Abstract:
A mount manager and supporting data structures enable automatic identification and re-establishment of logical volumes on non-removable storage devices in a computer system across multiple reboots and reconfigurations. The mount manager generates a redirected name for a new logical volume when a unique volume identifier is presented to the mount manager by the operating system. The mount manager stores the unique volume identifier and the associated redirected name in a persistent mount manager data structure The mount manager establishes a symbolic link between the persistent redirected name, which is used by higher layers of the operating system and user applications to address the logical volume, and a non-persistent device name used by the operating system. During the boot process, the mount manager uses the data structure entries identified by the unique volume identifiers of the arriving logical volumes to reconstruct the symbolic links so that references to the redirected name will resolve to the correct non-persistent device name. When the system undergoes physical reconfiguration, the mount manager associates an existing redirected name to a different non-persistent device name if the unique volume identifier is present in the data structure. In this fashion, logical volumes can be removed and restored in the computer without the knowledge of higher layers of the operating system and user applications. Optionally, the mount manager builds an in-memory data structure from the persistent data structure to increase the speed of the identification process.

Description:
RELATED APPLICATIONS 
     This application is related to the co-assigned and co-filed U.S. patent applications titled “Persistent Names for Logical Volumes” (U.S. patent application Ser. No. 09/096,540, now U.S. Pat. No. 6,496,839) and “Persistent Volume Mount Points” (U.S. Pat. No. 6,119,131), which are hereby incorporated by reference. 
    
    
     FIELD OF THE INVENTION 
     This invention relates generally to computer storage device configuration, and more particularly to managing logical volumes mounted in a computer system. 
     BACKGROUND OF THE INVENTION 
     Most operating systems identify a logical unit of mass storage through a “well-known” and system compatible name which defines an actual physical path to the logical unit, i.e., \device0\partition1\. The operating system then associates a user-friendly name, such as a drive letter, with the well-known name so that the data on the storage device can be easily accessible by higher layers of the operating system and user applications. The higher layers of the operating system and applications assume that the well-known names, and thus the associated user-friendly names, are persistent across boot sessions. In actuality, the names are persistent only as long as the physical configuration of the computer does not change. Persistence cannot be guaranteed because such operating systems assign the well-known names in the order in which the storage devices are detected when booting. When the physical locations of the storage devices change, these operating systems will assign the well-known names to different devices. Therefore, the consistency of name assignments across multiple boot sessions is not preserved under all circumstances, and the higher operating system layers and user applications will be unable to access the data on the devices without modification. 
     Furthermore, most operation systems assume that only the storage devices found during the boot process will be present during the boot session. Thus, new storage devices added after booting cannot be recognized. This limitation also means that a logical device unit will not be recognized if the underlying storage device is removed and then reinserted during a boot session. 
     Therefore, there is a need in the art for a operating system that tracks logical device units during and across boot sessions, and provides persistent names despite physical configuration changes. 
     SUMMARY OF THE INVENTION 
     The above-mentioned shortcomings, disadvantages and problems are addressed by the present invention, which will be understood by reading and studying the following specification. 
     A logical volume mount manager is responsible for identifying and tracking logical volumes created from a physical storage device by the operating system, and for determining a redirected name for a logical volume which is used by higher layers of the operating system and user applications. The mount manager builds and maintains a persistent data structure based on a unique volume identifier which identifies the logical volume. Optionally, the mount manager also creates an in-memory data structure as well. Each entry in the data structure(s) consists of the redirected name and the unique volume identifier for a logical volume so that the redirected name persists across boot sessions. Because the operating system addresses a logical volume through a non-persistent device name, the mount manager causes the operating system to create a symbolic link between the device name and the redirected name when the mount manager first identifies the logical volume during a boot session so that the higher layers of the operating system and user applications can access the logical volume through the persistent redirected name. 
     When the physical configuration of the computer changes, the device name changes but the unique volume identifier does not. The mount manager uses the unique volume identifier to locate the appropriate redirected name in its data structure(s) and causes a new symbolic link to be created with the new device name so that the symbolic link resolves the redirected name to the correct logical volume under all circumstances. 
     Because the mount manager identifies the logical volume through its unique volume identifier and does not rely on the devices being located in any particular order in the system, or being discovered in any particular order during the boot process, or being present only during the boot process, changes in the physical configuration of the computer between boots, or during a boot session, have no effect on the higher layers of the operating system and user applications which rely on the redirected name. Thus, the level of indirection provided by the mount manager and supporting data structures guarantees that the higher layers of the operating system and user applications will be able to access data on a logical volume for the life of the logical volume without modifications. 
     The present application describes computer systems, methods, and computer-readable media of varying scope. The mount manager is variously described as causing the processor of a computer to perform certain actions, as a series of steps executed from a computer-readable medium, and in terms of its interaction with objects and other system components in an object-based operating system. In addition to the aspects and advantages of the present invention described in this summary, further aspects and advantages of the invention will become apparent by reference to the drawings and by reading the detailed description that follows. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 shows a diagram of the hardware and operating environment in conjunction with which embodiments of the invention can be practiced; 
     FIG. 2A is a diagram illustrating a system-level overview of an exemplary embodiment of the invention; 
     FIGS. 2B and 2C illustrate mount manager data structures for use in the exemplary embodiment of the invention shown in FIG. 2A; 
     FIGS. 3A,  3 B,  3 C and  3 D are flowcharts of method to be performed by a computer system according to an exemplary embodiment of the invention; and 
     FIG. 4 is a diagram illustrating a particular embodiment of the invention in a Microsoft Windows NT environment. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     In the following detailed description of exemplary embodiments of the invention, reference is made to the accompanying drawings which form a part hereof, and in which is shown by way of illustration specific exemplary embodiments in which the invention may be practiced. These embodiments are described in sufficient detail to enable those skilled in the art to practice the invention, and it is to be understood that other embodiments may be utilized and that logical, mechanical, electrical and other changes may be made without departing from the spirit or scope of the present invention. The following detailed description is, therefore, not to be taken in a limiting sense, and the scope of the present invention is defined only by the appended claims. 
     The detailed description is divided into five sections. In the first section, the hardware and the operating environment in conjunction with which embodiments of the invention may be practiced are described. In the second section, a system level overview of the invention is presented. In the third section, methods for an exemplary embodiment of the invention are provided. In the fourth section, a particular Microsoft Windows NT 5.0 implementation of the invention is described. Finally, in the fifth section, a conclusion of the detailed description is provided. 
     Hardware and Operating Environment 
     FIG. 1 is a diagram of the hardware and operating environment in conjunction with which embodiments of the invention may be practiced. The description of FIG. 1 is intended to provide a brief, general description of suitable computer hardware and a suitable computing environment in conjunction with which the invention may be implemented. Although not required, the invention is described in the general context of computer-executable instructions, such as program modules, being executed by a computer, such as a personal computer. Generally, program modules include routines, programs, objects, components, data structures, etc., that perform particular tasks or implement particular abstract data types. 
     Moreover, those skilled in the art will appreciate that the invention may be practiced with other computer system configurations, including hand-held devices, multiprocessor systems, microprocessor-based or programmable consumer electronics, network PCs, minicomputers, mainframe computers, and the like. The invention may also be practiced in distributed computing environments where tasks are performed by remote processing devices that are linked through a communications network. In a distributed computing environment, program modules may be located in both local and remote memory storage devices. 
     The exemplary hardware and operating environment of FIG. 1 for implementing the invention includes a general purpose computing device in the form of a computer  20 , including a processing unit  21 , a system memory  22 , and a system bus  23  that operatively couples various system components, including the system memory  22 , to the processing unit  21 . There may be only one or there may be more than one processing unit  21 , such that the processor of computer  20  comprises a single central-processing unit (CPU), or a plurality of processing units, commonly referred to as a parallel processing environment. The computer  20  may be a conventional computer, a distributed computer, or any other type of computer; the invention is not so limited. 
     The system bus  23  may be any of several types of bus structures including a memory bus or memory controller, a peripheral bus, and a local bus using any of a variety of bus architectures. The system memory may also be referred to as simply the memory, and includes read only memory (ROM)  24  and random access memory (RAM)  25 . A basic input/output system (BIOS)  26 , containing the basic routines that help to transfer information between elements within the computer  20 , such as during start-up, is stored in ROM  24 . The computer  20  further includes a hard disk drive  27  for reading from and writing to a hard disk, not shown, a magnetic disk drive  28  for reading from or writing to a removable magnetic disk  29 , and an optical disk drive  30  for reading from or writing to a removable optical disk  31  such as a CD ROM or other optical media. 
     The hard disk drive  27 , magnetic disk drive  28 , and optical disk drive  30  are connected to the system bus  23  by a hard disk drive interface  32 , a magnetic disk drive interface  33 , and an optical disk drive interface  34 , respectively. The drives and their associated computer-readable media provide nonvolatile storage of computer-readable instructions, data structures, program modules and other data for the computer  20 . It should be appreciated by those skilled in the art that any type of computer-readable media which can store data that is accessible by a computer, such as magnetic cassettes, flash memory cards, digital video disks, Bernoulli cartridges, random access memories (RAMs), read only memories (ROMs), and the like, may be used in the exemplary operating environment. 
     A number of program modules may be stored on the hard disk, magnetic disk  29 , optical disk  31 , ROM  24 , or RAM  25 , including an operating system  35 , one or more application programs  36 , other program modules  37 , and program data  38 . A user may enter commands and information into the personal computer  20  through input devices such as a keyboard  40  and pointing device  42 . Other input devices (not shown) may include a microphone, joystick, game pad, satellite dish, scanner, or the like. These and other input devices are often connected to the processing unit  21  through a serial port interface  46  that is coupled to the system bus, but may be connected by other interfaces, such as a parallel port, game port, or a universal serial bus (USB). A monitor  47  or other type of display device is also connected to the system bus  23  via an interface, such as a video adapter  48 . In addition to the monitor, computers typically include other peripheral output devices (not shown), such as speakers and printers. 
     The computer  20  may operate in a networked environment using logical connections to one or more remote computers, such as remote computer  49 . These logical connections are achieved by a communication device coupled to or a part of the computer  20 , the local computer; the invention is not limited to a particular type of communications device. The remote computer  49  may be another computer, a server, a router, a network PC, a client, a peer device or other common network node, and typically includes many or all of the elements described above relative to the computer  20 , although only a memory storage device  50  has been illustrated in FIG.  1 . The logical connections depicted in FIG. 1 include a local-area network (LAN)  51  and a wide-area network (WAN)  52 . Such networking environments are commonplace in offices, enterprise-wide computer networks, intranets and the Internet. 
     When used in a LAN-networking environment, the computer  20  is connected to the local network  51  through a network interface or adapter  53 , which is one type of communications device. When used in a WAN-networking environment, the computer  20  typically includes a modem  54 , a type of communications device, or any other type of communications device for establishing communications over the wide area network  52 , such as the Internet. The modem  54 , which may be internal or external, is connected to the system bus  23  via the serial port interface  46 . In a networked environment, program modules depicted relative to the personal computer  20 , or portions thereof, may be stored in the remote memory storage device. It is appreciated that the network connections shown are exemplary and other means of and communications devices for establishing a communications link between the computers may be used. 
     The hardware and operating environment in conjunction with which embodiments of the invention may be practiced has been described. The computer in conjunction with which embodiments of the invention may be practiced may be a conventional computer, a distributed computer, or any other type of computer; the invention is not so limited. Such a computer typically includes one or more processing units as its processor, and a computer-readable medium such as a memory. The computer may also include a communications device such as a network adapter or a modem, so that it is able to communicatively couple to other computers. 
     System Level Overview 
     A system level overview of the operation of an exemplary embodiment of the invention is described by reference to FIGS. 2A-C. FIG. 2A shows one embodiment of a logical volume mounting subsystem  200  executing in a computer such as local computer  20  or remote computer  49  in FIG.  1 . The physical media, such as hard disk drive  27  in local computer  20 , contains one or more logical volumes. The process of associating a logical volume with the appropriate underlying physical media is commonly referred to in the art as “mounting” the logical volume on the physical media. The logical volume must be mounted before the data on the physical media can be accessed. 
     The logical volume mounting subsystem  200  shown in FIG. 2A comprises a mount manager  201  and a persistent mount manager data structure  203 , and is responsible for associating “redirected” names, used by the higher layers of the operating system and user applications, with mounted logical storage volumes  207 ,  208  and  209  so that the data on the underlying physical devices can be accessed through the redirected names. In FIG. 2A, the redirected names are represented by drive letters, such as commonly used by personal computer applications. The redirected names are not limited to drive letters as will be readily apparent to one skilled in the art and described in more detail below in conjunction with FIGS. 2B and 2C. 
     The operating system  205  creates the logical volumes  207 - 209  from removable or fixed physical media devices, such as hard disk drive  27 . Each logical volume is identified by a unique volume identifier, such as  994  for logical volume  207 , which is stored on the physical device, or devices, that make up the logical volume, and which is guaranteed to be unique on the particular computer. Each logical volume is also assigned a device name, such as Vol1 for logical volume  207 , during the boot process. The device name can change across boot sessions but is unique for a particular boot session. 
     The mount manager data structure  203  is maintained by the operating system  205  with other configuration data so that it is persistent across boot sessions. The operating system  205  presents each logical volume  207 - 209  to the mount manager  201  in the order in which the operating system  205  locates each logical volume in the computer when the computer is booted. The mount manager  201  queries each logical volume  207 - 209  for its device name and unique volume identifier. The mount manager  201  searches the mount manager data structure  203  to find an entry that contains the unique volume identifier for the logical volume  207 - 209 . 
     As shown in FIG. 2A, the unique volume identifier  991  for logical volume  207  appears in entry  211  in the mount manager data structure  203 . Entry  211  also contains a redirected name (“C:”) for logical volume  207 . Therefore, the mount manager  201  informs the operating system  205  of the association between the redirected name, C:, and the device name, Vol1, for logical volume  207 . The operating system  205  creates a logical, symbolic link between the redirected name and the device name, and maintains that link in a symbolic link data structure  215 . Similarly, the mount manager  201  causes the operating system  205  to create symbolic links between the device names, Vol3 and Vol2, and the appropriate redirected names, E: and D: respectively, when logical volumes  208 ,  209  are presented to it. 
     When a new logical volume is introduced into the system, the mount manager  201  creates an entry in the data structure  203  and a redirected name for the logical volume, and causes the operating system to create the corresponding symbolic link. The redirected name for the logical volume can stored on the physical device, or devices, that make up the logical volume so it can be recalled if queried. 
     Because the unique volume identifier  998  for logical volume  209  is associated with the redirected name D: in the persistent mount manager data structure  203 , logical volume  209  will be always be assigned the same redirected name even if the logical volume  209  is presented to the mount manager  201  after the logical volume  208  which is associated with redirected name E:. The order in which the logical volumes are presented is dependent upon the order in which they are detected by the operating system  205  so the order changes if the underlying physical devices are rearranged in the computer between boots. The device name of the logical volumes also change when the physical configuration changes. Because the mount manager data structure  203  depends on the unique volume identifiers rather than the device names to identify the logical volume, the mount manager data structure  203  ensures consistency between redirected names and logical volumes across boot sessions regardless of the underlying physical configuration of the computer or the order in which the devices are recognized as long as the logical volume is valid. 
     If a logical volume is permanently removed from the system, the operating system  205  notifies the mount manager  201  which deletes the corresponding entry from the data structure  203  and breaks the symbolic link. However, if the logical volume is only removed temporarily, as illustrated by logical volume  209  in FIG. 2A, the mount manager  201  only breaks the corresponding symbolic link. 
     As shown in phantom in FIG. 2A, if the logical volume  209  is re-introduced in the same boot session, the device name, Vol4, is different because a device name is used only once during a boot session, but the mount manager  201  finds the entry  212  in the data structure  203  based on the unique volume identifier  998 . Because the entry is present, the mount manager requests that the operating system  205  re-establish the corresponding symbolic link between logical volume  209  and its redirected name D:. 
     Thus, the volume mounting subsystem  200  guarantees that symbolic links will always resolve to the correct logical volume, both during a boot session and across multiple boot sessions, during the life of the logical volume. 
     FIG. 2B illustrates an alternate embodiment of a mount manager data structure  221  in which the redirected name assigned to a logical volume is a mount manager identifier (“persistent mount name”)  222 ,  223 , and  224  which is guaranteed to be unique across all computers. Certain logical volumes, illustrated by entries  228  and  229 , are also assigned an additional “user-friendly” redirected name, such as a drive letter. The alternate embodiment shown in FIG. 2B is particularly applicable in operating system environments in which the number of user-friendly names for logical volumes is limited because a logical volume without a user-friendly name can be addressed by its persistent mount name  223  and also benefits from the consistency provided by the redirected names of the present invention. 
     In an alternate embodiment of the logical volume mounting subsystem, the mount manager  201  copies the persistent mount manager data structure into memory so that the time required to mount logical volumes and to detect configuration changes during a boot session is decreased. The in-memory data structure is, by its nature, non-persistent across boot sessions and is recreated during the boot process. 
     One embodiment of an in-memory mount manager data structure  231  is shown in FIG.  2 C. The in-memory mount manager data structure  231  has been created from the persistent mount manager data structure  221  illustrated in FIG. 2B but could also be created from the persistent mount manager data structure  203  shown in FIG.  2 A. 
     Each entry  233  in the in-memory mount manager data structure  231  is composed of three fields: a redirected name field  235 , a unique volume identifier field  237 , and a device name field  239 . The redirected name field  235  and the unique volume identifier field  237  are copied from the persistent mount manager data structure  221  upon system boot. As each logical volume is presented to the mount manager  201 , the mount manager  201  stores the boot session device name in the appropriate device name field  239  in the in-memory mount manager data structure  231 . 
     When logical volume  209  is temporarily removed from the system, its device name, Vol2, is deleted from the appropriate entries  233  in the mount manager data structure  231  but the unique volume identifier  998  is maintained in the entries. Thus, when logical volume  209  is re-introduced into the system and assigned the new device name, Vol4, the mount manager  201  is able to identify it as a logical volume that it was previously present, update the data structure  231  appropriately, and re-establish the corresponding symbolic links. 
     In an alternate embodiment, the in-memory mount manager data structure is an identical copy of the persistent mount manager data structure without the device name field  239 . 
     The system level overview of the operation of an exemplary embodiment of the invention has been described in this section of the detailed description. A mount manager and supporting data structures enable consistent identification and addressing of logical volumes despite physical configuration changes for the life of the logical volumes. While the invention is not limited to any particular arrangement of data in the data structures, for sake of clarity exemplary embodiments of persistent and in-memory data structures have been illustrated and described. 
     Methods of an Exemplary Embodiment of the Invention 
     In the previous section, a system level overview of the operation of an exemplary embodiment of the invention was described. In this section, the particular methods performed by a computer executing an exemplary embodiment is described by reference to a series of flowcharts. The methods to be performed by a computer constitutes computer programs made up of computer-executable instructions. Describing the methods by reference to a flowchart enables one skilled in the art to develop such programs including such instructions to carry out the methods on suitable computers (the processor of the computers executing the instructions from computer-readable media). The methods are illustrated in FIGS. 3A-D and are inclusive of the steps or acts required to be taken by the mount manager  201  operating in the environment shown in FIGS. 2A-C. 
     Referring first to FIG. 3A, when the computer is booted, the mount manager  201  creates the in-memory data structure  231  from the persistent mount manager data structure  221  (step  301 ). Upon initial boot of the computer, the persistent mount manager data structure  221  contains only user-friendly redirected names as no logical volumes have yet been configured in the system. Upon subsequent boots, the persistent mount manager data structure  221  contains user-friendly names, persistent mount names, and unique volume identifiers for the logical volumes that were not known to be permanently deleted from the system when it was shut-down. 
     After initializing its in-memory data structure  231 , the mount manager  201  waits for notification from the operating system  205  that a logical volume has been detected in the computer (step  303 ). When the notification arrives, either during the boot process or during the boot session, the mount manager  201  queries the volume for its unique volume identifier and its boot session device name (step  305 ). The mount manager  201  uses the unique volume identifier to search the in-memory data structure  231  for a matching entry (step  307 ). If a matching entry is found (step  309 ), the mount manager  201  checks the entry to determine if any redirected name has been previously assigned to the volume (step  311 ). 
     If the entry contains a redirected name(s), the mount manager  201  updates the in-memory data structure  221  with the device name at step  315 , and causes the operating system  205  to create a symbolic link between the redirected name(s) and the boot session device name (step  317 ). Thus, the redirected name(s) used by the higher layers of the operating system  205  and user applications are preserved across boot sessions, even though the device names may change when the physical configuration of the computer is modified. In an alternate embodiment in which the in-memory data structure does not contain the device name field, the mount manager  201  skips step  315 , as both the in-memory and persistent data structures already contain the correct unique volume identifier and redirected name(s) (illustrated by a phantom logic flow path in FIG.  3 A). 
     When a logical volume is re-introduced into the system, either during a boot or during a boot session, if the entry does not contain redirected name(s) (step  311 ) as discussed above, then the mount manager  201  queries the logical volume for its “desired” redirected name(s) (step  313 ) and updates the corresponding entries in the data structures  221 ,  231  with the device names and/or redirected name(s) as appropriate. The mount manager  201  then requests the creation of the symbolic links (step  317 ). 
     If there is no existing data structure entry that matches the unique volume identifier (step  309 ), the mount manager  201  creates an entry for the logical volume (step  319 ) by inserting the unique volume identifier in an empty entry. The mount manager  201  creates a persistent mount name the new logical volume (step  313 ). If requested to do so by the operating system  205 , the mount manager  201  also assigns the next available user-friendly name to the new logical volume at step  313 . The mount manager  201  informs the logical volume of the assigned redirected name(s) so they can be stored for later use. The mount manager data structures  221 ,  231  are updated with the redirected name(s) (step  315 ), and the corresponding symbolic links created (step  317 ). 
     The mount manager  201  is also notified by the operating system  205  when a logical volume is temporarily removed from the computer as shown in FIG.  3 B. All symbolic links associated with the logical volume are retired (step  321 ). The mount manager  201  updates its data structures  221 ,  231  by deleting the device name for the logical volume from all entries associated with the logical volume. The unique volume identifier is maintained in the entries to detect the re-introduction of the logical volume (refer to step  309  in FIG.  3 A). 
     The mount manager  201  also provides for deleting the symbolic links and mount manager data structure entries associated with the logical volume as illustrated in FIG.  3 C. When requested to do so by the operating system  205 , the mount manager  201  retired the corresponding symbolic links (step  327 ), and deletes the unique volume identifier, device name, and persistent mount name for the logical volume from all the appropriate entries in the mount manager data structures  221 ,  231  (step  329 ). The operating system makes this request when a logical volume is permanently deleted from the system but the operating system can also make the request without deleting the logical volume. However, if the logical volume is being accessed by a higher-layer application, only the data structure entries are modified as described above (step  325 ). The symbolic link(s) are maintained so that the applications can continue to access the data on the logical volume. 
     FIG. 3D illustrates the query function of the mount manager  201 . The operating system  205  can query the mount manager  201  regarding a mounted logical volume by passing the symbolic link name, the unique volume identifier, or the boot session device name of a logical volume to the mount manager  201  at step  331 . The mount manager  201  searches its in-memory data structure, if present, or its persistent data structure (step  333 ) and returns the corresponding entry if one is found (step  337 ). If no entry matches the search criteria, the operating system  205  is so informed (step  339 ). If the device name is used as the search criteria and there is no in-memory data structure, or if the in-memory data structure does not contain the device name, the mount manager  201  uses the symbolic link to determine the redirected name in order to perform the search. 
     The methods performed by a mount manager of an exemplary embodiment of the invention have been described with reference to a series of flowcharts illustrated in FIGS. 3A-C, including all the steps from  301  until  339  shown therein. In particular, the methods of associating redirected names with logical volumes, and the management of such associations have been described. 
     Microsoft Windows NT 5.0 Implementation 
     In this section of the detailed description, a particular implementation of the invention is described that executes as part of the Microsoft Windows NT 5.0 operating system kernel. In the implementation illustrated in FIG. 4, the mount manager  401  and four other kernel modules work together to provide a user with access to data stored on a physical storage device  411  (shown as a fixed hard disk): a plug and play manager  403 , an object manager  405 , a partition manager  407 , and at least one volume manager  409 . The mount manager  401  is not limited to use with only devices that adhere to the partition manager and volume manager architectures described below. The mount manager  401  will manage any device which registers with the plug and play manager  403  which has some mechanism for reporting a device name and a unique identifier that is persistent between boots. The partition manager  407  and the volume manager  409  are shown and described for the sake of clarity in understanding the invention. 
     As described above, the mount manager  401  is responsible for associating redirected names with unique volume identifiers for logical volumes so that higher layers of the operating system and user applications can easily access the data on the logical volume. In the NT 5.0 embodiment, the mount manager  401  persistent data structure is stored in the NT registry. Alternate embodiments in which the persistent data structure is stored in non-volatile memory, such as battery-backed RAM or flash memory, will be readily apparent to one skilled in the art and are contemplated as within the scope the invention. The mount manager  401  also builds an in-memory data structure from the persistent data structure to decrease the time required to react to configuration changes in the system. In FIG. 4, data structure  441  is representative of both the in-memory and persistent data structures. 
     Because NT 5.0 is an object-based operating system, every device, either physical, logical or virtual, within the system is represented by a device object. The objects are organized into a device hierarchy in a global namespace controlled by the object manager  405 . The object manager  405  is also responsible for creating and maintaining symbolic link objects which serve as aliases for named device objects. The mount manager redirected name is represented in the namespace by a symbolic link object which contains the non-persistent device name of the corresponding logical volume. Thus, an “Open” command operating on a redirected name symbolic link object is the same as an “Open” command on the logical volume device object having the device name contained in the symbolic link object. 
     The partition manager  407  is responsible for handling device objects associated with logical divisions, partitions  412 ,  413 ,  414  and  415 , of a physical device  411 . The partitions  412 - 415  are created when the physical device  411  is formatted. The partition  412  is the entire physical device  411  while the partitions  413 - 415  are sub-divisions of the physical device  411 . A device driver (not shown) for the physical device  411  “enumerates” corresponding partition device objects  421 ,  422 ,  423 , and  424  when the computer is booted. The partition manager  407  and at least one volume manager  409  cooperate to create logical volumes from the partitions  413 - 415 . The composition of a logical volume is defined when the physical device is formatted, and can comprise one or more partitions. Additionally, one partition can comprise more than one logical volume. A unique volume identifier for the logical volume is stored in a privileged section of the physical device, or devices, that contain the partitions making up the logical volume The volume manager, or the device driver in the case of a removable device, responsible for the volume device object creates the unique volume identifier. 
     When the partition manager  407  is initialized, it requests notification from the plug and play manager of all volume managers  409  registered in the system. As each volume manager  409  registers, the plug and play system notifies the partition manager  407  which maintains a list of the volume managers  409  ordered by their arrival in the system. 
     When the physical device  411  is detected by the plug and play manager  403  upon booting the system, the plug and play manager  403  determines the formatted characteristics of the physical device  411 . The plug and play manager  403  loads the appropriate device driver to handle I/O access to the device. The device driver enumerates the partition device objects  421 - 424  used to access the data. As each partition device object  422 - 424  not representative of the entire device is enumerated by the device driver, the partition manager  407  “captures” the partition device object  422 - 424  before the driver registers the object with the plug and play manager  403 . The partition manager  407  presents each partition device object  422 - 424  to the volume managers  409  in the order in which the volume managers  409  arrived in the system. Because each partition device object  424 - 424  is associated with at least one logical volume, the volume manager  409  responsible for the corresponding logical volume(s) accepts the device object. 
     When a volume manager  409  has received a sufficient number of partition device objects corresponding to a particular logical volume, the volume manager  409  assigns a device name to the logical volume and enumerates a volume device object  431 - 432  for the logical volume containing the device name and the unique volume identifier for the logical volume. In the NT 5.0 embodiment, the device name is guaranteed to be unique only during a boot session, while the unique volume identifier is guaranteed to be unique across boot sessions. A counted string is used as the unique volume identifier in the NT 5.0 environment but a fixed length string can be equally applicable in other operating system environments. The counted string is as long as necessary to uniquely identify the device in the computer across multiple boot sessions. The volume device object  431 - 432  is stored by the object manager  405  in the device hierarchy by its device name. The volume manager  409  informs the plug and play manager  403  of the creation of the volume device object  431 - 432 . 
     Each volume device object  431 - 432  is presented to the mount manager  401  by the plug and play manager  403 . The mount manager  401  queries the volume device object  431 - 432  for its device name and unique volume identifier. Because of the indeterminate length of the unique volume identifier, the volume device object returns a byte count along with the string. 
     The mount manager  401  scans its internal data structure  441  (in-memory or persistent) looking for a matching entry for the unique volume identifier of the volume device object  431 - 432 . If none is found, this particular logical volume is new to the system, so the mount manager  401  assigns it a unique persistent mount name, and creates an entry for the logical volume in the mount manager data structure(s)  441 . In the NT 5.0 embodiment, the persistent mount name is based on a 16-byte globally unique identifier (GUID) with following format: 
     \??\volume {GUID }\ 
     where \??\ designates the entry point in the global namespace device hierarchy for logical volumes and is synonymous with \DosDevices\. The hexadecimal representation of GUID is xxxxxxxx-xxxx-xxxx-xxxx-xxxxxxxxxxxx. A GUID is comparable to the UUID (universally unique identifier) specified by the Open System Foundation. The persistent mount name is generated by a mount manager subroutine called “CreateNewVolumeName.” 
     After a new entry is created, or if a matching entry is found, the mount manager  401  requests that the object manager  405  create a symbolic link object to represent the relationship between the device name and the persistent mount name for the logical volume. 
     In order to facilitate the assignment of user-friendly drive letters to logical volumes, the mount manager data structure(s)  441  contains an entry for each of the twenty-four drive letters assignable to fixed hard disks, i.e., \DosDevices\C:\-\DosDevices\Z:\. The entries are sorted in alphabetical order. Upon the initial boot of the computer, only the logical boot volume is assigned a drive letter (such as \DosDevices\C:\). When a drive letter is requested for a logical volume by the plug and play manager  403  during the initial boot process, the mount manager  401  assigns the next available drive letter by storing the unique volume identifier in the corresponding entry in the data structure(s)  441 . The mount manager  401  requests that the object manager  405  create a symbolic link object representing the association between the drive letter and the volume device name. 
     On subsequent boots, each logical device is assigned its previous drive letter if one is present in the data structure(s)  441 . If a new logical device is introduced into the system during the boot process, the plug and play manager  403  must request the assignment of a drive letter. On the other hand, a new logical volume that is introduced during a boot session is automatically assigned the next available drive letter. 
     The plug and play manager  403  also informs the mount manager  401  when a logical volume will be temporarily removed from the boot session. The mount manager  401  deletes the device names, if present, from the data structure(s)  441 . The mount manager  401  also causes the object manager  405  to retire the symbolic link objects relating the volume device name and the drive letter and/or persistent mount name. 
     If the logical volume is reintroduced into the system during the same boot session, the volume manager will assign a different device name because the device names are guaranteed not to be reused during a boot session, but the unique volume identifier will be the same. Because the mount manager  401  has not deleted the unique volume identifier from the entries in the data structure(s)  441 , the mount manager  401  recognizes the logical volume if it is reintroduced and uses the data structure entries to re-create the same symbolic link objects as before so that consistency can be maintained. 
     If the logical volume is permanently deleted, its unique volume identifier and persistent mount name are also removed from the data structure(s)  441  and any now-empty entries are freed. 
     The partitions comprising a logical volume can change without deleting the volume if the logical volume is a mirrored volume that has been broken or a striped set that has been rebuilt. Under such circumstances, the device name does not change, but the unique volume identifier associated with the logical volume does. The volume manager  409  so informs the mount manager  401  which updates the data structures to reflect the change in the unique volume identifier. 
     The mount manager  401  communicates with the logical volume device objects  431 - 432  through an application program interface (API) having six calls: 
     QueryDeviceName (pointer to device object); 
     QueryUniqueId (pointer to volume device object); 
     QueryDesiredName (pointer to volume device object); 
     QueryUniqueIdChangeNotify (pointer to volume device object); 
     LinkCreated (pointer to volume device object); and 
     LinkDeleted (pointer to volume device object), 
     where the pointer to a device object is passed to the mount manager  401  by the plug and play manager  403 . 
     The interface between the mount manager  401  and volume device objects is identified by a GUID called the “mounted_device_GUID.” Any device object that declares support for the mounted_device_GUID must implement at least QueryDeviceName and QueryUniqueId. 
     QueryDeviceName and QueryUniqueId return the device name and unique volume identifier for the specified volume device object. QueryDesiredName returns a recommended redirected name(s) for the mount manager  401  to use in the event that the mount manager data structure(s)  441  does not yet contain any entries for the specified volume device object. A physical device that supports QueryDesiredName usually stores the desired name(s) in the same privileged area of the physical device as the unique volume identifier. 
     The mount manager  401  uses QueryUniqueIdChangeNotify to determine if the unique volume identifier for the specified logical volume device object has changed. Such a change is usually due to a change in the location for the volume but other circumstances can also cause the unique volume identifier to change. The mount manager  401  then updates the unique volume identifier in the data structure entries associated with the device name. 
     LinkCreated and LinkDeleted are used by the mount manager  401  to inform the volume device object of the creation and deletion of the redirected names and the symbolic link objects that reference the volume device object. A physical device that supports LinkCreated and LinkDeleted can use the information to update any redirected name(s) it has internally stored. 
     The mount manager  401  also provides an API with the plug and play manager  403  for managing the association between unique volume identifiers and the redirected names: 
     QueryPoints (drive letter/*, unique volume identifier/*, device name/*); 
     DeletePoints (drive letter/*, unique volume identifier/*, device name/*); 
     DeletePointsDBOnly (drive letter/*, unique volume identifier/*, device name/*); 
     CreatePoint (drive letter, device name); 
     NextDriveLetter(device name); 
     AutoDriveLetter; 
     FindFirstVolume; and 
     FindNextVolume (persistent mount name). 
     QueryPoints is called by the plug and play manager  403  to retrieve the entry in the mount manager data structure  441  for a logical volume. The plug and play manager  403  specifies the drive letter, the unique volume identifier, or any device name associated with the logical volume as search criteria to be used by the mount manager  401 (* is a “wild card” that matches all entries). 
     DeletePoints causes the mount manager  401  to return the corresponding entries from the data structure  441  and then perform the deletion steps described above to delete all entries and symbolic link objects associated with the volume device object. If a drive letter for a logical volume is explicitly deleted, i.e., not as the result of a wildcard operation, then an indicator associated with the unique volume identifier is set to alert the mount manager  401  that the logical volume is not to be assigned a drive letter if it is re-introduced into the system. DeletePoints is used when a logical volume is permanently deleted from the system or to disassociate a logical volume from some or all of its existing redirected names even though the logical volume itself is still present in the system. DeletePointsDBOnly operates as does DeletePoints but does not delete the symbolic link object(s). DeletePointsDBOnly is used when a logical volume is not deleted or removed and a user application is currently accessing data on the logical volume. 
     CreatePoint causes the mount manager  401  to assign the specified drive letter to a logical volume which previously had its drive letter deleted through either DeletePoints or DeletePointsDBOnly. 
     In order to preserve the historical drive letter assignments across boot sessions, the mount manager  401  does not automatically assign a drive letter to a logical volume during the boot process unless the logical volume had previously been assigned a drive letter. Therefore, the plug and play manager  403  uses NextDriveLetter to request that the mount manager  401  assign a drive letter to the logical volume associated with the device name specified in the call. NextDriveLetter returns the current drive letter and an indication of whether a drive letter was assigned. A drive letter cannot be assigned if no drive letter is available or if the logical volume represented by the device name is already assigned a drive letter. The plug and play manager  403  can also use AutoDriveLetter once the historical assignments have been made to request the mount manager  401  assign drive letters to all subsequent logical volumes upon arrival. 
     Because the mount manager  401  does not necessarily assign a drive letter to a logical volume, the mount manager  401  provides two calls that enumerate the persistent mount names present in the system. The symbolic link objects can then be used to determine the device name of the associated logical volume. FindFirstVolume returns the persistent mount name found in the first entry in the mount manager data structure  441 . FindNextVolume returns the persistent mount name in the entry following the entry containing the specified persistent mount name. 
     Thus, the NT 5.0 mount manager and supporting data structures guarantee the same redirected name(s) will be associated with the same logical volume across any number of boots or reconfigurations as long as the logical volume itself remains valid. The persistence of the redirected names guarantees that I/O commands on a redirected name are resolved through the symbolic links to the current device name for the correct logical volume so that the higher layers of the operating system and user applications do not have to be modified when the underlying physical structure of the computer changes. 
     CONCLUSION 
     A logical volume mount manager and supporting data structures have been described. The logical volume mount manager creates persistent redirected names for logical volumes in a computer system to enable symbolic links between the redirected names, which are used by the higher layers of the operating system and user applications, and non-persistent device names that identify the logical volumes to the lower layers of the operating system during a single boot session. Because of the level of indirection provided by the mount manager, the symbolic links can be torn down and rebuilt to point to different device names for the same logical volumes when necessary without requiring modification of the higher levels of the operating system and user applications. Thus, the mount manager and supporting data structures guarantee configuration consistency across boot sessions of the computer and ensure that access through a redirected name is resolved through the symbolic link to the current device name for the correct logical volume. 
     Although specific embodiments have been illustrated and described herein, it will be appreciated by those of ordinary skill in the art that any arrangement which is calculated to achieve the same purpose may be substituted for the specific embodiments shown. This application is intended to cover any adaptations or variations of the present invention. 
     The terminology used in this application with respect to is meant to include all operating system and programming environments capable of implementing the mount manager and supporting data structures as described above. Therefore, it is manifestly intended that this invention be limited only by the following claims and equivalents thereof.