Patent Publication Number: US-7912866-B2

Title: System and method for detecting and storing file identity change information within a file system

Description:
This application is a continuation of U.S. patent application Ser. no. 10/723,704, entitled “System and Method for Detecting and Storing File Identity Change Information Within a File System”, filed Nov. 26, 2003, now U.S. Pat. No. 7,328,217, issued Feb. 5, 2008. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to computer systems and, more particularly, to file-based storage systems. 
     2. Description of the Related Art 
     Computer systems often process large quantities of information, including application data and executable code configured to process such data. In numerous embodiments, computer systems provide various types of mass storage devices configured to store data, such as magnetic and optical disk drives, tape drives, etc. To provide a regular and systematic interface through which to access their stored data, such storage devices are frequently organized into hierarchies of files by software such as an operating system. Often a file defines a minimum level of data granularity that a user can manipulate within a storage device, although various applications and operating system processes may operate on data within a file at a lower level of granularity than the entire file. 
     In many conventional file-based computer systems, files may be created, destroyed and manipulated with relatively few constraints. Typically, files may be arbitrarily named, subject to operating system conventions, and often, unlimited numbers of exact copies of existing files may be made with ease, subject only to available storage capacity. While such ease of data proliferation may simplify system operation for the user, it may also result in inefficient use of storage devices. For example, storage devoted to multiple identical copies of a given file may be redundant and therefore wasted. Further, if a user creates multiple copies of a given file, gives each a unique identity, and then proceeds to work with each file individually, the relationships among files (such as their common origin, type, and degree of common content) may be obscured over time. Still further, not all types of files may be equally well suited to a given type of storage available in a system. For example, recently used data files may be more likely to be used again in the future and therefore good candidates to be stored in faster storage such as a disk drive, but files unlikely to be used again may be better suited to be stored on a tape drive. 
     Attempting to track file operations as they occur, to thereby gather greater information about such operations, is complicated by the problem of how such operations may be detected. In most operating system embodiments, application programs may be isolated from one another during execution such that one application may only detect the effects of another, such as a write to a given file, after the fact. However, at the point a file operation (e.g., a modification or copy operation) is visible to another application, the operation may have already occurred and information regarding the source of the operation may no longer be available. 
     SUMMARY OF THE INVENTION 
     Various embodiments of a system and method for detecting and storing file identity change information within a file system are disclosed. In one embodiment, the system may include a storage device configured to store a plurality of files and a file system configured to manage access to the storage device. The file system may be configured to detect an operation to modify an identity of a first file stored on the storage device and, subsequent to detecting the operation, store a record of the operation associated with the first file, where the record includes a signature corresponding to the first file. 
     In one specific implementation of the system, the operation may correspond to a file create operation, a file delete operation, a file rename operation, or a file copy operation. In another specific implementation of the system, the record may be stored in a named stream corresponding to the first file, the file system may include a history stream, and wherein the file system may be further configured to store an indication of the operation in the history stream in response to storing the record in the named stream. In yet another specific implementation of the system, the record is stored in a database configured to store a plurality of entries, and wherein the database is further configured to respond to a query of the plurality of entries. 
     A method is also contemplated which, in one embodiment, may include storing a plurality of files, detecting an operation to modify an identity of a first stored file, and subsequent to detecting the operation, storing a record of the operation associated with the first stored file, wherein the record includes a signature corresponding to the first stored file. 
     According to another aspect of the invention, a system is contemplated that may include a storage device configured to store a plurality of files and a file system configured to manage access to the storage device. The file system may be further configured to determine a file lineage relationship between a first file and a second file. In one specific implementation of the system, determining the file lineage relationship may include determining whether the first file and the second file are members of the same lineage pool. In another specific implementation of the system, determining the file lineage relationship may include determining whether the first file is an ancestor of the second file. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a block diagram illustrating one embodiment of a storage system. 
         FIG. 2  is a block diagram illustrating one embodiment of an operating system architecture and its interface to storage devices. 
         FIG. 3  is a block diagram illustrating one embodiment of a file system configured to detect identity-modifying operations on files. 
         FIG. 4A  is a flow diagram illustrating one embodiment of a method of generating and storing records corresponding to identity-modifying file operations. 
         FIG. 4B  is a flow diagram illustrating one embodiment of a method of importing records corresponding to identity-modifying file operations into a file mutation database. 
         FIG. 4C  is a flow diagram illustrating one embodiment of a method of determining whether two files are in the same lineage pool. 
         FIG. 4D  is a flow diagram illustrating one embodiment of a method of determining whether one file is an ancestor of another file. 
         FIG. 5  is a block diagram illustrating one embodiment of a file system configured to detect content access operations on files. 
         FIG. 6A  is a flow diagram illustrating one embodiment of a method of generating and storing records corresponding to content access file operations. 
         FIG. 6B  is a flow diagram illustrating one embodiment of a method of importing records corresponding to content access file operations into a file mutation database. 
     
    
    
     While the invention is susceptible to various modifications and alternative forms, specific embodiments thereof are shown by way of example in the drawings and will herein be described in detail. It should be understood, however, that the drawings and detailed description thereto are not intended to limit the invention to the particular form disclosed, but on the contrary, the intention is to cover all modifications, equivalents and alternatives falling within the spirit and scope of the present invention as defined by the appended claims. 
     DETAILED DESCRIPTION 
     Storage System and File System Overview 
     Turning now to  FIG. 1 , a block diagram of one embodiment of a storage system is shown. In the illustrated embodiment, storage system  10  includes a plurality of host devices  20   a  and  20   b  coupled to a plurality of storage devices  30   a  and  30   b  via a system interconnect  40 . Further, host device  20   b  includes a system memory  25  in the illustrated embodiment. For simplicity of reference, elements referred to herein by a reference number followed by a letter may be referred to collectively by the reference number alone. For example, host devices  20   a  and  20   b  and storage devices  30   a  and  30   b  may be referred to collectively as host devices  20  and storage devices  30 . 
     In various embodiments of storage system  10 , host devices  20  may be configured to access data stored on one or more of storage devices  30 . In one embodiment, storage system  10  may be implemented within a single computer system, for example as an integrated storage server. In such an embodiment, for example, host devices  20  may be individual processors, system memory  25  may be a cache memory such as a static RAM (SRAM), storage devices  30  may be mass storage devices such as hard disk drives or other writable or rewritable media, and system interconnect  40  may include a peripheral bus interconnect such as a Peripheral Component Interface (PCI) bus. In some such embodiments, system interconnect  40  may include several types of interconnect between host devices  20  and storage devices  30 . For example, system interconnect  40  may include one or more processor buses (not shown) configured for coupling to host devices  20 , one or more bus bridges (not shown) configured to couple the processor buses to one or more peripheral buses, and one or more storage device interfaces (not shown) configured to couple the peripheral buses to storage devices  30 . Storage device interface types may in various embodiments include the Small Computer System Interface (SCSI), AT Attachment Packet Interface (ATAPI), Firewire, and/or Universal Serial Bus (USB), for example, although numerous alternative embodiments including other interface types are possible and contemplated. 
     In an embodiment of storage system  10  implemented within a single computer system, storage system  10  may be configured to provide most of the data storage requirements for one or more other computer systems (not shown), and may be configured to communicate with such other computer systems. In an alternative embodiment, storage system  10  may be configured as a distributed storage system, such as a storage area network (SAN), for example. In such an embodiment, for example, host devices  20  may be individual computer systems such as server systems, system memory  25  may be comprised of one or more types of dynamic RAM (DRAM), storage devices  30  may be standalone storage nodes each including one or more hard disk drives or other types of storage, and system interconnect  40  may be a communication network such as Ethernet or Fibre Channel. A distributed storage configuration of storage system  10  may facilitate scaling of storage system capacity as well as data bandwidth between host and storage devices. 
     In still another embodiment, storage system  10  may be configured as a hybrid storage system, where some storage devices  30  are integrated within the same computer system as some host devices  20 , while other storage devices  30  are configured as standalone devices coupled across a network to other host devices  20 . In such a hybrid storage system, system interconnect  40  may encompass a variety of interconnect mechanisms, such as the peripheral bus and network interconnect described above. 
     It is noted that although two host devices  20  and two storage devices  30  are illustrated in  FIG. 1 , it is contemplated that storage system  10  may have an arbitrary number of each of these types of devices in alternative embodiments. Also, in some embodiments of storage system  10 , more than one instance of system memory  25  may be employed, for example in other host devices  20  or storage devices  30 . Further, in some embodiments, a given system memory  25  may reside externally to host devices  20  and storage devices  30  and may be coupled directly to a given host device  20  or storage device  30  or indirectly through system interconnect  40 . 
     In many embodiments of storage system  10 , one or more host devices  20  may be configured to execute program instructions and to reference data, thereby performing a computational function. In some embodiments, system memory  25  may be one embodiment of a computer-accessible medium configured to store such program instructions and data. However, in other embodiments, program instructions and/or data may be received, sent or stored upon different types of computer-accessible media. Generally speaking, a computer-accessible medium may include storage media or memory media such as magnetic or optical media, e.g., disk or CD-ROM included in storage system  10  as storage devices  30 . A computer-accessible medium may also include volatile or non-volatile media such as RAM (e.g. SDRAM, DDR SDRAM, RDRAM, SRAM, etc.), ROM, etc, that may be included in some embodiments of storage system  10  as system memory  25 . Further, a computer-accessible medium may include transmission media or signals such as electrical, electromagnetic, or digital signals, conveyed via a communication medium such as network and/or a wireless link, which may be included in some embodiments of storage system  10  as system interconnect  40 . 
     In some embodiments, program instructions and data stored within a computer-accessible medium as described above may implement an operating system that may in turn provide an environment for execution of various application programs. For example, a given host device  20  may be configured to execute a version of the Microsoft Windows operating system, the Unix operating system, the Apple Macintosh operating system, or another suitable operating system. Additionally, a given host device may be configured to execute application programs such as word processors, web browsers and/or servers, email clients and/or servers, and multimedia applications, among many other possible applications. 
     During execution on a given host device  20 , either the operating system or a given application may generate requests for data to be loaded from or stored to a given storage device  30 . For example, code corresponding to portions of the operating system or an application itself may be stored on a given storage device  30 , so in response to invocation of the desired operation system routine or application program, the corresponding code may be retrieved for execution. Similarly, operating system or application execution may produce data to be stored. 
     Many operating system embodiments provide data and control structures for organizing the storage space provided by storage devices  30  into files. In various embodiments, the data structures may include one or more tables configured to store information such as, for example, the identity of each file, its location within storage devices  30  (e.g., a mapping to a particular physical location within a particular storage device), as well as other information about each file as described in greater detail below. Also, in various embodiments, the control structures may include executable routines for manipulating files, such as, for example, function calls for changing file identity and for modifying file content as described in greater detail below. Collectively, these data and control structures may be referred to herein as a file system, and the particular data formats and protocols implemented by a given file system may be referred to herein as the format of the file system. 
     In some embodiments, a file system may be integrated into the operating system such that any access to data stored on storage devices  30  is governed by the control and data structures of the file system. Different operating systems may implement different native file systems using different formats, but in some embodiments, a given operating system may include a file system that supports multiple different types of file system formats, including file system formats native to other operating systems. In such embodiments, the various file system formats supported by the file system may be referred to herein as local file systems. Additionally, in some embodiments, a file system may be implemented using multiple layers of functionality arranged in a hierarchy, as illustrated in  FIG. 2 . 
       FIG. 2  illustrates one embodiment of an operating system architecture and its interface to storage devices. In the illustrated embodiment, operating system  200  includes a user space  210  and a kernel space  220 . User space  210  includes a plurality of processes  212 A-C, each of which may correspond to a given user application. In some embodiments, some application processes  212  within user space  210  may be distinct from operating system  200 . Such processes may be said to operate within an environment provided by operating system  200 , or to operate “on top of” operating system  200 . Each of processes  212  may be configured to access storage devices  230 A-C through calls to application programming interface (API)  214 . API  214  provides processes  212  with access to file system  205 , which is configured to operate within kernel space  220 . In one embodiment, storage devices  230  may be illustrative of storage devices  30  of  FIG. 1 . Also, in one embodiment, operating system  200 , any of its components, and/or any of processes  212  may be configured to execute on one or more host devices  20  of  FIG. 1 , for example as program instructions and data stored within a computer-accessible medium such as system memory  25  of  FIG. 1 . 
     As described above with respect to storage system  10  of  FIG. 1 , a given host device  20  may reside in a different computer system from a given storage device  30 , and may access that storage device via a network. Likewise, with respect to operating system  200 , in one embodiment a given process such as process  212 A may execute remotely and may access storage devices  230  over a network. In the illustrated embodiment, file system  200  includes network protocols  225  to support access to the file system by remote processes. In some embodiments, network protocols  225  may include support for the Network File System (NFS) protocol or the Common Internet File System (CIFS) protocol, for example, although it is contemplated that any suitable network protocol may be employed, and that multiple such protocols may be supported in some embodiments. 
     File system  205  may be configured to support a plurality of local file systems. In the illustrated embodiment, file system  205  includes a VERITAS (VxFS) format local file system  240 A, a fast file system (FFS) format local file system  240 B, and a proprietary (X) format local file system  240 X. However, it is contemplated that in other embodiments, any number or combination of local file system formats may be supported by file system  205 . To provide a common interface to the various local file systems  240 , file system  205  includes a virtual file system  222 . In one embodiment, virtual file system  222  may be configured to translate file system operations originating from processes  212  to a format applicable to the particular local file system  240  targeted by each operation. Additionally, in the illustrated embodiment operating system  200  includes device drivers  224  through which local file systems  240  may access storage devices  230 . Device drivers  224  may implement data transfer protocols specific to the types of interfaces employed by storage devices  230 . For example, in one embodiment device drivers  224  may provide support for transferring data across SCSI and ATAPI interfaces, though in other embodiments device drivers  224  may support other types and combinations of interfaces. 
     In the illustrated embodiment, file system  205  also includes filter driver  221 . In some embodiments, filter driver  221  may be configured to monitor each operation entering file system  205  and, subsequent to detecting particular types of operations, to cause additional operations to be performed or to alter the behavior of the detected operation. For example, in one embodiment filter driver  221  may be configured to combine multiple write operations into a single write operation to improve file system performance. In another embodiment, filter driver  221  may be configured to compute a signature of a file subsequent to detecting a write to that file. In still another embodiment, filter driver  221  may be configured to store information, such as records, associated with particular files subsequent to detecting certain kinds of operations on those files, as described in greater detail below. It is contemplated that in some embodiments, filter driver  221  may be configured to implement one or more combinations of the aforementioned operations, including other filter operations not specifically mentioned. 
     It is noted that filter driver  221  is part of file system  205  and not an application or process within user space  210 . Consequently, filter driver  221  may be configured to operate independent of applications and processes within the user space  210 . Alternatively, or in addition to the above, filter driver  221  may be configured to perform operations in response to requests received from applications or processes within the user space  210 . 
     It is further noted that in some embodiments, kernel space  220  may include processes (not shown) that generate accesses to storage devices  230 , similar to user space processes  212 . In such embodiments, processes executing in kernel space  220  may be configured to access file system  205  through a kernel-mode API (not shown), in a manner similar to user space processes  212 . Thus, in some embodiments, all accesses to storage devices  230  may be processed by file system  205 , regardless of the type or space of the process originating the access operation. 
     Numerous alternative embodiments of operating system  200  and file system  205  are possible and contemplated. For example, file system  205  may support different numbers and formats of local file systems  240 , or only a single local file system  240 . In some embodiments, network protocol  225  may be omitted or integrated into a portion of operating system  200  external to file system  205 . Likewise, in some embodiments virtual file system  222  may be omitted or disabled, for example if only a single local file system  240  is in use. Additionally, in some embodiments filter driver  221  may be implemented within a different layer of file system  205 . For example, in one embodiment, filter driver  221  may be integrated into virtual file system  222 , while in another embodiment, an instance of filter driver  221  may be implemented in each of local file systems  240 . 
     Tracking File Identity Change Operations 
     As described above, file system  205  may be configured to manage access to a plurality of files stored on storage devices  230 . In many embodiments, each stored file may have an associated identity used by the file system to distinguish each file from other files. In one embodiment of file system  205 , the identity of a file may be a file name, which may for example include a string of characters such as “filename.txt”. In embodiments of file system  205  that implement a file hierarchy, such as a hierarchy of folders or directories, all or part of the file hierarchy may be included in the file identity. 
     In the course of execution, operating system  200  and/or processes  212  may generate operations configured to modify the identity of one or more files managed by file system  205 . In one embodiment, such identity-modifying operations may include any of the following: a file create operation, a file delete operation, a file rename operation, or a file copy operation. For example, a given process such as process  212 A may receive a directive from a user to save work in a file with a corresponding identity that does not currently exist within file system  205 , or to delete a specified file. Process  212 A may then respectively generate a file create operation to create a file with the specified file identity, or a file delete operation to delete the specified file. Similarly, process  212 A may receive a directive from a user to rename or copy a given file to a file with a specified identity. Process  212 A may then respectively generate a file rename operation or a file copy operation. In some embodiments, certain identity-modifying operations may be implemented using other identity-modifying operations. For example, a file rename operation may be implemented as a file create operation (specifying the identity of the target file of the rename) followed by a file delete operation (specifying the identity of the source file of the rename). 
     In one embodiment, file system  205  may be configured to detect various kinds of identity-modifying operations on files, and to store records of such operations.  FIG. 3  illustrates one such embodiment of a file system. The embodiment of file system  205  shown in  FIG. 3  may include those elements illustrated in the embodiment of  FIG. 2 ; however, for sake of clarity, some of these elements are not shown. In the illustrated embodiment, file system  205  includes filter driver  221 , an arbitrary number of files  310   a - n , and a respective named stream  320   a - n  associated with each of files  310   a - n . File system  205  further includes a history stream  330 , a file mutation database  340 , and an update daemon  350 . It is noted that a generic instance of one of files  310   a - n  or named streams  320   a - n  may be referred to respectively as a file  310  or a named stream  320 , and that files  310   a - n  and named streams  320   a - n  may be referred to collectively as files  310  and named streams  320 , respectively. 
     Files  310  may be representative of files managed by file system  205 . Each of files  310  has a corresponding named stream  320 . Each of named streams  320  may be configured to store information about its corresponding file, which may be referred to herein as metadata. In various embodiments, metadata may include information such as (but not limited to) the file identity, size, ownership, and file access permissions, as well as records corresponding to detected identity-modifying operations, as described below. It is noted that files  310  and named streams  320  may be physically stored on one or more storage devices, such as storage devices  230  of  FIG. 2 . However, for purposes of illustration, files  310  and named streams  320  are shown as conceptually residing within file system  205 . 
     Identity-Modifying Operation Record Generation and Format 
     In one particular embodiment, file system  205  may be configured to detect an operation to modify the identity of a file  310 , such as one of the identity-modifying operations described above. In such an embodiment, filter driver  221  may be configured to detect the identity-modifying operation when it is received by file system  205 , or at some later time. Subsequent to detecting the identity-modifying operation, filter driver  221  may be configured to store a record of the detected operation in a named stream  320  corresponding to the target file of the operation. For example, if file  310   a  is the target of the detected operation, filter driver  221  may store a record of the operation in corresponding named stream  320   a . It is contemplated that storage of a record may take place at any time subsequent to detection of the relevant operation. For example, in one embodiment, storage of the record may be delayed until the operation on file  310   a  is complete, while in another embodiment, storage of the record may occur prior to completion of the operation. In the latter case, if the operation is not guaranteed to complete (i.e., is speculative), filter driver  221  may provide a mechanism to delete a record stored in advance of its corresponding operation in case the operation does not complete. 
     The record stored by filter driver  221  subsequent to detecting an identity-modifying operation may in various embodiments include various kinds of information about the file  310  and the identity-modifying operation detected, such as the file identity, file type, file size, file owner, and/or file permissions, for example. In one embodiment, the record may include a file signature indicative of the content of file  310 . A file signature may be a hash-type function of all or a portion of the file contents and may have the property that minor differences in file content yield quantifiably distinct file signatures. For example, the file signature may employ the Message Digest 5 (MD5) algorithm, which may yield different signatures for files differing in content by as little as a single bit, although it is contemplated that any suitable signature-generating algorithm may be employed. In some embodiments, filter driver  221  may compute the file signature at the time the record of the identity-modifying operation is detected or stored, while in other embodiments filter driver  221  may use a file signature that was computed prior to detection of the operation. 
     In one embodiment, the record stored by filter driver  221  subsequent to detecting an identity-modifying operation may be generated and stored in Extensible Markup Language (XML) format, although it is contemplated that in other embodiments, any suitable format may be used. One example of an XML-format record is as follows: 
                                            &lt;record sequence=“1”&gt;             &lt;path&gt;/test1/foo.pdf&lt;/path&gt;             &lt;type&gt;application/pdf&lt;/type&gt;             &lt;user id=1598&gt;username&lt;/user&gt;             &lt;group id=119&gt;groupname&lt;/group&gt;             &lt;perm&gt;rw-r--r--&lt;/perm&gt;             &lt;md5&gt;d41d8cd98f00b204e9800998ecf8427e&lt;/md5&gt;             &lt;size&gt;0&lt;/size&gt;           &lt;/record&gt;                        
Such a record may be appended to the named stream (for example, named stream  320   a ) associated with the file (for example, file  310   a ) having the file identity “/test1/foo.pdf” subsequent to a file create operation. In this case, the number associated with the “record sequence” field indicates that this record is the first record associated with file  310   a . The “path” field includes the file identity, and the “type” field indicates the file type, which in one embodiment may be provided by the process issuing the file create operation, and in other embodiments may be determined from the extension of the file name or from header information within the file, for example. The “user id” field records both the numerical user id and the textual user name of the user associated with the process issuing the file create operation, and the “group id” field records both the numerical group id and the textual group name of that user. The “perm” field records file permissions associated with file  310   a  in a format specific to the file system  205  and/or the operating system. The “md5” field records an MD5 signature corresponding to the file contents, and the “size” field records the length of file  310   a  in bytes. It is contemplated that in alternative embodiments, filter driver  221  may store records corresponding to identity-modifying operations that include more or fewer fields, as well as fields having different definitions and content.
 
     Filter driver  221  may be configured to append a record similar to the one illustrated above to the named stream  320  corresponding to a file  310  subsequent to detecting an identity-modifying operation of that file such as a create, delete, rename, or copy operation. Additionally, filter driver  221  may be configured to append a similar record to a named stream  320  corresponding to a file  310  when a process modifies the contents of file  310  without issuing an identity-modifying operation to the file. For example, in one embodiment filter driver  221  may be configured to detect a file close operation to a file  310  whose contents have been modified, where the file close operation is issued by the last process having the modified file open. In other words, multiple processes may have issued file open operations to a file  310  that is subsequently modified, and filter driver  221  may be configured to detect the last of such processes to issue a file close operation. Subsequent to detecting such a “last close” of the modified file  310 , filter driver  221  may be configured to update the signature associated with the file  310  and to append a record including the updated signature to the named stream  320  corresponding to file  310 . Filter driver  221  may thereby ensure that signatures reflected in records in named streams of files remain current without tracking each individual write of such files. Referring to the above example record, filter driver  221  may write the following example record to the named stream  320   a  of file  310   a  “/test1/foo.pdf” upon detecting the last close of the file following modification: 
                                            &lt;record sequence=“2”&gt;             &lt;path&gt;/test1/foo.pdf&lt;/path&gt;             &lt;type&gt;application/pdf&lt;/type&gt;             &lt;user id=1598&gt;username&lt;/user&gt;             &lt;group id=119&gt;groupname&lt;/group&gt;             &lt;perm&gt;rw-r--r--&lt;/perm&gt;             &lt;md5&gt;b42455dadf928643d8df3171cca9216a&lt;/md5&gt;             &lt;size&gt;10597&lt;/size&gt;           &lt;/record&gt;                        
As illustrated in this example record, the “md5” and “size” fields have been updated to reflect the modification to file  310   a.  
 
     Certain identity-modifying file operations may involve more than one of files  310 . For example, file rename and file copy operations may involve one or more source files and a destination file, where the destination file may or may not exist at the time the operation is performed. Subsequent to detecting identity-modifying operations involving more than one file, filter driver  221  may be configured to mark the existing records (if any) in the named stream  320  corresponding to the destination file  310  as “old,” and to append each record in the named streams corresponding to each source file to the named stream corresponding to the destination file. For example, subsequent to the modification of file  310   a  “/test1/foo.pdf” shown above, filter driver  221  may detect a file rename operation to rename file  310   a  “/test1/foo.pdf” to file  310   b  “/test1/destination.pdf”, which latter file may already exist and may have a number of records in its associated named stream  320   b . Subsequently, filter driver  221  may mark the existing records associated with file  310   b  “/test1/destination.pdf” as old and associate each of the records associated with file  310   a  “/test1/foo.pdf” in its named stream  320   a  to the named stream  320   b  of file  310   b  “/test1/destination.pdf”, along with a new record indicating the identity change. Following this activity, the content of the named stream  320   b  of file  310   b  “/test1/destination.pdf” may include the following records: 
                                            &lt;record sequence=“1”&gt;             &lt;path&gt;/test1/foo.pdf&lt;/path&gt;             ...           &lt;/record&gt;           &lt;record sequence=“2”&gt;             &lt;path&gt;/test1/foo.pdf&lt;/path&gt;             ...           &lt;/record&gt;           &lt;record sequence=“3”&gt;             &lt;path&gt;/test1/destination.pdf&lt;/path&gt;             &lt;type&gt;application/pdf&lt;/type&gt;             &lt;user id=1598&gt;username&lt;/user&gt;             &lt;group id=119&gt;groupname&lt;/group&gt;             &lt;perm&gt;rw-r--r--&lt;/perm&gt;             &lt;md5&gt;b42455dadf928643d8df3171cca9216a&lt;/md5&gt;             &lt;size&gt;10597&lt;/size&gt;            &lt;oldrecord&gt;               &lt;record sequence=“1”&gt;                  &lt;path&gt;/test1/destination.pdf&lt;/path&gt;                  ...               &lt;/record&gt;               ...            &lt;/oldrecord&gt;           &lt;/record&gt;                        
where the first two records listed (with content omitted for clarity) may be identical to the first two records of file  310   a  “/test1/foo.pdf” as shown above, and the third record indicates the change in file identity to “/test1/destination.pdf”. The other fields of the third record may be copied or linked from the most recent record of file  310   a  “/test1/foo.pdf” as indicated above. Further, records corresponding to file  310   b  “/test1/destination.pdf” prior to the identity change are shown being preserved (though their specific content is omitted for clarity) and delimited with the &lt;oldrecord&gt; indicator. As shown above, the preserved old records are associated with a particular record (in this case, the third record), although in other embodiments it is contemplated that the old records may be associated with a different record or may constitute a standalone record separate from and not within the scope of another record.
 
     It is noted that in some embodiments, following the aforementioned processing of records, filter driver  221  may be configured to delete the records associated with the source file  310  if the identity change operation is a file rename operation and to preserve the records associated with the source file  310  if the identity-modifying operation is a file copy operation. It is further noted that in some embodiments, file rename or copy operations may result in associated metadata records being duplicated in multiple named streams, whereas in other embodiments, metadata records may be associated with additional files by linking a pointer to an existing record into a named stream of a destination file rather than copying the record to the named stream of the destination file. 
     History Stream and File Mutation Database 
     In the illustrated embodiment, file system  205  includes history stream  330 . History stream  330  may be a named stream similar to named streams  320 ; however, rather than being associated with a particular file, history stream  330  may be associated directly with file system  205 . In some embodiments, file system  205  may include only one history stream  330 , while in other embodiments, more than one history stream  330  may be provided. For example, in one embodiment of file system  205  including a plurality of local file systems  240  as illustrated in  FIG. 2 , one history stream per local file system  240  may be provided. 
     In some embodiments, filter driver  221  may be configured to store a record in history stream  330  in response to storing a record corresponding to an identity-modifying operation in a given named stream  320 . For example, in response to storing a record subsequent to detecting an operation to modify the identity or the content of a file  310  as described above, filter driver  221  may store a record indicative of the operation in history stream  330  as well as the identity of the file operated on. History stream  330  may thereby provide a centralized history of the identity-modifying operations transpiring within file system  205 . 
     In one embodiment, the record stored by filter driver  221  in history stream  330  may be generated in Extensible Markup Language (XML) format, although it is contemplated that in other embodiments, any suitable format may be used. Referring to the example above in which file  310   a  “/test1/foo.pdf” was created, modified, and then renamed to file  310   b  “/test1/destination.pdf”, in one embodiment history stream  330  may include the following example records subsequent to the rename operation: 
                                            &lt;record&gt;               &lt;op&gt;create&lt;/op&gt;               &lt;path&gt;/test1/foo.pdf&lt;/path&gt;           &lt;/record&gt;           &lt;record&gt;               &lt;op&gt;modify&lt;/op&gt;               &lt;path&gt;/test1/foo.pdf&lt;/path&gt;           &lt;/record&gt;           &lt;record&gt;               &lt;op&gt;rename&lt;/op&gt;               &lt;path&gt;/test1/destination.pdf&lt;/path&gt;               &lt;oldpath&gt;/test1/foo.pdf&lt;/oldpath&gt;           &lt;/record&gt;                        
In this example, the “op” field of each record indicates the operation performed, while the “path” field indicates the file identity of the file  310   a  operated on. In the case of the file rename operation, the “path” field indicates the file identity of the destination file  310   b  of the rename operation, and the “oldpath” field indicates the file identity of the source file  310   a . It is contemplated that in alternative embodiments, filter driver  221  may store within history stream  330  records including more or fewer fields, as well as fields having different definitions and content.
 
     Update daemon  350  may be configured as either a kernel-mode or a user-mode process operating within file system  205 , although it is contemplated that in some embodiments, update daemon  350  may be implemented external to file system  205 . In the illustrated embodiment, update daemon  350  may scan the records stored in history stream  330  at regular or irregular intervals. If a valid record is found, for each destination file  310  recorded in the history stream (i.e., the file identified by the “path” field in the above example), update daemon  350  may be configured to access the corresponding named stream  320 , and to convey the records stored therein to file mutation database  340 . (In case a record stored in history stream  330  indicates that a given file  310  has been deleted, update daemon  350  may in one embodiment convey only that indication to file mutation database  340 , as the named stream  320  corresponding to the deleted file  310  may have also been deleted.) In one embodiment, update daemon  350  may convey all records stored in the corresponding named stream  320 , while in other embodiments, update daemon  350  may convey only those records not previously conveyed to file mutation database  340 . For example, in one embodiment each record in each named stream  320  may include a “scanned” field that may be tested and set by update daemon  350 , such that only unscanned records are conveyed to file mutation database  340 . Similarly, in various embodiments update daemon  350  may mark records in history stream  330  as they are scanned, processing only unmarked records, or may delete them from history stream  330  after scanning. 
     It is noted that in an alternative embodiment, history stream  330  may be omitted from file system  205 . In such an embodiment, update daemon  350  may be configured to scan all named streams  320  within file system  205  at regular or irregular intervals, conveying all or only modified records to file mutation database  340 . Further, in another alternative embodiment, both history stream  330  and update daemon  350  may be omitted from file system  205 . In such an embodiment, filter driver  221  may signal file mutation database  340  directly upon generating a record, such as via a software interrupt or function call, for example. Filter driver  221  may be configured to directly convey records to file mutation database  340 , in which case records may not be stored within named streams  320 . Alternatively, file mutation database  340  may be configured to retrieve records directly from named streams  320  in response to receiving notification from filter driver  221  to do so. 
     In the illustrated embodiment, file mutation database (FMD)  340  is a database integrated with file system  205 , although it is contemplated that in other embodiments, FMD  340  may be implemented externally to file system  205 . In various embodiments, FMD  340  may be configured as a kernel-mode or a user-mode process. FMD  340  may be configured to store records in the same format as the records stored in named streams  320  and history stream  330 , such as XML format records. However, it is contemplated that file mutation database  340  may implement any suitable database format or architecture. Further, in some embodiments, FMD  340  or update daemon  350  may be configured to convert records stored in one format within named streams  320  and history stream  330  to another format for storage within FMD  340 . File system  205  may provide an API through which various processes may submit database queries to FMD  340 , which may in turn be configured to respond to such queries. 
     Numerous types of queries of FMD  340  are possible and contemplated, dependent on the type of information included in the records of identity-modifying operations generated by filter driver  221 . For example, in one embodiment, whenever update daemon  350  conveys a record from a named stream  320  to FMD  340 , FMD  340  may build a list identifying all files  310  having file signatures identical to the one included in the conveyed record. Subsequently, FMD  340  may be queried to identify all files sharing the file signature corresponding to a given file identity. 
     Other types of queries may include queries to determine file lineage relationships among two or more files. Generally speaking, file lineage relationships refer to the relationships created among files as a result of identity-modifying operations. Queries to determine file lineage relationships may include lineage pool queries and file ancestor queries, although other lineage relationships and associated queries are possible and contemplated. Files  310  may be considered to be members of the same lineage pool if they share a common file signature at some point in time, i.e., if each file has a record indicating the same file signature. A given file  310   a  may be considered to be an ancestor of a given file  310   b  if the first valid file signature of file  310   b  (i.e., the earliest record of file  310   b  including a file signature) matches some file signature of file  310   a . Using such records and queries, file system  205  may be configured to detect and track the identities of files as those identities evolve through the execution of identity-modifying file operations. Such tracking may be useful, for example, in tracking the origins of properly or improperly modified files, or in implementing effective storage policies such as allowing files with identical content but different identities to share storage. 
     Other embodiments of file system  205  may be configured to determine file lineage relationships. For example, in one embodiment, FMD  340  may be omitted, and a query process may be configured to operate directly on records stored within named streams  320  to determine file lineage relationships. In another embodiment, named streams  320  may be omitted and records may be stored at the time of generation directly within FMD  340  or another type of repository for subsequent determination of file lineage relationships. 
       FIG. 4A  and  FIG. 4B  illustrate embodiments of methods of generating and storing records corresponding to identity-modifying file operations and of importing such records into a file mutation database, respectively. Referring collectively to  FIG. 1  through  FIG. 3  and  FIG. 4A , operation begins in block  400  where an operation to modify the identity of a file is detected. In one embodiment, filter driver  221  of file system  205  may be configured to detect an identity-modifying operation as described above. 
     Subsequent to detection of the identity-modifying operation, a record of the operation is generated (block  402 ). In some embodiments, filter driver  221  may be configured to generate this record, and as described above, in some embodiments the record may be in the XML format and may include information about the operation, the file identity, a signature corresponding to the file, and other information as desired. 
     After the operation record is generated, it is stored in a named stream corresponding to the file (block  404 ). Additionally, a history record of the operation is stored in a history stream (block  406 ). As noted above, filter driver  221  may be configured in some embodiments to store the generated record in a named stream  320  corresponding to the file  310  targeted by the operation, and may additionally be configured to store a history record such as described above in history stream  330 . 
     The method of  FIG. 4B  may in some embodiments operate in parallel to the method illustrated in  FIG. 4A . For example, the method of  FIG. 4B  may be implemented within update daemon  350 . Referring collectively to  FIG. 1  through  FIG. 3  and  FIG. 4B , operation begins in block  410  where a history record corresponding to an identity-modifying operation is detected within the history stream. As described above, in one embodiment update daemon  350  may be configured to scan history stream  330  to detect history records not previously processed. 
     Once a history record is detected, the records stored in the named stream of the file indicated in the history record are accessed and conveyed to the file mutation database (block  412 ). As described above, in one embodiment update daemon  350  may be configured to access the named stream corresponding to a file indicated in the history record and convey the records included therein to file mutation database  340 . 
     Other embodiments of these methods are possible and contemplated. For example, as noted above, in some embodiments of file system  205 , history stream  330  may be omitted, and update daemon  350  may be configured to scan the entire file system to determine the presence of updated records. Also, in some embodiments update daemon  350  may be omitted and filter driver  221  may communicate directly with file mutation database  340 . 
       FIG. 4C  illustrates an embodiment of a method of determining whether two files are in the same lineage pool. Referring collectively to  FIG. 1  through  FIG. 3  and  FIG. 4C , operation begins in block  420  where a request to determine whether two or more files are members of the same lineage pool is detected. For example, in one embodiment FMD  340  may be configured to detect a query corresponding to such a request. 
     Upon detecting such a request, the records corresponding to each file subject to the request may be examined (block  422 ). For example, in one embodiment FMD  340  may be configured to identify the database records corresponding to each subject file. In another embodiment, the records stored in named streams  320  corresponding to the subject files  310  may be scanned. 
     Subsequent to examination of the appropriate records, it may be determined whether the subject files share a common signature in any of their collective records (block  424 ). For example, in one embodiment FMD  340  may be configured to compare each unique signature indicated in the records of each subject file with each signature indicated in the records of every other subject file and to note signature matches. If a common signature exists among all subject files, the subject files may be determined to be members of the same lineage pool (block  426 ). Otherwise, the subject files may be determined to be members of different lineage pools (block  428 ). 
       FIG. 4D  illustrates an embodiment of a method of determining whether one file is an ancestor of another file. Referring collectively to  FIG. 1  through  FIG. 3  and  FIG. 4D , operation begins in block  430  where a request to determine whether a first file is an ancestor of a second file is detected. For example, in one embodiment FMD  340  may be configured to detect a query corresponding to such a request. 
     Upon detecting such a request, the records corresponding to each file subject to the request may be examined (block  432 ). For example, in one embodiment FMD  340  may be configured to identify the database records corresponding to each subject file. In another embodiment, the records stored in named streams  320  corresponding to the subject files  310  may be scanned. 
     Subsequent to examination of the appropriate records, it may be determined whether the first valid signature of the second file is included as a signature of the first file (block  434 ). For example, in one embodiment FMD  340  may be configured to compare the first valid signature of the second file with each unique signature indicated in the records of the first file and to note signature matches. If a matching signature exists, the first file may be determined to be an ancestor of the second file (block  436 ). Otherwise, it may be determined that the first file is not an ancestor of the second file (block  438 ). It is contemplated that in an alternative embodiment, the method of  FIG. 4D  may also be configured to determine whether the second file is an ancestor of the first file, for example by modifying the step at block  436  to include comparing the first valid signature of the first file with each unique signature indicated in the records of the second file and noting signature matches. 
     Tracking Content Access Operations 
     Referring once again to  FIG. 2 , as described above, file system  205  may be configured to manage access to a plurality of files stored on storage devices  230 . In addition to each file having an associated file identity as described above, each file may have corresponding content. In various embodiments, such content may include data such as text data, image data, sound data, or application-specific data such as Microsoft Word data, for example. In other embodiments, file content may include executable code. For example, the content of a given file may include instructions that, when executed, perform the various functions of a program or application. File content may be stored via file system  205  on storage devices  230  using any encoding suitable for storage devices  230 . For example, file content may be stored on storage devices  230  using a binary encoding. 
     In the course of execution, operating system  200  and/or processes  212  may generate input/output (I/O) operations configured to access the content of one or more files managed by file system  205 . In some embodiments, such I/O operations may include a file read operation or a file write operation, and in one embodiment a file write operation may be further categorized as either an appending write operation (i.e., a write operation that appends content to a file) or a random write operation (i.e., a write operation that may overwrite the content of a file). For example, a given process such as process  212 A may receive a directive from a user to open an existing file to read its content, or to save work in an existing file. Process  212 A may then respectively generate a file read operation to read the content of the specified file, or a file write operation (such as an appending write operation) to modify the content of the specified file. In some embodiments, certain I/O operations may invoke or be invoked by some of the identify-modifying operations described above. For example, if a given file identity does not exist, a file write operation to that file identity may result in a file create operation being performed, followed by a file write operation. 
     In some embodiments, file system  205  may be configured to aggregate file I/O operations on a per-process basis. For example, file system  205  may be configured to aggregate I/O operations on a given file performed by a particular process  212  from the time the file is opened until the time the file is closed by that particular process. This aggregation of I/O operations may be referred to herein as a content access operation. It is contemplated that the degree of aggregation of I/O operations into a single content access operation may vary in various embodiments. For example, in one embodiment all read and write I/O operations to a given file by a process  212  between the opening and closing of the given file may be aggregated into a single content access operation. In another embodiment, all such read I/O operations may be aggregated into one content access operation, and all such write I/O operations may be aggregated into a second content access operation. In yet another embodiment, each individual file I/O operation may correspond to a single content access operation. 
     In some embodiments, file system  205  may be configured to detect various kinds of content access operations on files, and to store records of such operations.  FIG. 5  illustrates one such embodiment of a file system. The embodiment of file system  205  shown in  FIG. 5  may include those elements illustrated in the embodiment of  FIG. 2 ; however, for sake of clarity, some of these elements are not shown. Like the embodiment of  FIG. 3 , the embodiment of file system  205  illustrated in  FIG. 5  includes filter driver  221 , an arbitrary number of files  310   a - n , and a respective named stream  320   a - n  associated with each of files  310   a - n . File system  205  further includes a history stream  330 , a file mutation database  340 , and an update daemon  350 . As above, a generic instance of one of files  310   a - n  or named streams  320   a - n  may be referred to respectively as a file  310  or a named stream  320 , and that files  310   a - n  and named streams  320   a - n  may be referred to collectively as files  310  and named streams  320 , respectively. 
     Files  310  may be representative of files managed by file system  205 . Each of files  310  has a corresponding named stream  320 . Each of named streams  320  may be configured to store metadata about its corresponding file, as described above in conjunction with the description of  FIG. 3 . As described in greater detail below, in various embodiments, metadata may include records corresponding to detected content access operations, as well as the other kinds of information mentioned previously. As with the embodiment of  FIG. 3 , it is noted that files  310  and named streams  320  may be physically stored on one or more storage devices, such as storage devices  230  of  FIG. 2 . However, for purposes of illustration, files  310  and named streams  320  are shown as conceptually residing within file system  205 . 
     Content Access Operation Record Generation and Format 
     In one particular embodiment, file system  205  may be configured to detect an operation by a particular process  212  of  FIG. 2  to access content of a file  310 , such as one of the content access operations described above. In such an embodiment, filter driver  221  may be configured to detect the content access operation when it is received by file system  205 , or at some later time. Subsequent to detecting the content access operation, filter driver  221  may be configured to store a record of the detected operation in a named stream  320  corresponding to the target file of the operation. For example, if file  310   a  is the target of the detected operation, filter driver  221  may store a record of the operation in corresponding named stream  320   a . It is contemplated that storage of a record may take place at any time subsequent to detection of the relevant operation. For example, in one embodiment, storage of the record may be delayed until the operation on file  310   a  is complete, while in another embodiment, storage of the record may occur prior to completion of the operation. In the latter case, if the operation is not guaranteed to complete (i.e., is speculative), filter driver  221  may provide a mechanism to delete a record stored in advance of its corresponding operation in case the operation does not complete. 
     The record stored by filter driver  221  subsequent to detecting a content access operation may in various embodiments include various kinds of information about the file  310  and the content access operation detected, such as the file identity, file type, file size, file owner, file permissions, content access type, process identity, and/or process arguments, for example. In one embodiment, the record may include a file signature indicative of the content of file  310  as described in detail above, such as an MD5 signature, for example. 
     In one embodiment, the record stored by filter driver  221  subsequent to detecting a content access operation may be generated and stored in Extensible Markup Language (XML) format, although it is contemplated that in other embodiments, any suitable format may be used. One example of an XML-format record is as follows: 
                                            &lt;record sequence=“4”&gt;            &lt;path&gt;/test1/file.xls&lt;/path&gt;            &lt;type&gt;application/vnd.ms-excel&lt;/type&gt;            &lt;user id=“1598”&gt;username&lt;/user&gt;            &lt;group id=“119”&gt;groupname&lt;/group&gt;            &lt;perm&gt;rwxrwxr-x&lt;/perm&gt;            &lt;md5&gt;af662188a09d0b9998f710d744918bfe&lt;/md5&gt;            &lt;size&gt;15360&lt;/size&gt;            &lt;date sec=“1055278487”&gt;2003-06-10T20:54:47Z&lt;/date&gt;            &lt;io&gt;                &lt;write&gt;append&lt;/write&gt;            &lt;/io&gt;            &lt;process&gt;                &lt;name&gt;smbd&lt;/name&gt;                &lt;args&gt;/opt/VRTSsamba/bin/smbd -D                 -s/opt/VRTSsamba/lib/smb.conf&lt;/args&gt;                &lt;pid&gt;393&lt;/pid&gt;                &lt;ppid&gt;376&lt;/ppid&gt;                &lt;pgrpid&gt;376&lt;/pgrpid&gt;            &lt;/process&gt;           &lt;/record&gt;                        
Such a record may be appended to the named stream (for example, named stream  320   a ) associated with the file (for example, file  310   a ) having the file identity “/test1/file.xls” subsequent to an appending write operation. In this case, the number associated with the “record sequence” field indicates that this record is the fourth record associated with file  310   a . The “path” field includes the file identity, and the “type” field indicates the file type which in one embodiment may be provided by the process issuing the file create operation, and in other embodiments may be determined from the extension of the file name or from header information within the file, for example. The “user id” field records both the numerical user id and the textual user name of the user associated with the process issuing the file create operation, and the “group id” field records both the numerical group id and the textual group name of that user. The “perm” field records file permissions associated with file  310   a  in a format specific to the file system  205  and/or the operating system. The “md5” field records an MD5 signature corresponding to the file contents, and the “size” field records the length of file  310   a  in bytes.
 
     Additionally, the “date” field records the date and time the record was created. The “io” field records information about the type of content access operation performed, and may include subfields specific to the operation type such as “read” and/or “write”; the “write” subfield may further delimit information regarding the type of write, such as “append” or “random.” The “process” field may include subfields recording information about the process performing the content access operation. The “name” subfield records the name of the process, and the “args” subfield records the arguments given when the process was invoked. The “pid,” “ppid,” and “pgrpid” subfields record the process ID, the ID of the parent of the process, and the group ID of the process, respectively. It is contemplated that in alternative embodiments, filter driver  221  may store records corresponding to content access operations that include more or fewer fields, as well as fields having different definitions and content. 
     It is noted that in some embodiments, file system  205  may be configured to store records subsequent to detecting file content access operations, as just described, whereas in other embodiments, file system  205  may be configured to store records subsequent to detecting file identity-modifying operations as described above in conjunction with the description of  FIG. 3 . It is contemplated that in still other embodiments, file system  205  may be configured to store records corresponding to both content access operations and identity-modifying operations subsequent to detecting each respective type of operation. In one such embodiment, both types of records may be stored within a single named stream  320  corresponding to the file operated on, while in another such embodiment, each type of record may be stored in a distinct named stream corresponding to the file operated on. Further, in some embodiments storing both types of records, all stored records may follow the conventions described above for identity-modifying operations regardless of record type. For example, all stored records associated with a source file may be copied to the named stream of a destination file in the event of a file copy operation, and all stored records associated with a given file may be marked as “old” subsequent to that file changing identity, such as due to a file rename operation as described above. 
     History Stream and File Mutation Database 
     In the illustrated embodiment, file system  205  includes history stream  330 , which may be exemplary of history stream  330  of  FIG. 3  and described in detail above. In some embodiments, filter driver  221  may be configured to store a record in history stream  330  in response to storing a record corresponding to a content access operation in a given named stream  320 . For example, in response to storing a record subsequent to detecting an operation to access the content of a file  310  as described above, filter driver  221  may store a record indicative of the operation in history stream  330  as well as the identity of the file operated on. History stream  330  may thereby provide a centralized history of the content access operations transpiring within file system  205 . 
     As noted above, in one embodiment the record stored by filter driver  221  in history stream  330  may be generated in Extensible Markup Language (XML) format, although it is contemplated that in other embodiments, any suitable format may be used. Referring to the example above in which file  310   a  “/test1/file.xls” underwent an appending write operation, in one embodiment history stream  330  may include the following example record subsequent to the appending write operation: 
     
       
         
           
               
               
             
               
                   
                   
               
             
            
               
                   
                 &lt;record&gt; 
               
               
                   
                     &lt;op&gt;append_write&lt;/op&gt; 
               
               
                   
                     &lt;path&gt;/test1/file.xls&lt;/path&gt; 
               
               
                   
                 &lt;/record&gt; 
               
               
                   
                   
               
            
           
         
       
     
     In this example, as in the previous history record example, the “op” field of each record indicates the operation performed, while the “path” field indicates the file identity of the file  310   a  operated on. It is contemplated that in alternative embodiments, filter driver  221  may store within history stream  330  records including more or fewer fields, as well as fields having different definitions and content. For example, in one embodiment records corresponding to all types of write content access operations (e.g., appending and random) may be indicated simply as “modify” records within history stream  330  such as shown in the previous history record example. 
     Update daemon  350  may be exemplary of update daemon  350  of  FIG. 3 , described in detail above. As in that embodiment, update daemon  350  may be configured as a kernel-mode or user-mode process operating within file system  205  that may scan the records stored in history stream  330  at regular or irregular intervals. If a valid record is found, then for each destination file  310  recorded in the history stream (i.e., the file identified by the “path” field in the above example), update daemon  350  may be configured to access the corresponding named stream  320 , and to convey the records stored therein to file mutation database  340 . As described in detail above, update daemon  350  may be configured to convey all records stored in the corresponding named stream  320 , or only records newly created since the named stream  320  was last accessed by update daemon  350 . 
     As in the embodiment of  FIG. 3 , it is noted that in an alternative embodiment, history stream  330  may be omitted from file system  205 . In such an embodiment, update daemon  350  may be configured to scan all named streams  320  within file system  205  at regular or irregular intervals, conveying all or only modified records to file mutation database  340 . Further, in another alternative embodiment, both history stream  330  and update daemon  350  may be omitted from file system  205 . In such an embodiment, filter driver  221  may signal file mutation database  340  directly upon generating a record, such as via a software interrupt or function call, for example. Additionally, in some embodiments, filter driver  221  may be configured to store records subsequent to detected either identity-modifying operations or content access operations. As noted above, filter driver  221  may store both types of records in a single named stream  320  corresponding to a given file  310 , or in separate named streams. Update daemon  350  may be appropriately configured to retrieve records from one or more named streams  320  according to each such embodiment. 
     In the illustrated embodiment, as for the embodiment illustrated in  FIG. 3 , file mutation database (FMD)  340  is a database integrated with file system  205 , although it is contemplated that in other embodiments, FMD  340  may be implemented externally to file system  205 . FMD  340  may be configured to store records in the same format as the records stored in named streams  320  and history stream  330 , such as XML format records. However, it is contemplated that file mutation database  340  may implement any suitable database format or architecture. Further, in some embodiments, FMD  340  or update daemon  350  may be configured to convert records stored in one format within named streams  320  and history stream  330  to another format for storage within FMD  340 . File system  205  may provide an API through which various processes may submit database queries to FMD  340 , which may in turn be configured to respond to such queries. 
     Numerous types of queries of FMD  340  are possible and contemplated, dependent on the type of information included in the records of content access operations generated by filter driver  221 . Such queries may be configured to classify sets of files based on how content access operations indicate that such files are used. For example, in one embodiment, the class of log files (i.e., files used to log information regarding some aspect of a system&#39;s continuing operation) may exhibit a common set of characteristics, such as having appending writes but not random writes, having far fewer reads than writes, and having writes originating from a single process group rather than multiple process groups. In such an embodiment, a query may be designed and issued to FMD  340  to identify the files  310  having records of content access operations satisfying these characteristics. Based on this and similar classifications, different storage policies for file classes may be implemented. For example, in one embodiment file system  205  may assign files identified as log files through such a query to a lower-speed class of storage device, based on the heuristic that log files are typically infrequently accessed and therefore relatively performance-insensitive. Numerous other queries corresponding to various file classes as well as storage policies for such file classes are possible and contemplated. 
       FIG. 6A  and  FIG. 6B  illustrate embodiments of methods of generating and storing records corresponding to content access file operations and of importing such records into a file mutation database, respectively. Referring collectively to  FIG. 1 ,  FIG. 2 ,  FIG. 5 , and  FIG. 6A , operation begins in block  600  where an operation to access content of a file is detected. In one embodiment, filter driver  221  of file system  205  may be configured to detect an content access operation as described above. 
     Subsequent to detection of the identity-modifying operation, a record of the operation is generated (block  602 ). In some embodiments, filter driver  221  may be configured to generate this record, and as described above, in some embodiments the record may be in the XML format and may include information about the operation, the file identity, a signature corresponding to the file, and other information as desired. 
     After the operation record is generated, it is stored in a named stream corresponding to the file (block  604 ). Additionally, a history record of the operation is stored in a history stream (block  606 ). As noted above, filter driver  221  may be configured in some embodiments to store the generated record in a named stream  320  corresponding to the file  310  targeted by the operation, and may additionally be configured to store a history record such as described above in history stream  330 . 
     As in the case of identity-modifying operations described above, the method of  FIG. 6B  may in some embodiments operate in parallel to the method illustrated in  FIG. 6A . For example, the method of  FIG. 6B  may be implemented within update daemon  350 . Referring collectively to  FIG. 1 ,  FIG. 2 ,  FIG. 5 , and  FIG. 6B , operation begins in block  610  where a history record corresponding to a content access operation is detected within the history stream. As described above, in one embodiment update daemon  350  may be configured to scan history stream  330  to detect history records not previously processed. 
     Once a history record is detected, the records stored in the named stream of the file indicated in the history record are accessed and conveyed to the file mutation database (block  612 ). As described above, in one embodiment update daemon  350  may be configured to access the named stream corresponding to a file indicated in the history record and convey the records included therein to file mutation database  340 . 
     Other embodiments of these methods are possible and contemplated. For example, as noted above, in some embodiments of file system  205 , history stream  330  may be omitted, and update daemon  350  may be configured to scan the entire file system to determine the presence of updated records. Also, in some embodiments update daemon  350  may be omitted and filter driver  221  may communicate directly with file mutation database  340 . 
     Although the embodiments above have been described in considerable detail, numerous variations and modifications will become apparent to those skilled in the art once the above disclosure is fully appreciated. It is intended that the following claims be interpreted to embrace all such variations and modifications.