path consists of a virtual file system (“VFS”)  50  and the data path consists of a distributed storage cluster  70 . The control path is used to conduct all directory operations. The VFS includes, in part, client assigned filenames and network storage system assigned unique file identifiers for each rich media object. The unique file identifiers are embedded into storage resource locators (“SRLs”). 
   An article of manufacture may comprise a distributed storage cluster  70 . The distributed storage cluster  70  is used to store the object files for the system (i.e., all client data). As shown in  FIG. 1 , the VFS and the storage cluster  70  are coupled to communicate information so as to coordinate file system information with the physical storage of the object files. 
   As shown in  FIG. 1 , file system control  60  issues directory operation requests to the VFS  50 . As is described more fully below, file system control  60  may comprise software that uses a library to essentially “translate” file system requests from the client&#39;s local file system to file system requests compatible with the network storage system. In other embodiments, file system control  60  consists of a storage port coupled to the client&#39;s system (e.g., the client&#39;s application or web server). In general, the storage port, implemented in either hardware or software, translates file system commands from the client&#39;s local file system (e.g., NFS or CIFS) to file system requests compatible with the network storage system. In one embodiment, to interface the client&#39;s file system to the network storage system, a client need only mount the storage port as a network drive. The storage port then provides complete access to the network storage system. A detailed discussion of the storage port is set forth below. 
   As shown in  FIG. 1 , object recipient  80  receives, in response to object requests, objects downloaded from storage cluster  70 . The object recipient  80  may comprise the client, or the object recipient  80  may consist of one or more end-users. Embodiments for transferring objects from the storage cluster  70  to object recipients, including both end-users and clients, are described more fully below. 
   The network storage system has applications for use as an Internet based media storage service. For this application, the network storage system is an integral part of the Internet infrastructure used by rich media content owners and delivery networks.  FIG. 2  illustrates one embodiment for use of the network storage system as a media storage service. In general, the storage service  130  provides a single consistent worldwide image of a client&#39;s (e.g., a company operating a web site) entire directory of rich objects. For this embodiment, an end-user  100  is coupled to both the content origin server  120  and storage service  130  through a network. For example, the end-user  100  may be coupled to the content origin server  120  and storage service  130  via the Internet. The storage service  130  includes processing and networking facilities, such as a server  140 , and data store  150 . The storage service  130  and content origin server  120  communicate to conduct file directory operations and object file operations. The data store  150 , part of the storage service  130 , stores large data files, such as rich media data files, illustrated as multimedia files  160 ,  170  and  180  in  FIG. 2 . In one embodiment, the data store  150  consists of a cluster of intelligent storage nodes. 
   In one embodiment, the storage service communicates with web servers (e.g., content origin server  120 ) and browsers (e.g., Microsoft Explorer or Netscape Navigator) operating on end-user computer  100  via the standard Internet hypertext transfer protocol (“HTTP”) and universal resource locators (“URLs”). Although the use of HTTP is described herein, any transport protocol may be used without deviating from the spirit or scope of the invention. For the configuration of  FIG. 2 , the end-user, through end-user computer  100 , generates hyper text transfer protocol (“HTTP”) requests to the content origin server  120  to obtain hyper text mark-up language (“HTML”) files. In addition, to obtain large data objects associated with those text files, the end-user, through end user computer  100 , generates HTTP requests to the storage service  130 . For example, the end-user may download from the content origins server  120  a few kilobytes of textual data describing a rich object, such as text describing an upcoming film. When the user “clicks” on a URL to download a film snippet from the upcoming film, an HTTP request is generated to the storage service  130 , and a storage service  130  downloads the film snippet to the end-user computer  100 . The network configuration of  FIG. 2  permits off loading the storage of rich objects from the content origin server  120  to the storage service  130 . This configuration greatly reduces the size and complexity of content origin servers needed to store, manage and serve rich objects to end-users. 
   Distributed Storage Cluster: 
   In one embodiment, the storage cluster utilizes distributed systems technology that harnesses the throughput of hundreds of CPUs and the storage of thousands of disk drives.  FIG. 3  is a block diagram illustrating one embodiment for the storage cluster. The storage cluster  300  receives upload, download, and delete operations that include the storage resource locator (“SRL”). The SRL is then used to uniquely identify a client file. As shown in  FIG. 3 , the storage cluster consists of distributed object storage managers (“DOSMs”)  320  and intelligent storage nodes  340 . There are “n” distributed object storage managers  320 , wherein “n” is any integer value greater than one. Similarly, there are “n” intelligent storage nodes for the intelligent storage nodes  340  component (i.e., wherein “n” is also any integer value greater than one). 
   As shown in  FIG. 3 , file upload and download operations are input to a load balancing fabric  310 . In one embodiment, the load balancing fabric  310  is a layer four (“L4”) switch. In general, L4 switches are capable of effectively prioritizing TCP and UDP traffic. In addition, L4 switches, which incorporate load balancing capabilities, distribute requests for HTTP sessions among a number of resources, such as servers. For this embodiment, the load balancing fabric  310  distributes upload and download requests to one of a plurality of DOSMs based on DOSM availability. The load balancing capability in an L4 switch is currently commercially available. 
   Each DOSM independently handles hundreds of simultaneous download transactions. In one embodiment described below, each DOSM has a local high-speed disk cache to store frequently accessed file objects. Each DOSM has a map, dynamically generated, of the storage system. The map identifies a correspondence between an intelligent storage node address and an object finger print. In one embodiment, the DOSMs record all usage and performance data gathered by a separate accounting system and monitoring system. 
   The DOSMs  320  communicate with the intelligent storage nodes  340  via an interconnect fabric  330 . The interconnect fabric  330  consists of a high-speed, high bandwidth fabric to ensure that all the DOSMs  320  communicate with every intelligent storage node at all times. In one embodiment, the DOSMs  320  communicate with the intelligent storage node over the interconnect fabric via a protocol, entitled the distributed object storage protocol (“DOSP”). Effectively, the DOSP links hundreds of intelligent storage nodes into one large storage cluster. As described more fully below, the DOSP consist of a multi-cast protocol as well as a point-to-point protocol. 
   In general, the intelligent storage nodes  340  provide the persistent store for the objects or files. The intelligent storage nodes contain thousands of high-density disk drives. The intelligent storage nodes are described more fully below in conjunction with the discussion of  FIG. 7 . 
   In one embodiment, the network storage system uses the storage resource locators (“SRLs”) to process requests. In one embodiment, the network storage system uses the following format for the SRL: 
   http://&lt;storage-cluster&gt;/&lt;encoded-request&gt;/&lt;digital-signature&gt;/&lt;arbitrary-customer-uri, 
   wherein: 
   the “storage-cluster” field includes the name or IP address of a storage center DSM pool; 
   the “encoded-request” field comprises a base64 encoded op code and arguments; 
   the “digital-signature” field consists of a certificate derived from the following expression: md5(shared-secret+md5(shared-secret+encoded-request)); and 
   the “arbitrary-customer-uri” field contains arbitrary information added to the SRL by the network storage system clients. For example, the arbitrary-customer-uri field may include the filename and extension of the file being downloaded to enable browsers to send the content to an appropriate plug-in. 
   In one embodiment, the “encoded request” field is encoded using base64 encoding. As shown in Table 1, the encoded request consists of a URL type field, a version field, and type/version specific payload field. 
   
     
       
         
             
             
             
           
             
               TABLE 1 
             
             
                 
             
             
               Field 
               Datatype 
               Comment 
             
             
                 
             
           
          
             
               Type 
               Numeric 
               Type of the URL, i.e. Standard, CDN, etc. 
             
             
               Version 
               Numeric 
               Version of the URL 
             
             
               Payload 
               NA 
               Payload specific to the Type/Version of the URL. 
             
             
                 
             
          
         
       
     
   
   In one embodiment, the type/version specific payload field consists of a series of ‘/’ delimited fields that contain accounting information, an op code, and an op code dependent argument list. Table 2 shows one embodiment for the type/version specific payload field. 
                           TABLE 2               Field   Datatype   Comment                  Expires   Numeric   Number of seconds since the epoc that the link               expires. If 0, the link has an infinite duration               and will not be checked for expiration.       Access   Numeric   The access method associated with the SRL,       method       i.e. Storage Port, end user SRL, CDN, etc.       Client Id   Numeric   The client id of the client performing the               operation.       Op Code   Numeric   The opcode of the operation to be performed.       Arguments   NA   An opcode specific argument list.                    
Table 3 includes two access method types for the access method field.
 
                                   TABLE 3                       Access method   Encoding   Comment                          SRL   0x0001   End user SRL request.           Storage Port   0x0002   Internal Storage Port request.                        
Table 4 includes operational codes for the op code field.
 
   
     
       
         
             
             
             
           
             
               TABLE 4 
             
             
                 
             
             
               Operation 
               Encoding 
               Arguments 
             
             
                 
             
           
          
             
               NO_OP 
               0x0000 
               None 
             
             
               STORE 
               0x0010 
               Pfid - numeric Parent folder id to upload 
             
             
                 
                 
               the file to. Other arguments are mime 
             
             
                 
                 
               encoded. 
             
             
               FETCH 
               0x0020 
               Md5 - alphanumeric Hexadecimal 
             
             
                 
                 
               representation of the md5 hash of the file 
             
             
                 
                 
               to be downloaded. 
             
             
               FETCH_AUTH 
               0x0021 
               Md5 - alphanumeric Hexadecimal 
             
             
                 
                 
               representation of the md5 hash of the file 
             
             
                 
                 
               to be downloaded. 
             
             
                 
                 
               Authentication Callback URI - 
             
             
                 
                 
               alphanumeric URL encoded callback URI 
             
             
               DELETE 
               0x0050 
               Md5 - alphanumeric Hexadecimal 
             
             
                 
                 
               representation of the md5 hash of the file 
             
             
                 
                 
               to be deleted. 
             
             
               CONTROL 
               0x1000 
               ControlTicket - alphanumeric 
             
             
                 
                 
               Hexadecimal representation of the digital 
             
             
                 
                 
               signature of the XML control document. 
             
             
                 
             
          
         
       
     
   
   The object files, stored in one or more storage clusters, are not associated with a “central authority” that specifies a physical location for the object files. The VFS, in part, stores an object fingerprint for a file, but does not indicate a location for the file. Because of this, the network storage system may be referred to as a “stateless” or a “soft state” system. Instead of using a central authority to locate files, the physical address for the files is identified in the storage cluster through a dynamically generated reference. However, the reference does not necessarily identify the location for all the object files (i.e., the reference, at any one time, potentially identifies only a subset of the object files in the system). Since the network storage system does not use a central authority, object files may be added, updated or stored in multiple locations in the storage system, and the location of the object files in the intelligent storage nodes may be discovered in response to a specific request. 
     FIG. 4  is a flow diagram illustrating one embodiment for the download operation in the storage cluster. For purposes of nomenclature, the “recipient” in a download operation is the destination of the file for the download operation. The storage cluster receives a download request, including the unique file identifier (e.g., SRL) (block  400 ,  FIG. 4 ). When the storage cluster receives a download request, the load balancing fabric  310  ( FIG. 3 ), such as an IA switch, selects an available DOSM (block  410 ,  FIG. 4 ). The DOSM parses the SRL to extract the certificate and the encoded request (block  415 ,  FIG. 4 ). From the encoded request, a certificate is calculated, and the calculated certificate is compared to the SRL certificate. If the SRL does not authenticate, then an error message is sent to the recipient (blocks  420  and  425 ,  FIG. 4 ). Alternatively, if the SRL does authenticate, then the DOSM determines whether the object identified by the SRL resides in the corresponding DOSM&#39;s data cache (blocks  420  and  430 ,  FIG. 4 ). If the data object is cached, then the object is transmitted from the storage cluster to the recipient (e.g., via the Internet using HTTP protocol) (blocks  430  and  495 ,  FIG. 4 ). If the object is not cached at the DOSM, then the DOSM attempts to identify the location of the object in one of the intelligent storage nodes (blocks  430  and  440 ,  FIG. 4 ). 
   If the DOSM knows the location of the object (e.g., the object file is an entry in the DOSM look-up table) and the storage node is readable, then the DOSM obtains a connection with the storage node that stores the object, and transmits the object from the storage cluster to the recipient (blocks  442 ,  435  and  495 ,  FIG. 4 ). In one embodiment, to determine whether the storage node is readable, the DOSM queries the storage node for the object file a predetermined number of times. Alternatively, if the DOSM does not know the storage location of the object in the intelligent storage nodes, then the DOSM broadcasts a request to the intelligent storage nodes to locate the object (blocks  440  and  450 ,  FIG. 4 ). Each intelligent storage node determines whether the object is stored on one of its disk drives (block  460 ,  FIG. 4 ). If the object file is located in one of the intelligent storage nodes, then the intelligent storage node, which stores the requested object, broadcasts identification information to all of the distributed object storage managers (blocks  462  and  470 ,  FIG. 4 ). For example, if intelligent storage node “1” of intelligent storage nodes  340  stores the requested object in disk “3”, then intelligent storage node “1” broadcasts to all “n” DOSMs that the object file is located in disk “3” of intelligent storage node “1.” All DOSMs snoop the packets on the network to obtain file identification information. In response to the intelligent storage nodes broadcast, each DOSM updates its reference (e.g., lookup table or file system directory) with the proper file identification information. 
   If the DOSM broadcasts a request to the intelligent storage nodes to locate the object and the object is not located from the request, then the DOSM establishes a point-to-point connection with an intelligent storage node to individually query the storage node for the object (blocks  462  and  464 ,  FIG. 4 ). This process is repeated until all intelligent storage nodes have been queried or the object has been located. If the object is located in one of the intelligent storage nodes, then the intelligent storage node, which stores the requested object, broadcasts identification information to all of the distributed object storage managers (blocks  466  and  470 ,  FIG. 4 ). Alternatively, if the object is not located in one of the intelligent storage nodes, then a failover procedure is executed to locate the object in a different storage center (blocks  466  and  468 ,  FIG. 4 ). 
   When the intelligent storage node is located, the DOSM obtains a connection with the intelligent storage node, and opens the file with the requested object. If the storage node is readable (i.e., the DOSM successfully reads the file from the storage node), then the object is transmitted from the intelligent storage node to the recipient via a network (e.g., using HTTP protocol over the Internet). If the object file is not readable, then a failover procedure is executed to obtain the object in a different storage node and/or storage center, and the DOSM obtains a connection with the new storage node (blocks  442 ,  468  and  435 ,  FIG. 4 ). Thereafter, the object is transmitted from the storage cluster to the recipient (block  495 ,  FIG. 4 ). 
   In one embodiment, accesses to the network storage system require a valid authentication certificate. In one embodiment utilizing CDNs, the certificate is based on the object file&#39;s unique user filename and a secure key assigned to each client account. In other embodiments, the network storage system supports full HTTPS and SSL protocols for secure communications between clients/end-users and the network storage system. 
     FIG. 5  is a flowchart illustrating one embodiment for authentication in the network storage system. To authenticate a request, the network storage system decodes the SRL to extract the client identification, the SRL certificate and the client filename or object fingerprint (block  500 ,  FIG. 5 ). The network storage system (i.e., virtual file system or storage cluster) extracts a “secret” or secure key corresponding to the client identified with the request. In general, the “secret” or secure key is a password supplied by the client to authenticate operations in the network storage system. Using the secure key and object fingerprint, the network storage system generates a calculated certificate (block  520 ,  FIG. 5 ). In one embodiment, the network storage system generates a calculated certificate for the request in accordance with the following expression:
 MD5 Hash(Secure Key+MD5 Hash(Secure Key+Encoded SRL)) 
   As shown above, a first MD5 hash calculation is performed on the object fingerprint and the secure key to obtain a first result, and a second MD5 hash calculation is performed on the first result and the secure key to obtain the calculated certificate. The network storage system compares the calculated certificate with the SRL certificate (i.e., the certificate transmitted with the SRL request) (block  530 ,  FIG. 5 ). If the certificates match, then the SRL is authenticated, and the request is performed (blocks  540  and  560 ,  FIG. 5 ). Alternatively, if the calculated certificate does not match the SRL certificate, then the network storage system generates an error message to the requester (blocks  540  and  550 ,  FIG. 5 ). 
     FIG. 6  illustrates one embodiment of a distributed object storage manager (“DOSM”). For this embodiment, the processes and functions of each DOSM (i.e., also referred to herein as a “control node”) are implemented in software for execution on a computer, such as a server  600 . In other embodiments, the distributed object storage managers  320  may be implemented in a combination of hardware and software on one or more computers. Each DOSM maintains a file lookup table to identify the location of object files stored in the intelligent storage nodes  340 . Table  610  of  FIG. 6  illustrates one embodiment for a DOSM file lookup table. For this embodiment, each entry of the table identifies a corresponding object file stored in an intelligent storage node. Specifically, each entry includes a file identification, an IP address, and a disk identification. The file identification, also referred to herein as the object fingerprint, is derived by performing an MD5 hash calculation on the contents of the object file. The result of this MD5 hash calculation is a 128 bit string. For this embodiment, the DOSM file lookup table stores, in the file identification column, the 128 bit string, with the file designation “MD5.” The second column of the DOSM file lookup table stores the IP address of the intelligent storage node that stores the object file (e.g., “10.3.100.1”). The third column, labeled disk ID, stores an integer value that identifies the specific disk drive on the intelligent storage node that stores the object file. In one embodiment, when the look-up table is at full capacity, the DOSM uses a least recently used (“LRU”) caching algorithm to replace existing entries in the DOSM lookup table with new entries received. 
   As shown in  FIG. 6 , the DOSM also includes a data cache  620 . In general, the data cache  620  stores objects (i.e., client data) to permit the DOSM to streamline data directly to the recipient in response to a download request. During a download request, in the event of a cache miss, when the object is transferred from the intelligent storage node to the recipient, the object is also stored in the data cache  620 . Similar to the DOSM file lookup table, the data cache  620  uses a least recently used (“LRU”) caching algorithm to replace existing entries with new data objects when the data cache is full. 
   The DOSM also maintains a state table  630 . In general, the state table  630  provides the state of the system by storing information on the overall capacity and health of the intelligent storage nodes  340 . In one embodiment, the state tables are built using the multicast protocol to obtain, from the intelligent storage nodes, information about the corresponding intelligent storage node. The state information indicates whether disks on the intelligent storage nodes are healthy, how much space is on the disks, etc. In one embodiment, as shown in  FIG. 6 , state table  630  stores: read-write state of the storage nodes; health of the storage nodes (including an identification of failed nodes); and the current load of the storage nodes, including available storage capacity and the number of input/output (“I/O”) operations per second. The DOSM uses state information to select, in an upload operation, the appropriate intelligent storage node for storage of a new object file. For example, the DOSM uses information on the number of input/output (“I/O”) operations per second to load balance the storage nodes. The DOSM also uses information on available storage capacity to select an intelligent storage node to store a new object file. 
     FIG. 7  is a block diagram illustrating one embodiment for an intelligent storage node. For this embodiment, the intelligent storage node is implemented on a computer, including software to perform the functions described herein. An intelligent storage node  700  includes a processing core  710  that consists of one or more central processing units (“CPUs”). In one embodiment, the processing core  710  comprises two CPUs. The intelligent storage node  700  also includes volatile memory, labeled  730  in  FIG. 7 . The memory  730  is used to store instructions executed by the processing core  710 , as well as data used by the intelligent storage node. The intelligent storage node  700  further includes a network interface  720  to interface the intelligent storage node to the plurality of distributed object storage managers  320  via the interconnect fabric  330 . The elements of the intelligent storage node  700  communicate via a computer transport mechanism  750  (e.g., a peripheral component interconnect (“PCI”) bus, processor bus, etc.). The computer transport mechanism  750  is intended to represent a broad category of one or more computer busses, such as peripheral component interconnect (“PCI”) bus or the industry standard association (“ISA”) bus. 
   The intelligent storage node  700  further includes a plurality of disk drives  740  to store the object files. As shown in  FIG. 7 , the number of disks in an intelligent storage node is represented as “n”, such that “n” is an integer value greater than one. In one embodiment, the processing core  710  communicates with the disk drives  740  using the ISA protocol. However, any protocol used to access disk drives, including standard computer serial interface (“SCSI”) protocol, may be used without deviating from the spirit or scope of the invention. 
   The intelligent storage node contains information to identify object files that it stores. In one embodiment, the information to identify object files is stored in the file system directory of the intelligent storage node. In other embodiments, the information to identify object files is cached. Table 5 illustrates example entries to cache the identification of object files in an intelligent storage node. 
   
     
       
         
             
             
             
           
             
                 
               TABLE 5 
             
             
                 
                 
             
             
                 
               FILE ID 
               DISK ID 
             
             
                 
                 
             
           
          
             
                 
               File1.MD5 
               1 
             
             
                 
               File6.MD5 
               2 
             
             
                 
               File4.MD5 
               2 
             
             
                 
               File5.MD5 
               “n” 
             
             
                 
                 
             
          
         
       
     
   
   Table 5 includes a file identifier and a disk identifier. The file identifier, or file ID, stores the unique file handle corresponding to the object file. In one embodiment, the unique file handle is the object fingerprint obtained from performing an MD5 hash function on the contents of the object file. For the first example entry in Table 5, the unique file handle is represented as “file1.MD5.” The second column, labeled disk id, identifies the specific disk drive on the intelligent storage node that stores the object file. For the second example entry in Table 5, the object file, “file6.MD5”, is stored on the second disk drive on that intelligent storage node. On initial start-up of the intelligent storage node, the intelligent storage node builds the file identification table. 
   The storage cluster also processes upload requests.  FIG. 8  is a flow diagram illustrating one embodiment for processing upload requests in the storage cluster. For purposes of nomenclature, the “source”, as used herein, refers to the source of the object file for the upload operation. If the storage cluster receives an upload request, then the load balancing fabric  320  ( FIG. 3 ) selects an available DOSM to process the upload request (blocks  805  and  810 ,  FIG. 8 ). The VFS creates a file identification (e.g., storage system node) and the appropriate directory for the new object file (block  805 ,  FIG. 8 ). The selected DOSM parses the upload request to extract the certificate, object file, as well as client and directory information (block  820 ,  FIG. 8 ). If the upload request does not authenticate, then the DOSM transmits an error message to the source (block  835 ,  FIG. 8 ). Alternatively, if the upload request does authenticate, then the DOSM selects at least one intelligent storage node to store the object file (block  840 ,  FIG. 8 ). In one embodiment, the upload operation stores the object file in two storage nodes. The “mirroring” of the object files ensures accessibility to the object in the event a failure occurs in an intelligent storage node. In one embodiment for “mirroring” the object files, the network storage system stores the object file at different geographic locations (e.g., different storage centers). If access to the geographically disparate storage center is unavailable at the time the object file is uploaded, then an additional copy of the file is stored at the local storage center. 
   In one embodiment, the DOSM uses a state table ( FIG. 6 ) to select the intelligent storage nodes most appropriate to store the new object. For purposes of discussion, the selected intelligent storage nodes are referred to herein as the “destination intelligent storage nodes.” The DOSM establishes a connection with the destination intelligent storage node (block  850 ,  FIG. 8 ). In one embodiment, the DOSM establishes a DOSP point-to-point connection with the destination source node. The object file is then transferred to the destination intelligent storage node (block  860 ,  FIG. 8 ). In addition, after transferring the file to the intelligent storage node, the DOSM receives a status message as part of the DOSP point-to-point protocol. The status message indicates whether the transfer operation was successful. 
   In one embodiment, the destination intelligent storage node generates a unique fingerprint for the object file (block  870 ,  FIG. 8 ). Specifically, the destination intelligent storage node computes an MD5 hash of the contents of the object file. The intelligent storage node also verifies the object file. After receiving the successful status at the DOSM, the DOSM establishes a connection to the virtual file system (“VFS”). The DOSM communicates file information (e.g., the 128 bit.MD5 unique object fingerprint, file size, etc.), directory information (e.g., folder ID, parent folder ID, etc.), as well as client information and metadata (block  880 ,  FIG. 8 ). The VFS attempts to verify the upload. If the VFS does not verify the upload, then an error message is sent to the source of the upload request (blocks  890  and  835 ,  FIG. 8 ). If the VFS does verify the upload, then the verification is transmitted to the DOSM. In turn, the DOSM verifies the upload to the source (block  895 ,  FIG. 8 ). Also, the storage system returns, to the source, a file handle that uniquely identifies the file to the network storage system. 
   If the source of the upload request is an end-user, then the DOSM re-directs the end-user to the client. For example, the DOM may redirect the end-user to a predetermined URL at the client&#39;s web site. In other embodiments, if the source was a storage port, then the DOSM transmits a storage system node (i.e., handle used only for the storage system) and the unique object file fingerprint. 
   As discussed above, as part of the upload operation, the network storage system generates a unique fingerprint of the object file.  FIG. 9  is a flow diagram illustrating one embodiment for generating unique fingerprints of object files. First, the destination intelligent storage node creates a temporary file with the contents of the object file (block  900 ,  FIG. 9 ). An MD5 hash calculation is performed on the contents of the temporary file (block  910 ,  FIG. 9 ). The DOSM determines whether the unique fingerprint, generated from the MD5 hash operation, currently exists in the network storage system (block  920 ,  FIG. 9 ). If the fingerprint currently exists, the temporary file, which holds the contents of the object, is deleted (blocks  930  and  940 ,  FIG. 9 ). Also, a reference count associated with the existing fingerprint file is incremented (block  950 ,  FIG. 9 ). The use of reference counts is described more fully below in conjunction with a discussion of the delete operation. If the fingerprint generated from the temporary file does not exist, then the temporary file is converted to a permanent file, and the unique fingerprint is used to identify the file in the storage cluster (block  960 ,  FIG. 9 ). 
   Virtual File System: 
   In one embodiment, directory operations are performed in the virtual file system (“VFS”).  FIG. 11  is a block diagram illustrating one embodiment for implementing a VFS for use with a network storage system. In general, the VFS is the control path for maintaining the network storage system. The VFS maintains, for each object file, the customer file directory including the customer assigned filenames and the unique network storage system file identifiers. In one embodiment discussed above, the unique network storage system file identifiers consist of a 128 bit digital fingerprint obtained from performing an MD5 hash calculation on the contents of the object file. As shown in  FIG. 11 , the VFS consists of distributed directory managers (“DDMs”)  1110  and distributed directories  1120 . There are “n” DDMs and “n” distributed directories, wherein “n” represents any integer one or greater. In one embodiment, each client is mapped to a distributed directory. 
   The DDMs support common directory operations, such as “open file”, “move file”, “delete file”, “open folder”, “move folder”, and “create folder.” The arrows of  FIG. 11  depict multi-directory requests and operations. The requests may originate from the end-user or the client, via a storage port or a web store. In one implementation, the requests to the VFS are transported using HTTP requests and encoded using the extended markup language (“XML”). Although the VFS is described using the HTTP protocol with XML encoded requests, any network protocol with any type of request format may be used without deviating from the spirit or scope of the invention. 
   In one embodiment, the VFS employs a database to implement the file system. For the database implementation, each directory operations request is converted into the database operation. Alternatively, the VFS may implement the file system using a local file system (i.e., a file system local to the VFS). For the file system embodiment, files are generated to store information stored in the database implementation. Also, the DDMs include a lookup table to locate the files in the distributed directories. The files or database tables are replicated in a remote storage center. 
   The network storage file system consists of files arranged in directories or folders (hereafter referred to as folders). Similar to most file systems, the network storage file system is a hierarchical file system. In a hierarchical file system, directories or folders are arranged in levels, starting with a root or base folder. Additional folders or sub folders are then arranged under the root folder. The file system may comprise any number of levels, such that additional layers of sub folders fall beneath other sub folders. For purposes of nomenclature used herein, a parent folder to a folder is the folder arranged above the folder in the hierarchy of folders or directories. 
     FIG. 12  illustrates example database tables for implementing the file system with a database. For the database embodiment, the VFS maintains a customer table  1200 , folder table  1210  and file table  1220 . The customer table  1200  includes fields for “customer ID”, “customer name”, and “customer reserved fields.” The customer ID is a network storage system identifier used to uniquely identify the client. The customer name is the real name associated with a customer. For the first example entry in the customer table  1200 , “customer A” has a customer ID of “1.” The customer reserved fields provide storage reserved for use by the client. 
   The folder table  1210  includes fields for “customer ID”, “folder ID”, “folder parent ID”, and “metadata.” For this embodiment, each entry in the folder table corresponds to a folder in the network storage file system. The customer ID, the same customer ID stored in the customer table, uniquely identifies the client. For the example entries in folder table  1210 , the customer ID of “3” identifies that the folders have been assigned to “customer C.” The folder ID identifies the specific folder for that entry. For example, the first entry in folder table  1210  is for a folder identified by the identification of “2.” The third column, labeled “folder parent ID”, identifies the parent folder for the folder corresponding to the database entry or row. For example, the second entry in folder table  1210  is a sub folder to the first entry of table  1210  (i.e., folder “100” is in the next hierarchical level beneath folder “2”). Note that the first entry in folder table  1210  does not have a value for the folder parent ID. This signifies that folder “2” is a root folder. 
   The file table contains an entry for each object file stored in a network storage file system. The example file table  1220  includes columns or fields for “customer ID”, “file handle”, “folder ID”, “folder parent ID”, and “metadata.” Again, the customer ID identifies the customer that owns the file. The entries shown in file table  1220  are for files stored by customer C. The file handle field stores the fingerprint that the network file system uses to uniquely identify the file. Although the network file system stores 32 byte hexadecimal character sequences to identify files, for purposes of illustration, file handle entries for file table  1220  are shown as “52.MD5”, “55.MD5”, “99.MD5”, and “67.MD5.” The folder ID field identifies the folder that contains the file. For example, the first entry in file table  1220 , corresponding to file “55.MD5”, is organized or stored in folder 100. The folder parent ID identifies the parent folder to the folder that stores the file. The folder 100, which contains “52.MD5”, has a parent folder of “2.” 
     FIGS. 13A and 13B  are flow diagrams illustrating one embodiment for performing directory operations in the VFS. When a DDM receives a directory operation request, the DDM parses the request to extract the certificate, an operational code, as well as arguments corresponding to the operational code (blocks  1300  and  1310 ,  FIG. 13A ). The operational code specifies the directory operation requested. The DDM, using the certificate and the information contained in the request, validates the request. If the request does not validate, the DDM sends an error message to the requester (blocks  1320  and  1330 ,  FIG. 13A ). Alternatively, if the request does validate, the DDM parses the operational code and extracts the arguments, including the folder to perform the open operation (blocks  1320  and  1330 ,  FIG. 13A ). 
   In general, if the operation is for an “open folder” operation, then the DDM returns all sub folders and files contained in the folder identified by the argument. Specifically, the DDM extracts, from the appropriate distributed directory, the file and folder tables that correspond to the folder identified as an argument in the “open folder” request (blocks  1340  and  1345 ,  FIG. 13A ). Specifically, the DDM extracts all the files and sub folders that correspond to the folder identified as an argument with the request. Using the example of  FIG. 12 , if the “open folder” request included the arguments “folder ID=2” and “customer ID=3”, then the DDM extracts, from the folder table in the distributed directory, folder IDs  100  and  251  (i.e., folders  100  and  251  are sub folders of the root folder 2). If the “open folder” request included the arguments “folder ID=100”, then the DDM extracts from the file table file handles “52.MD5” and “55.MD5.” 
   If the operational code in a directory request is for an “open file” operation, subsequent to an “open folder” request, then file information is obtained from the file table (i.e., file handle) and the client table (i.e., client identification) to construct an authentication certificate and an SRL for the file. For the above example, if the argument with the “open file” operation specified the file “52.MD5”, then file and client information are obtained to construct the SRL for the “52.MD5” file. 
   If the operational code in a directory request is for a “move folder” operation, then a database operation is performed to revise the entries in the file and folder tables to reflect the new location of the folder. The “move folder” operation includes, as an argument, the new destination for the folder. Using the example of  FIG. 12 , if the “move folder” operation specified moving folder ID  166  from a sub folder of folder ID  251  to directly beneath the root folder 2, then the parent folder ID on the fourth entry of folder table  1210  is modified from “251” to “2.”Also, for file table  1220 , the parent folder ID for the third and fourth entries are modified from “251” to “2.” 
   If the directory operation is a “create folder” operation, then a new entry or row is generated for the folder table (blocks  1360  and  1365 ,  FIG. 13A ). The “create folder” operation includes a parent folder as an argument. As described below, the client&#39;s folder name is converted to the network storage system&#39;s folder identification. Using the example of  FIG. 12 , if the requester desires to create a new folder under the sub folder 166, then the DDM assigns a new folder identification for the folder, and enters a new row or entry for the folder table  1210  with a folder parent ID of 166. 
   If the directory operation is a “move file” operation, then a database operation is performed to revise an entry in the file table to reflect the new location of the file (blocks  1370  and  1375 ,  FIG. 13A ). The “move file” operation includes a new destination for the file as an argument in the directory request. For the example database tables in  FIG. 12 , if the “move file” operation specified moving file “52.MD5” from folder 100 to folder 166, then the folder ID and folder parent ID fields for the first entry of file table  1220  are revised to “166” and “251”, respectively. 
   As shown in block  1390  of  FIG. 13A , the arguments extracted from the database tables are returned to the requester. In one embodiment, the response from a DDM includes XML encoded documents with the list of files (i.e., in the form of a SRL) and/or directories. For example, in response to the “open folder” request, the VFS returns file and folder Ids for the files and subfolders arranged under the subject folder. 
     FIG. 13B  is a continuation of the flow diagram of  FIG. 13A  illustrating additional file system operations in the VFS. If the operational code is a “delete folder” operation, then the corresponding folder entry is deleted from the folder table (blocks  1372  and  1374 ,  FIG. 13B ). If the operational code designates a “delete file” operation, then the file entry, identified in the operation, is deleted from its file table (blocks  1376  and  1378 ,  FIG. 13B ). For a “create file” operation, the VFS adds an entry for a new file in the file table (blocks  1386  and  1388 ,  FIG. 13B ). If the operational code specifies an “update folder” operation, then the client metadata in the corresponding folder table for the folder entry is updated (blocks  1386  and  1388 ,  FIG. 13B ). For an “update file” operation, the VFS updates client metadata in the table for the corresponding file entry (blocks  1392  and  1394 ,  FIG. 13B ). After executing the appropriate database operation, the arguments for the operation are returned to the requester (blocks  1396 ,  FIG. 13B ). 
   In one embodiment, the network storage system uses a reference count to manage up loading and deleting existing files. In general, when a new file is uploaded to the network storage system or a file request is received to upload an existing file, the reference count is incremented by one. Conversely, when a file request is received to delete a file, the reference count is decremented by one. The network storage system uses the reference count to delete an object file when the reference count is zero. For example, a client may transmit a first request to upload an object file, entitled “my file.” After the upload operation is complete, the reference count to “my file” is one. Thereafter, a client may transmit a second request to upload “my file.” Instead of storing a second copy of “my file”, the network storage system increments the reference count of “my file” to “2.” For this example, the client may then transmit a first request to delete “my file.” In response to this request, the network storage system does not delete “my file.” Instead, the network storage system decrements the reference count to “1.” Thereafter, if the client transmits a second request to delete “my file”, the reference count is decremented to “0”, and the network storage system deletes “my file.” 
     FIG. 14  is a flow diagram illustrating one embodiment for the delete file operation for the network storage system. If the VFS receives a delete request, then a DDM performs a validation check (blocks  1400  and  1405 ,  FIG. 14 ). If the delete request is not valid, then an error message is transmitted to the requester (blocks  1410  and  1415 ,  FIG. 14 ). If the request is validated, then the DDM extracts a file handle (i.e., MD5 file handle) from the file table in the database (block  1420 ,  FIG. 14 ). The DDM deletes the file identification from the file table in the database (block  1450 ,  FIG. 14 ). In addition, the DDM constructs a delete SRL, and transmits the delete SRL to the storage cluster (block  1460 ,  FIG. 14 ). In response to the delete operation, the storage cluster extracts the reference count for the corresponding file. If the reference count is greater than 1, the storage cluster decrements the reference count by one (blocks  1430  and  1440 ,  FIG. 14 ). Alternatively, if the reference count is one, the storage cluster decrements the reference count to zero, and deletes the file, identified by the SRL, in the appropriate intelligent storage node (block  1470 ,  FIG. 14 ). 
   Dynamic Data Caching: 
     FIG. 10  is a block diagram illustrating one embodiment for caching data in the storage cluster. As shown in  FIG. 10 , there are “n” DOSMs. Each DOSM (i.e., DOSM 1, DOSM 2, DOSM 3 . . . DOSM “n”) contains a corresponding data cache (i.e., data cache 1, data cache 2, data cache 3 . . . data cache “n”). The network storage system file upload and download operations are received by the load balancing fabric  310  (also see  FIG. 3 ). A switch, such as an L4 switch, with load balancing capabilities, allocates resources among a pool of resources. For the network storage system, the load balancing fabric  310  efficiently allocates requests among the “n” DOSMs. In one embodiment, when a DOSM transfers an object from the intelligent storage node to a destination, the object is cached in the data cache of the corresponding DOSM. Objects are deleted from the data cache in order to store objects more recently requested via a least recently used (“LRU”) caching policy. 
   Load balancing the DOSMs in the network storage system permits an “automatic” caching of objects in high demand. In prior art systems, elaborate mechanisms are employed to identify data in high demand. Based on these decision mechanisms, data is cached in an attempt to meet the needs of the high demand. For example, an object may be in high demand when a movie studio offers, over its web site, a video preview of a newly released or upcoming film. For this example, the movie studio uses the network storage system to deliver the media rich object, “New Film Preview.” The “New Film Preview” may be available to the end-user if the end-user “clicks” on a URL in the movie studio&#39;s web site. For this example, if the movie is very popular, when the movie studio client offers the “New Film Preview” through its web site, many end-users may attempt to download the rich object, “New Film Preview.” 
   For an initial request to download the object “New Film Preview”, the load balancing fabric  310  selects a DOSM to manage the request. For this example, the load balancing fabric  310  selects DOSM 1 to fulfill the request. Assuming the DOSM 1 does not currently store the object in its data cache, the DOSM 1 acquires the object from the appropriate intelligent storage node. As the object is delivered to satisfy the initial request, the object is stored in the DOSM 1 data cache 1. For this example, the storage cluster receives a second request for the “New Film Preview” object, and the load balancing fabric  310 , based on availability, selects DOSM 3 to process the request. Again, assuming DOSM 3 does not currently store the object in its data cache, the DOSM 3 obtains the object from the appropriate intelligent storage node, and transfers the object to the requestor as well as stores the object in the data cache 3. Similarly, for this example, additional requests are made to the storage cluster to download the “New Film Preview” object. Based on available resources, the load balancing fabric  310  selects, for two separate requests, the DOSM 2 and the DOSM “n” to handle the two requests. Again, assuming DOSM 2 and DOSM “n” do not currently store the object in their data caches, both DOSMs acquire the “New Film Preview” object from the appropriate intelligent storage node, transfer the New Film Preview to the requester, and store the object and their respective data caches (i.e., data cache 2 and data cache “n”). As illustrated by the previous example, if an object is in high demand, the storage cluster, using a load balancing fabric that selects the different DOSMs, fetches, for storage in each of the DOSM data caches, a copy of the high demand object. Thus, the distribution of DOSM resources results in fast access to an object highly requested. 
   The dynamic caching of object files in the DOSM also occurs for object files retrieved from different storage centers. For example, an object file, “New Film Preview”, may be stored in an intelligent storage node at storage center 1. In storage center 2, DOSMs receive requests for the object file, “New Film Preview.” For this example, the DOSMs in storage center 2 retrieve the object file, “New Film Preview”, from storage center 1. Similar to the example provided above, the DOSMs in storage center 2 cache the object file, “New Film Preview.” Thus, object files in high demand are cached in DOSMs globally, as required by demand. 
   As shown in the example of  FIG. 10 , each data cache stores potentially different objects depending upon requests processed in the respective DOSMs. For example, in addition to the “New Film Preview” object, data cache 1 stores “Photos Y” and “BLOB X”; data cache 2 stores “Ad  5 ” and “Video Snippet 8”; data cache three stores “Photos Z” and “Advertisement 10”; and data cache “n” stores “BLOB A” and “Video Snippet 2.” 
   Geographic Replication of Storage Centers: 
   The network storage system is optimized to support a massive number of simultaneous download transactions. The network storage system relies upon a single virtual directory of all file objects. From any location on the Internet, clients see the exact same view of their private file system. Thus, the network storage system supports simultaneous downloads of a single object that appears identical to users worldwide. In one implementation, the network storage system spans multiple continents with storage repositories or storage centers. The automatic geographic load balancing between storage centers ensures that all requests are directed to the nearest storage center. However, to provide fail over and enhanced performance, the storage center, including the storage cluster and VFS, are replicated. The physical replication across multiple locations includes a traffic management service. The traffic management service provides geographic load balancing of user transactions among geographic locations. 
     FIG. 15  illustrates geographical replications of storage centers. For this example, there is a North American storage center  1510 , an Asian storage center  1530 , and a European storage center  1520 . As shown in the example of  FIG. 15 , clients and end-users in North America have optimal access to the storage center through the North American storage center  1510 . Also, clients and end users in Europe have optimal access to European storage center  1520 . Similarly, clients and end-users in Asia have optimal access to be Asian storage center  1530 . In this configuration, the storage center is coupled to a wide area network to provide the maximum bandwidth for the delivery of objects. If a particular storage center becomes overloaded with requests, new requests are automatically diverted to the next closest storage center. All objects are geographically mirrored to provide one hundred percent disaster protection. Also, if access to the geographically disparate storage center is unavailable at the time a file is stored, then an additional copy of the file is stored at the local storage center (i.e., the object file is mirrored locally). 
   The components within the network storage system are fully redundant with automatic recovery. Thus, the system supports extremely high level of service availability. 
   Download requests to each geographic storage center are continuously distributed across the DOSMs to deliver the fastest possible response time. In addition, in one embodiment, a global load balancing system ensures that the worldwide load across all storage centers is evenly spread to eliminate any “hot spots” and alleviate transitory demand spikes. The storage system operates far more quickly than the network itself, and thus introduces negligible delay to the overall file transit time. Thus, the worse case elapsed time for the individual object download is primarily determined by the speed of the wide area network used to transfer the object. 
   All components within the network storage system are replicated and redundant to provide complete recoverability in the event of a failure. In one embodiment, each storage center attaches to multiple network back bone providers to ensure continuous network access. All files and the control path directory structure are geographically replicated at the time of upload to prevent any possible loss of data. As is described more fully below, the system maintains coherency among disparate storage centers through use of the distributed object storage protocol (“DOSP”). 
     FIG. 16  is a block diagram illustrating one embodiment for replicating the storage centers. For this example; two storage centers, labeled  1510  and  1520 , are shown. However, based on the distributed architecture of the network storage system, any number of storage centers may be replicated. Storage centers  1510  and  1520  both include, for the storage cluster, load balancing fabric  320 , distributed objects storage managers (“DOSMs”)  320 , interconnect fabric  330 , and intelligent storage nodes  340 . Storage center  1510  stores the same object files as storage center  1520 . For example, if “object file 1” is stored in storage node 10 storage center  1510 , then “object file 1” is stored in storage node “1” in storage center  1520 . For the control path, the storage centers and  1510  and  1520  include the virtual file system (“VFS”)  50 . Similarly, the VFS in storage center  1510  stores the same directory information as the VFS in storage center  1520 . Accordingly, the storage centers are replicated. Although the VFS and the storage clusters are shown in the same geographic “storage center”, the VFS and storage cluster may be located at geographically disparate locations. 
   For this example, intelligent storage nodes in storage cluster  1510  (i.e., storage node 1, storage node 2, . . . storage node “n”) are accessed via Internet protocol (“IP”) addresses IP addr 1 , IP addr 2 , and IP addr n , respectively. Thus, when a DOSM communicates with an intelligent storage node in storage center  1510 , the DOSM uses these IP addresses to access the specific intelligent storage node. Storage center  1520  includes storage nodes (i.e., storage node 1, storage node 2, . . . storage node “n”) addressed by IP address IP addr 1 ′, IP addr 2 ′, and IP addr n ′, respectively. Thus, in storage center  1520 , when a DOSM communicates with the storage node, the DOSM uses an IP addr across the interconnect fabric  330 . Although the replication of storage centers is described using an TCP/IP network protocol, any network protocol and corresponding addressing scheme may be used to replicate the storage centers. 
   As shown in  FIG. 16 , the distributed objects storage managers of storage center  1510  are coupled to the interconnect fabric of storage center  1520 . Similarly, the distributed object storage managers of storage center  1520  are coupled to the interconnect fabric of storage center  1510 . Based on this configuration, the distributed objects storage managers of storage center  1510  have access to the intelligent storage nodes of storage center  1520 . Likewise, the distributed object storage managers of storage center  1520  have access to the intelligent storage nodes of storage center  1510 . As discussed above, each DOSM maintains a lookup table that correlates a file to an IP address (See  FIG. 6 ). For example, if a file specified in a download request resides on storage node 1 in storage center  1510 , then an entry of the DOSM lookup table specifies IP addr 1 . Similarly, in storage center  1520 , if a file resides in storage node 1, an entry for the DOSM lookup table specifies IP addr 1 ′. 
   The storage center architecture supports a “dynamic” fail over. If a storage node, or a disk drive on a storage node, renders the access to a file inaccessible, then the DOSM may obtain the file from the replicated storage center. In one embodiment, to perform “dynamic” fail over, a mapping is stored between intelligent storage nodes in storage center  1510  and intelligent storage nodes in storage center  1520 . Table 6 below shows a mapping for the example in configuration of  FIG. 16 . 
   
     
       
         
             
             
             
           
             
                 
               TABLE 6 
             
             
                 
                 
             
             
                 
               IP Address 
               IP Address′ 
             
             
                 
                 
             
           
          
             
                 
               IP Addr 1   
               IP Addr 1 ′ 
             
             
                 
               IP Addr 2   
               IP Addr 2 ′ 
             
             
                 
               . 
               . 
             
             
                 
               . 
               . 
             
             
                 
               . 
               . 
             
             
                 
               IP Addr n   
               IP Addr n ′ 
             
             
                 
                 
             
          
         
       
     
   
   For this example, IP addr 1  maps to IP addr 1 ′. If there is a failure in storage node 1 in storage center  1510 , then DOSMs of storage center  1510  access storage node 1 of storage center  1520  using IP addr 1 ′. In one embodiment, the IP mapping between storage centers is implemented by modifying only the subnet address portion between the two IP addresses mapped. For example, if IP addr 1  is 10.3.100.1, then IP addr 1 ′ is derived by changing, as appropriate, the subnet portion of the address (e.g., 10.10.100.1). 
   The directory information stored in the VFS is replicated between storage center  1510  and  1520  in a similar manner. Thus, if a failure occurs in a distributed directory of storage center  1510 , then the distributed directory manager in storage center  1510 , using an IP address mapping, accesses the replicated distributed directory in storage center  1520 . 
   In one embodiment, to further implement geographic replication for a fail over mode, if one disk fails, then a DOSM attempts to identify the file in the same node at a different storage center. If a storage node is rendered inoperable, then the DOSM clears the entry in the DOSM file lookup table, and attempts to locate the file at a remote storage center. For example, if disk “2” of storage node “1” in storage center  1510  fails, a DOSM  320  attempts to locate the file in storage node “1”, disk “2”, in storage center  1520 . If the file is not located in storage node “1”, disk 2, of storage center  1520 , the DOSM, using the multicast protocol, attempts to locate the file locally (i.e., in the storage center  1510 ). If the file is not located locally, the DOSM, using the multicast protocol, attempts to locate the file at a remote storage center (e.g., storage center  1520 ). 
   Accessing the Network Storage System: 
   The network storage system has application for use in content delivery networks. In general, content owners and providers often employ the services of a content delivery network. Content delivery networks attempt to optimize the delivery of commonly accessed rich media objects. In order to maximize the delivery of the rich media objects, content delivery networks employ local caches at the edges of the wide area network. 
   The network storage system has applications to complement content delivery networks by providing the underlying content for the content origin web site. In one embodiment, each cache at the content delivery network directly accesses the geographically closest storage center to locate the desired object to eliminate the need for content delivery network to access the content owner&#39;s/provider&#39;s web site. 
     FIG. 17  illustrates one embodiment for use of the storage center in a content delivery network. For the example of  FIG. 17 , the content delivery network  1700  includes an end-user computer  1740  coupled over a network (e.g., Internet) to a content origin web server  1720 . The content origin web server  1720  implements or hosts a web site. The web site permits the end-user to select content, such as rich media objects. A content delivery network includes a (“CDN”) server  1730 . The CDN server  1730  delivers content published on the web site by the content origin web server  1720 . Specifically, the end-user computer  1740  is coupled to the CDN server  1730  to maximize the delivery of content, including rich media objects associated with the web site, to the end-user. The CDN server  1730  caches, at the CDN, a portion of the content associated with the web site  1730 . 
   For purposes of illustration, a wide area network  1750  is shown as including satellite communication networks  1760 , wireless communication networks  1770 , and fiber-optic networks  1780 . As illustrated in  FIG. 17 , the CDN server  1730  is located close to the edges of the wide area network  1750 . The location of CDN server  1730  close to the wide area network  1750  optimizes the delivery of objects cached at the CDN server  1730 . For this embodiment, one or more storage center(s)  1710  are coupled to the CDN server  1730 . In the event of a cache miss at the CDN server  1730 , the CDN server  1730  obtains the content (e.g., object file) from storage center(s)  1710 . This configuration allows the CDN server  1730  to bypass the slower content origin web server  1720  in the event that content, requested by end-user computer  1740 , is not located at the CDN server  1730 . According, the storage center(s)  1710  optimize routing of content through the Internet back to the CDN when the desired content is not located in the local cache. 
     FIG. 18  is a flow diagram illustrating one embodiment for use of the storage center with a content delivery network. The end-user, through the end-user computer, generates an HTTP request to the content origin web server (block  1800 ,  FIG. 18 ). In response to the user request, the content origin server returns to the end-user computer HTML with embedded file URLs (block  1810 ,  FIG. 18 ). The embedded file URLs identify the rich media objects stored at the CDN server. To obtain the rich media objects, the end-user computer generates HTTP file requests to the content delivery network (e.g., CDN server  1730 ) (block  1820 ,  FIG. 18 ). If the file identified by the URL is located in a cache at the CDN server site, then the CDN server delivers the file to the end-user computer (blocks  1825  and  1850 ,  FIG. 18 ). Alternatively, if the file is not cached at the CDN server site, the CDN server generates an HTTP file request to the storage center (blocks  1825  and  1830 ,  FIG. 18 ). In one embodiment, the HTTP file request includes the network storage system&#39;s SRL, to uniquely identify the file. In response to the CDN server&#39;s request, the storage center downloads the file to the CDN cache (block  1840 ,  FIG. 18 ). The CDN server delivers the file to the end-user computer (block  1850 ,  FIG. 18 ). 
   Accessing the Network Storage System Using a Storage Port: 
   There are multiple ways to access the network storage system. In one embodiment, the client uses a “storage port.” The storage port provides access to the network storage system through a standard file system interface (e.g., network file system (“NFS”) or Microsoft NT CIFS). The storage port may be configured by the client in various ways for different applications to optimize the delivery of rich media objects. In one embodiment, the storage port is configured at the client site to provide seamless integration from the client site to the network storage system. In another embodiment, to further off load rich media object traffic from a web site, the storage port may be used as a file system manager that downloads files to the end-user directly from the network storage system. In other embodiments, the network storage system may be directly interfaced with a private file structure. 
   The storage port device provides a transparent gateway connection into the network storage system. In one application, the storage port device is installed at the client site, and interfaces to local web servers via standard NFS or CIFS protocols over a local area network (“LAN”) connection. Specifically, in one embodiment, the user mounts the storage port as a storage device on the client network. In this configuration, the storage port effectively provides the user with a virtual NFS or CIFS file system with storage capacity at the storage center (i.e., provides the user with hundreds of terabytes in storage capacity). In one embodiment, the storage port device occupies only approximately 1.75 inches of rack height. As described more fully below, multiple storage ports may be installed at a single client site to increase aggregate throughput. 
     FIG. 19  illustrates one embodiment for use of the storage port in the network storage system. An end-user  1900  communicates with a client site  1910  over a wide area network  1920 . The end-user computer  1900  generates requests (e.g., HTTP requests) for files accessed through the client&#39;s web site. A content web server  1925 , located at the client site  1910 , processes requests to the client web site, including requests to download rich media objects. Content web server  1925  is intended to represent a broad category of computers and software used to implement a web site, such as multiple web servers and/or application servers, and any hardware/software configuration may be used without deviating from the spirit or scope the invention. 
   The content web server  1925  is coupled to the storage port  1930  over a network, such as a local area network at the client site  1910 . Specifically, the content web server  1925  generates file and directory operation requests in accordance with the format of the “local” file system. As used herein, a “local” file system connotes one or more file systems or file structures used at the client site. For example, the content web server  1925  may generate NFS or Microsoft NT CIFS requests for files and directory operations. To interface the storage port  1930  with the content web server  1925 , the storage port  1930  is mounted as a storage device. In one embodiment, one directory is mounted for object files and a second directory is mounted for SRLs. As shown in  FIG. 19 , the storage port  1930  communicates with the storage center  1950  to conduct file and directory operations. 
     FIG. 20  is a flow diagram illustrating one embodiment for use of a storage port to deliver content. The client site receives a URL file request from an end-user computer (block  2010 ,  FIG. 20 ). The URL identifies an object file associated with the client&#39;s web site. In response to the end user&#39;s URL file request, the client site (e.g., content web server) generates a local file system request for the object file (block  2020 ,  FIG. 20 ). The local file system request is received by the storage port. The storage port includes a cache to store both object files and directory information. If the object file is stored locally in the storage port, then the storage port retrieves the object file from the data cache, and returns the object file to the content web server in response to the local file system request (blocks  2030 ,  2040 , and  2070 ,  FIG. 20 ). Alternatively, if the storage port does not store a copy of the object file in its data cache, then the storage port requests the object file from the storage center (blocks  2030  and  2050 ,  FIG. 20 ). In response to the local file system request, the storage center downloads the object file to the storage port, and the object file is returned to the content web server (blocks  2060  and  2070 ,  FIG. 20 ). Thereafter, the content web server delivers the object file to the end-user in response to the URL file request (block  2080 ,  FIG. 20 ). 
   The storage port may be implemented in either hardware or software.  FIG. 21   a  illustrates one hardware configuration for a storage port device. As shown in  FIG. 21   a , the content web server  2100  communicates with the storage port  2110  over a communications link  2120 , such as a local area network. The storage port  2110  conducts file and directory operations with storage center  2130 . 
     FIG. 21   b  illustrates embodiments for implementing the storage port in software. In one embodiment, the network storage system is accessed through library calls or through application program interface (“API”) calls. For these embodiments, the software provides translation between the client&#39;s local file system and the network storage file system. As discussed above, the storage center  2160  includes software running on computers for performing the functions of the VFS and intelligent storage clusters. This software includes entry points (i.e., APIs) to permit interfacing of external software. In part, the APIs on the storage center software permit the client to conduct file and directory operations as described herein. As shown in  FIG. 21   b , content web server  2140  runs, in addition to software to operate the client site, software to call APIs in the network storage center. Thus, for this embodiment, the content web server  2140  executes network storage system file and directory operations over the wide area network  2180  through remote program calls. 
   In another embodiment, shown as storage system library calls  2155 , a customized network storage system library includes a collection of file system operations. For example, one library function may permit software operating at the client (e.g., on content web server  2140 ) to request an object file download to the storage center through use of the library function. For this example, to perform the file download operation, the client software calls the file download function and passes the SRL as an argument to the function call. A library of functions provides an additional means to interface client software to directly access the network storage system. 
     FIG. 22  is a block diagram illustrating one embodiment for a storage port. As shown in  FIG. 22 , a storage port  2200  includes a processing core  2210 , memory  2230 , storage port data store  2240 , and network interface(s)  2220 . These components are coupled via a bus transport  2250  that may include one or more busses (e.g., ISA, PCI, or microprocessor buses). Processing core  2210  includes one or more central processing units (“CPUs”). In one embodiment, the storage port includes two CPUs. Memory  2330  is used to store, during operation of the device, software to perform the functions of the storage port described herein. The storage port data store  2240  contains one or more hard disk drives (i.e., “n” hard disk drives, wherein “n” is any number one or greater), used, in part, to cache file system information (i.e., directory cache) and object files (i.e., data cache). The network interface(s)  2220 , which includes “n” network interface cards, couples the storage port  2200  to client devices (e.g., content web server). In addition, to support a fail over architecture, the network interface cards are used to connect one or more storage ports together. In one embodiment, the storage port includes three network interface cards. 
     FIG. 23  is a block diagram illustrating one embodiment for file system translation in the storage port. The network storage system issues “file handles” unique to the network storage system. In one embodiment, a network storage system file handle identifies, for a corresponding file: a) client identification; b) parent directory; c) metadata and d) the unique digital fingerprint (i.e., 128 bit MD5 identification). In general, the file system translation software  2300  converts local file system operations to network storage system file system operations. In one embodiment, to perform this function, the software includes file system translator  2320  and storage system access processes  2330 . The file system translator  2320  includes local file system interception  2340  and storage system kernel processes  2350 . 
   In operation, local client file system  2310 , which may include operating system software running at the client&#39;s site, issues local file system operations. For example, the client software may issue requests, in accordance with UNIX or Microsoft NT to open a file. The file open operation includes a file descriptor that identifies the file in the local file system. Typically, file system calls are processed by the operating system kernel (labeled  2360  in  FIG. 23 ). The operating system kernel software maintains a mapping between file descriptors and directories to “inodes.” The inodes provide the system a physical pointer to the file data in the system (e.g., a pointer to the file stored on a hard disk drive). 
   For the embodiment of  FIG. 23 , when the local client file system  2310  issues a file system operation, local file system interception  2340  “traps” or intercepts the call, and passes the thread of execution to the storage system kernel processes  2350 . In one embodiment, the local file system interception  2340  comprises CODA software, developed at Carnegie Mellon University. In general, CODA is a type of distributed file system. A portion of the functionality provided by the CODA software exports an underlying file system. Specifically, CODA exports file system operations, typically executed in the kernel level, to applications programs accessible in the user portion of memory. Although file system translation is described using CODA to intercept local file system operations, any software that intercepts file system calls may be used without deviating to the spirit or scope of the invention. 
   In general, the storage system kernel processes  2350  obtains network storage system file handles (referred to herein as “storage handles”) for storage in operating system kernel  2360  to provide a mapping between local file system descriptors and storage handles. Thus, the file descriptors provide a handle to identify files and directories in the local file system, and the storage handles provide a handle to identify files and directories in the network storage system. 
   To maintain the mapping between local file system descriptors and storage handles, the storage system kernel processes  2350  obtains network storage file system information from storage system access processes  2330 . Specifically, storage system kernel processes  2350  obtains from storage system access processes  2330  storage handles and directory information. As shown in  FIG. 23 , storage system access processes  2330  obtain directory and storage handle information from directory cache  2370 . Alternatively, if directory and storage handle information is not cached at the storage port, storage system access processes  2330  query the network storage system (i.e., VFS) to obtain directory information and storage handles. Accordingly, the translation system  2300  provides a mapping between the client&#39;s local file system and the network storage file system. 
     FIG. 24  is a flow diagram illustrating one embodiment for translating a file system operation from a local file system to the network storage file system. The process is initiated by the client issuing a local file system request (block  2400 ,  FIG. 24 ). The local file system request is received by the operating system kernel, and dispatched to the file system translator ( FIG. 23 ). For example, if the file system operation is an open file operation for the file “foo.txt”, then the operating system kernel dispatches the open file operation with the file name “foo.txt” as an argument to the file system translator. If the file system operation is an “Open Folder” operation for the folder “dir1”, then the operating system kernel dispatches the open folder operation with the folder name “dir1” as an argument. 
   The process determines whether there is sufficient directory information in the storage port directory cache (block  2430 ,  FIG. 24 ). For the “Open Folder” example above, if the storage handles for all subfolders and files are not stored in the directory cache, then additional directory information is required to fulfill the request. For the “Open File” example, if the storage port has been recently initialized and thus does not contain information on the file, then additional directory information on the file (e.g., “foo.text”) is required to open the file. 
   If there is sufficient directory information in the directory cache, and the file system operation does not require retrieving data (i.e., the file system operation is not an “open file” operation) or updating directory information, then the appropriate directory information from the directory cache is retrieved and returned in response to the local file system operation (blocks  2430  and  2435 ,  FIG. 12 ) (blocks  2435  and  2437 ,  FIG. 24 ). For the “Open Folder” example above, storage handles for all subfolders and files in the subject folder are retrieved from the directory cache, the storage handles and corresponding file identifiers are stored in the operating system kernel, and the file identifiers are returned to local file system. 
   If additional directory information is required (i.e., the information is not in the storage port directory cache), then a request is generated to the VFS for the additional directory information (block  2070 ,  FIG. 24 ). In one embodiment, the storage port generates an XML encoded request For the “Open Folder” example, if the storage nodes and corresponding file identifiers are not stored in the directory cache, then the storage port generates an XML encoded “Open Folder” request to extract file and folder information for files and subfolders within the subject folders (i.e., the folder that is the subject of the “Open Folder” request). In one embodiment, in response to a request for folder information, the VFS returns name, folder identification, client metadata, upload SRL, and parent folder identification. In response to a request for file information, the VFS returns name, file identification, client metadata, download SRL, and parent folder identification. In one embodiment, the client metadata fields are used to track and maintain state information used in the local file system (e.g., information for UNIX, Microsoft Windows or NT, etc.). In addition to obtaining additional directory information, if the client local file system command is a directory operation (i.e., “move folder”, “delete folder”, etc.), then an XML request to the VFS is generated to perform the directory operation in the VFS. The directory information is received and stored in the directory cache (block  2480 ,  FIG. 24 ). 
   If the file system operation requires file data (e.g., open file, read file etc.), then the storage port determines whether the file is located in the data cache (block  2440 ,  FIG. 12 ). If the file is stored in the data cache, then the file, or appropriate portion, is transferred from the storage port to the client requestor (block  2090 ,  FIG. 12 ). Alternatively, if the file is not in the data cache, then the storage port generates a file download request to the storage cluster (block  2050 ,  FIG. 24 ). In response to the storage cluster request, the storage port receives and subsequently caches the object file in the data cache (block  2060 ,  FIG. 12 ). The object is then transferred from the storage port to the client requestor (block  2090 ,  FIG. 12 ). 
   End User Network Storage System Access Method: 
   In another embodiment, the storage port supports file downloads directly to the end-user or through a CDN partner. In one embodiment, the SRLs are directly embedded into the Web page HTML, and are sent to the end-user. This results in transferring objects directly from the storage center to the end-user browser.  FIG. 25  is a block diagram illustrating one embodiment for using the storage port to directly download object files to the end-user. For this configuration, an end-user computer  2610  communicates with a client site  2620  and the storage center  2650 . The client site  2620  maintains a web site. For this embodiment, the client site  2620  maintains a web site through a content web server  2630 . However, any configuration of servers, including remote web site hosting, may be used without deviating the spirit or scope of the invention. 
   The content web server  2630  communicates with the storage port  2640 , and in turn, the storage port  2640  communicates with the storage center  2650 . As illustrated in  FIG. 25 , the end-user, through end-user computer  2610 , generates URL requests to the client site  2620 , and receives, in return, HTML with one or more embedded SRLs. Using the embedded SRLs, the end-user computer  2610  generates SRL requests directly to the storage center  2650  over a wide area network  2660 . In response, the storage center  2650  serves object files directly to the end-user computer  2610 . 
     FIG. 26  is a flow diagram illustrating one embodiment for directly downloading object files to an end-user. The client site (e.g., content web server) generates local file system requests for SRL(s) corresponding to file(s) (block  2700 ,  FIG. 26 ). The file(s) contain content that the client desires to embed in the web page. In one embodiment, the storage port dynamically generates the SRL(s) in response to the request from the content web server (block  2710 ,  FIG. 26 ). In one embodiment, a time-out parameter is added to the SRL(s) (block  2720 ,  FIG. 26 ). The time-out parameter permits a client to specify a period of time that the SRL is valid (i.e., a period of time that the end-user may access the file). In one implementation, the time-out parameter specifies a period of time with a granularity in seconds. 
   The SRL(s) are embedded in the HTML of the client&#39;s web page (block  2730 ,  FIG. 26 ). The end-user issues web page requests to the client site (block  2740 ,  FIG. 26 ). The content web server then downloads the requested HTML with the embedded SRL(s) (block  2745 ,  FIG. 26 ). With the embedded SRL, the end-user generates HTTP requests to the storage center (block  2750 ,  FIG. 26 ). If the SRL(s) do not authenticate at the storage center, then the storage center transmits an error message to the end-user (block  2755 ,  FIG. 26 ). If the SRL(s) do authenticate, then the time-out parameter is checked to determine whether the file access is valid (block  2760 ,  FIG. 26 ). If the SRL is not valid (i.e., the time-out parameter is out of range), then the operation is ceased (block  2760 ,  FIG. 26 ). If the SRL is within the specified time range, then the storage center downloads the object file to the end-user (block  2770 ,  FIG. 26 ). 
   The storage port  2640  acts as a file system cache. For this embodiment, the storage port contains the client&#39;s SRL files stored in a standard NFS or CIFS directory format. Each NFS or CIFS file contains the corresponding SRLs, and the SRLs contain the unique file identifier and the SRL authentication certificate. 
   In one embodiment, to deliver the SRLs to the end-user, the network file system utilizes a second directory, in addition to the directory for the object files, that shadows the object file directory. The client uses the second directory to obtain shadow files. A shadow file contains an SRL to identify an object file of the network storage system. In one embodiment, to embed the SRL into the web page HTML, the client reads the contents of the shadow file for the corresponding object file. In one embodiment, the shadow file is generated during an upload operation. The client may access a shadow file by mounting the second directory. For example, a client may specify, for the file “foo.text”, the following directory-filename:
         storagefilesystem:/export/dir/foo.text.
 
The client uses this directory and filename to access the contents of the object file, “foo.text.” To obtain the SRL for the example file “foo.text”, a client mounts a different directory, such as the following example directory:
   storagefilesystem:/SRL/dir/foo.text,
 
wherein, the SRL file contains a unique file identifier and the SRL authentication certificate for the file, “foo.text.” To deliver the SRL to the end-user, the client reads the contents of a shadow file for the corresponding object file, and publishes the SRL to the user.
       

   Client Private File System Directory: 
   The network storage system of the present invention also supports using an existing private file directory to access the storage system. For this embodiment, the network storage system customer (e.g., client) may desire to use their own file structure in conjunction with the network storage system&#39;s file system. In other embodiments, a client of the network storage system may wish to develop a file system to track additional information beyond that information tracked using NFS or CIFS. 
     FIG. 27  is a block diagram illustrating one embodiment to interface a storage center to a client&#39;s private file directory system. In one embodiment, the storage port at the client site  2820  is replaced with a private file manager  2840 . For this embodiment, the private file manager  2840  generates SRLs for object files using a unique file identification assigned to the user file at the time of upload, as well as using a shared secret to authenticate file system operations. As shown in  FIG. 27 , the content web server  2830 , operating at the client site  2820 , generates file system requests to the private file manager  2840 . In turn, the private file manager  2840  issues SRLs corresponding to the object files that are the subject of the request. In one embodiment, the client supplies their own unique ID at the time the client uploads files to the storage center. In another embodiment, the client utilizes, in requests to download files, the object finger print returned by the storage center. 
   As shown in  FIG. 27 , the end-user, through end-user computer  2810 , generates URL requests to the client&#39;s web site. In turn, the client site  2820  returns HTML with embedded SRLs. With the embedded SRLs, the end-user computer  2810  generates SRL requests, over a wide area network  2860 , to the storage center  2850 . In turn, the storage center  2850  serves object files identified by the SRL. 
     FIG. 28  is a flow diagram illustrating one embodiment for accessing object files in a storage center using a client&#39;s private file system. The end-user issues the URL requests to the client web site (block  2900 ,  FIG. 28 ). In response, the client (e.g., content web server) generates file location requests to a file manager (block  2910 ,  FIG. 28 ). In general, the file manager services requests to issue SRLs corresponding to files in the client&#39;s private file system. A client may use any type of file system in conjunction with the network storage system. All that is required is that the client&#39;s private file system issues SRLs for files managed by the client&#39;s private file system. The file manager retrieves the SRL for the file associated with the HTML, and delivers the file to the content web server (block  2920 ,  FIG. 28 ). The content web server then transmits to the end-user HTML with the embedded SRL (block  2930 ,  FIG. 28 ). Thereafter, the end-user generates HTTP requests to the storage center with the SRL (block  2940 ,  FIG. 28 ). If the SRL does not authenticate, then the storage center issues an error message to the user. Alternatively, if the SRL authenticates, then the storage center generates an MD5 hash on the client supplied unique file ID to identify the file (block  2947 ,  FIG. 28 ). The storage center thereafter downloads the object file to the end-user (block  2950 ,  FIG. 28 ) 
   For the client&#39;s private file system access method, the client maintains a mapping between unique filenames and SRLs. In one embodiment, the unique filename is not obtained from an MD5 hash operation, but is a unique filename. Thus, the network storage system utilizes a technique to differentiate between MD5 file names, derived from the contents of the object file, and client unique file names. In one embodiment, to differentiate between these two types of file names, the network storage system assigns different storage fingerprint identifiers. For a filename generated by an MD5 hash operation on the contents of the object file, the file is designated “128bits.MD5.” To identify a customer unique filename, the file is designated as “MD5.UFID” (i.e., where “MD5” is the client&#39;s unique file name). This convention permits the network storage system to differentiate between the twp types of file identifiers, and allows the customer to interface with the network storage system by only designating unique file names. 
   Failover Architecture: 
   In one embodiment, the storage port supports failover or failsafe architectures.  FIG. 29  is a block diagram illustrating one embodiment for a storage port fail over configuration. For purposes of explanation,  FIG. 29  illustrates a fail over configuration with two storage ports. However, the storage port fail over configuration may be extended to any “2N” fail over configuration. For this embodiment, the fail over configuration includes an active storage port  3010  and a passive storage port  3020 . Each storage port includes a plurality of network interface cards. Both the active storage port  3010  and passive storage port  3020  communicate to storage center(s) over wide area network  3065 , through network interface cards  3045  and  3025 , respectively. The active storage port  3010  and passive storage port  3020  also communicate to the client site network via network interface cards  3050  and  3035 , respectively. As shown in  FIG. 29 , the client accesses the active storage port  3010  over client site network  3060  using IP Addr. 
   For the embodiment of  FIG. 29 , a third network interface card is contained on both the active storage port  3010  ( 3055 ) and passive storage port  3020  ( 3030 ) to communicate between the devices for fail over monitoring. The active storage port  3010  operates as current storage port at the client site. The passive storage port  3020  monitors the health of the active storage port  3010 . Specifically, active storage port  3010  includes health monitoring  3070  that continually executes a process to ascertain the health of the active storage port  3020  (e.g., health of the CPUs, hard disk drives, etc.). For this embodiment, the passive storage port  3020  queries the active storage port  3010  for health status. If a condition occurs in the active storage port  3010  that warrants a fail over condition, then the passive storage port  3020  becomes the active storage port (i.e., the passive storage port is used to interface the client site to storage center(s)). 
   In one embodiment, to support fail over, one IP address is used for the NFS/CIFS export. For this embodiment, a standard IP switch over scheme may be utilized. Specifically, when a fail over condition occurs, the passive storage port  3020  assumes the IP address of the active storage port  3010 . The health monitoring  3070  and  3080  include both active and passive processes, so that if a fail over condition occurs, the passive storage port may execute the active storage port process. 
     FIG. 30  is a flow diagram illustrating one embodiment for a storage port fail over process. When a storage port fail over occurs, the new storage port does not contain any directory information in its directory cache or any objects in its data cache. Thus, after a fail over operation, if a file is open and the storage port receives a read file request, the new storage port must execute a file open operation (blocks  3130  and  3140 ,  FIG. 30 ). After the storage port receives the file identification information (e.g., SRL), the storage port generates a request to the storage center to obtain the object file, in order to transmit a block of object data in response to the read file request. 
   After a fail over condition, when a file is requested (block  3120 ,  FIG. 30 ) or an open file operation is necessary, the storage port generates an XML to the VFS to obtain file identification information (block  3150 ,  FIG. 30 ). In response, the VFS returns file identification information (block  3160 ,  FIG. 30 ). With the file identification information, the storage port updates its directory cache (block  3170 ,  FIG. 30 ). With the file identification information (e.g., SRL), the storage port generates a request to the storage center for the object file (block  3180 ,  FIG. 30 ). In response, the storage center delivers the object file, and the storage port updates its data cache (block  3190 ,  FIG. 30 ). If the storage center download operation was in response to a read request to the storage port, the read request delivers data as specified in the read request. 
   Network Storage System Dynamic Failover: 
   In one embodiment, storage nodes monitor the health of their respective nodes (e.g., monitor hard disk drives, processor, network access, etc.). If the health of a storage node requires that the storage node should cease operation, then the storage cluster executes a fail over operation. In one embodiment, in a fail over operation, the storage node reports the failed status to the DOSMs, and the DOSMs update their state table. If this occurs, the DOSMs attempt to locate the replicated file at a different storage node (i.e., either locally or remotely). 
     FIG. 31  is a flow diagram illustrating one embodiment for using the multicast protocol after a storage node fail over condition. If a storage node fails, then the DOSMs update and their state tables to indicate that the storage node is no longer in use (blocks  3210  and  3220 ,  FIG. 31 ). If the DOSM receives a file request for a file previously stored on the failed storage node, then the DOSM, which received the download request, issues a multicast protocol request to the storage nodes (blocks  3225  and  3230 ,  FIG. 31 ). In one embodiment, the DOSM may issue the multicast protocol request to local storage nodes (i.e., storage nodes located at its storage center). 
   Each storage node that receives the multicast request determines whether it contains the requested object file (block  3240 ,  FIG. 31 ). If none of the storage nodes contain the object file, then the DOSM may issue another multicast protocol request at a remote storage location (blocks  3245  and  3247 ,  FIG. 31 ). Again, at the remote storage center, each storage node determines whether it contains the requested object file (block  3240 ,  FIG. 31 ). In another embodiment, if the DOSM does not locate the file using the multicast protocol, the DOSM may query each individual storage node using the DOSP point-to-point protocol. 
   When a storage node locates the requested object file, the storage node broadcasts the file identification information using the multicast protocol (block  3250 ,  FIG. 31 ). Each DOSM snoops, using the multicast protocol, to receive the file identification information (block  3260 ,  FIG. 31 ). As illustrated in the process embodiment of  FIG. 31 , the multicast protocol may be used to synchronize file location information in the DOSMs in the event of a fail over condition. 
   Multi-Cast Protocol: 
   The multi-cast protocol of the present invention supports the maintenance of file information in a distributed storage system. Since the network storage system consists of a plurality of storage nodes, the multicast protocol is used to track file information and synchronize file information throughout the network storage system. The tracking and maintaining of file and directory information includes maintaining information throughout geographically disparate storage centers. In one embodiment, the multi-cast protocol synchronizes cache information in the DOSMs. For example, if a new object file is loaded, the multi-cast protocol provides a means for all DOSMs in the network storage system to obtain information necessary to access the new object file. In addition, some file operations, including delete file or update file operations, require updating the DOSM lookup tables. Also, if a storage node fails, and a fail over condition is executed, the multi-cast protocol provides a means for the DOSMs to locate the file at the storage node the file has been replicated. 
   The Distributed Object Storage Protocol (DOSP): 
   In one embodiment, the DOSP includes daemon/master services and multicast-based monitoring communications. Communication between the daemon and master components is accomplished through a set of “request packets” and “response packets.” The request packets consist of three major subcomponents: an opcode that specifies the type of request; a header implemented via a C++ specific structure that provides information about the data that follows; and data transmitted, if any. 
   Each operation has an associated operation code and a pair of structures: one for issuance of the request, and a second separate structure for return values. Once the receiver has received and processed the request (sent data, deleted file, etc) it then sends a response consisting of the appropriate “Out Structure” indicating the status of the request (SUCCESS, FAILURE, etc) and any required return values. Currently, there are six service operations supported by the DOSP: null, store file, retrieve file, retrieve file range, delete file, and get contents. 
   The null operation provides a framework to develop future modifications of the protocol and to test basic functionality of the master/daemon request/response interaction. 
   When a file is ready for storing, the DOSM client sends a request id, followed by a request header. It then sends the data to the dosd in a series of chunks, each of which is preceded by a DosdStoreHeader which gives the size of the next chunk to be read, and a field indicating whether this is the last packet to be sent. 
   When a file is being retrieved from the Storage Cluster, the DOSM client sends a request Id, followed by a request structure. The DOSD responds by first sending the size of the data, the data requested, and finally an Out structure with the return value of the operation. 
   The get contents operation is used to acquire the contents of the storage node as a character based stream. The after the “In Structure” is passed to the dosd, the dosd first returns the length of the stream of md5 hash/node&amp;disk associations, followed by the stream of data, with the “Out structure” coming last. 
   The DOSP provides an extensible framework for any new services or additional functionality. There are essentially three steps to adding new functionality: defining a new pair of In/Out structures; assigning a new opcode, implementing a handle in the DOSM client; and adding a service handle for the dosd. 
   To facilitate gathering of information about the system, the DOSP provides several multicast-based services. In one embodiment, these services work in a manner very similar to the non-multicast aspect of the protocol. Specifically, requests consist of three parts: an opcode; a request In structure; and any additional data. 
   Responses consist of a response structure containing a RETURN value and any other return values required to satisfy the request. If data is streamed, a size field precedes the data, followed by the data, and then followed by the Out structure. 
   Since multicast traffic occurs on a completely separate port from point-to-point dosm/dosd traffic, the multicast In/Out structures are not multicast-specific. This makes it possible for the DOSM to query the entire dosd storage cluster or to query an individual machine with the same request/response structures and their associated operational sequencing. 
   One of the jobs of the DOSM is to monitor the current state of nodes in the cluster. There are several tools to facilitate this task. Primarily, the various dos daemons multicast heartbeats on a specific multicast port and group. The DOSM contains an option to query a specific disk, or all of the disks on a given a storage node. A “get disk state” function returns a value, and an array of disk state values (online, offline, down) with one entry per disk. A “get disk status” function contains an option to query a specific disk, or all of the disks on a given a node. The “get disk status” contains a RETURN value, and an array of disk statistics; one array per statistic (bytes free, bytes available, inodes used, inodes available, number of outstanding ops), with one entry per disk. The DOSP includes a load balancing function. 
   The DOSP includes a heartbeat function. This allows querying specific machines for a heartbeat in addition to providing system-wide tracking functionality via multicast methods. 
   Although the present invention has been described in terms of specific exemplary embodiments, it will be appreciated that various modifications and alterations might be made by those skilled in the art without departing from the spirit and scope of the invention.