Abstract:
A virtualized computer system employs a virtual disk. Multiple snapshots of the virtual disk can be created. After a snapshot is created, writes to the virtual disk are captured in delta disks. Two snapshots are consolidated by updating block references in snapshot meta data. Block reference update takes advantage of the fact that blocks for the two snapshot are managed within the same storage container and, therefore, can be moved in the snapshot logical space without incurring data copy operations. Consolidation of delta disks also gracefully handles failures during the consolidation operation and can be restarted anew after the system has recovered from failure.

Description:
BACKGROUND 
       [0001]    Computer virtualization is a technique that involves encapsulating a physical computing machine platform into a virtual machine that is executed under the control of virtualization software on a hardware computing platform, or “host.” A virtual machine has both virtual system hardware and guest operating system software. Virtual system hardware typically includes at least one “virtual disk,” which is represented as a single file or a set of files in the host&#39;s file system, and appear as a typical storage drive to the guest operating system. The virtual disk may be stored on the host platform&#39;s local storage device (if any) or on a remote storage device. Typically, a virtual machine uses the virtual disk in the same manner that a physical storage drive is used, to store the guest operating system, application programs, and application data. 
         [0002]    A snapshot of the virtual disk can be taken at a given point in time to preserve the content within the virtual disk at that point in time, referred to herein as a “point in time (PIT) copy of the virtual disk.” Once a snapshot of a virtual disk is created, subsequent writes received from the guest operating system to the virtual disk are captured in a “delta disk” so that the preserved content, i.e., the base PIT copy, is not modified. The delta disk is an additional file associated with the virtual disk. At any given time, represents the difference between the current state of the virtual disk and the state at the time of the previous snapshot. Thus, the base PIT copy remains intact and can be reverted back to or can be used as a base template to create writable virtual disk clones. Multiple PIT copies of the virtual disk can be created at various points in time by creating snapshots of snapshots. Each snapshot corresponds to a separate delta disk that is overlaid on a previous delta disk. 
         [0003]    Creating multiple snapshots of a virtual disk results in a long chain of delta disks, each corresponding to a snapshot of the virtual disk. Every read IO operation to the virtual disk has to traverse through each delta disk associated with the virtual disk to get the latest copy of the data from a delta disk. Therefore, an increased number of delta disks negatively impacts the performance of read IO operations to the virtual disk. Performance of such IO operations may be increased when redundant delta disks are consolidated to reduce the number of delta disk in a given chain. Redundant delta disks are associated with PIT copies of the virtual disk that are no longer needed. For example, a PIT copy of the virtual disk may created for backing up or testing purposes and becomes redundant upon backup completion or when the testing is successful. 
         [0004]    Delta disks are consolidated by merging PIT copies such that a particular delta disk can be deleted. Merging the PIT in copies typically involves copying out data from the delta disk to be deleted (the “source delta disk”) to the main primary disk or an adjacent delta disk (either, referred to generally as the “destination delta disk”). Copying data in such a manner from the source delta disk to the destination delta disk involves data movement operations that cost a significant amount of IO and CPU resources. As the size of data in the source delta disk increases, the data movement operations that are necessary to consolidate two delta disks becomes very IO intensive. Thus, during consolidation, the IO performance for the virtual disk as a whole degrades drastically when a delta disk consolidation operation is in process. 
         [0005]    As the foregoing illustrates, what is needed in the art is a mechanism for consolidating delta disks with minimal impact to IO operation performance within the virtual disk and minimal data transfer overheads. 
       SUMMARY 
       [0006]    One or more embodiments of the present invention provide techniques for consolidating snapshots of a virtual disk in a manner that eliminates the need of data movement from one delta disk to another. 
         [0007]    A method for consolidating a plurality of delta disk included in a delta disk chain associated with a virtual disk. The method includes the step of determining that a first delta disk included in the delta disk chain is to be consolidated with a second delta disk included in the delta disk chain, where, once consolidated, the second delta disk is to be removed from the delta disk chain. The method also includes the steps of determining that a first data block in the first delta disk corresponds to a second data block in the second delta disk, wherein the second data block stores data reflecting an update to data stored in the first data block, and modifying a reference included in metadata for the first delta disk that points to the first data block to point to a second data block included in the second delta disk 
         [0008]    Embodiments of the present invention further include a non-transitory computer-readable storage medium storing instructions that when executed by a computer system cause the computer system to perform one or more of the methods set forth above, and a computer system that is configured to carry out one or more of the methods set forth above. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0009]      FIG. 1  is a computer system configuration utilizing a shared file system, according to an embodiment. 
           [0010]      FIG. 2  is a virtual machine based computer system, according to an embodiment. 
           [0011]      FIG. 3  illustrates a detailed view of the base disk of  FIG. 2  and a file Mode associated with the base disk. 
           [0012]      FIG. 4A  illustrates delta disks generated after a snapshot of virtual disk of  FIG. 3  is taken. 
           [0013]      FIG. 4B  illustrates the consolidation of the delta disks shown in  FIG. 4A . 
           [0014]      FIGS. 5A and 5B  set forth a flow diagram of method steps for consolidating two delta disks corresponding to different snapshots of a virtual disk, according to one embodiment. 
       
    
    
     DETAILED DESCRIPTION 
       [0015]      FIG. 1  is a computer system configuration utilizing a shared file system, according to an embodiment. The computer system configuration of  FIG. 1  includes multiple servers  100   A  to  100   N  each of which is connected to storage area network (SAN)  105 . Operating systems  110   A  and  110   B  on servers  100   A  and  100   B  interact with a shared file system  115  that resides on a data storage unit (DSU)  120  accessible through SAN  105 . In particular, data storage unit  120  is a logical unit (LUN) of a data storage system  125  (e.g., disk array) connected to SAN  105 . While DSU  120  is exposed to operating systems  110   A  to  110   B  by system storage manager  130  (e.g., disk controller) as a contiguous logical storage space, the actual physical data blocks upon which shared file system  115  may be stored is dispersed across the various physical disk drives  135   X  to  135   Z  of data storage system  125 . 
         [0016]    Data in DSU  120  (and possibly other DSUs exposed by the data storage systems) are accessed and stored in accordance with structures and conventions imposed by shared file system  115  which, for example, stores such data as a plurality of files of various types, typically organized into one or more directories. Shared file system  115  further includes metadata data structures that store or otherwise specify information, for example, about how data is stored within shared file system  115 , such as block bitmaps that indicate which data blocks in shared file system  115  remain available for use, along with other metadata data structures indicating the directories and files in shared file system  115 , along with their location. Such meta data structures are typically stored in an information node (Mode) associated with a file. So that data blocks can be shared across multiple files in the file system, each data block is also associated with a reference count indicating the number of files that reference the data block. When data blocks are shared, techniques such as copy on write (COW) are implemented to achieve isolation properties. 
         [0017]      FIG. 2  is a virtual machine based computer system  200 , according to an embodiment. A computer system  201 , generally corresponding to one of the servers  100 , is constructed on a conventional, typically server-class hardware platform  224 , including, for example, host bus adapters (HBAs)  226  that network computer system  201  to remote data storage systems, in addition to conventional platform processor, memory, and other standard peripheral components (not separately shown). Hardware platform  224  is used to execute a hypervisor  214  (also referred to as virtualization software) supporting a virtual machine execution space  202  within which virtual machines (VMs)  203  can be instantiated and executed. For example, in one embodiment, hypervisor  214  may correspond to the vSphere product (and related utilities) developed and distributed by VMware, Inc., Palo Alto, Calif. although it should be recognized that vSphere is not required in the practice of the teachings herein. 
         [0018]    Hypervisor  214  provides the services and support that enable concurrent execution of virtual machines  203 . Each virtual machine  203  supports the execution of a guest operating system  208 , which, in turn, supports the execution of applications  206 . Examples of guest operating system  208  include Microsoft® Windows®, the Linux® operating system, and NetWare®-based operating systems, although it should be recognized that any other operating system may be used in embodiments. Guest operating system  208  includes a native or guest file system, such as, for example, an NTFS or ext3FS type file system. The guest file system may utilize a host bus adapter driver (not shown) in guest operating system  208  to interact with a host bus adapter emulator  213  in a virtual machine monitor (VMM) component  204  of hypervisor  214 . Conceptually, this interaction provides guest operating system  208  (and the guest file system) with the perception that it is interacting with actual hardware. 
         [0019]      FIG. 2  also depicts a virtual hardware platform  210  as a conceptual layer in virtual machine  203 ( 0 ) that includes virtual devices, such as virtual host bus adapter (HBA)  212  and virtual disk  220 , which itself may be accessed by guest operating system  208  through virtual HBA  212 . In one embodiment, the perception of a virtual machine that includes such virtual devices is effectuated through the interaction of device driver components in guest operating system  208  with device emulation components (such as host bus adapter emulator  213 ) in VMM  204 ( 0 ) (and other components in hypervisor  214 ). 
         [0020]    File system calls initiated by guest operating system  208  to perform file system-related data transfer and control operations are processed and passed to virtual machine monitor (VMM) components  204  and other components of hypervisor  214  that implement the virtual system support necessary to coordinate operation with hardware platform  224 . For example, HBA emulator  213  functionally enables data transfer and control operations to be ultimately passed to the host bus adapters  226 . File system calls for performing data transfer and control operations generated, for example, by one of applications  206  are translated and passed to a virtual machine file system (VMFS) driver  216  that manages access to files (e.g., virtual disks, etc.) stored in data storage systems (such as data storage system  125 ) that may be accessed by any of the virtual machines  203 . In one embodiment, access to DSU  120  is managed by VMFS driver  216  and shared file system  115  for LUN  120  is a virtual machine file system (VMFS) that imposes an organization of the files and directories stored in DSU  120 , in a manner understood by VMFS driver  216 . For example, guest operating system  208  receives file system calls and performs corresponding command and data transfer operations against virtual disks, such as virtual SCSI devices accessible through HBA emulator  213 , that are visible to guest operating system  208 . Each such virtual disk may be maintained as a file or set of files stored on VMFS, for example, in DSU  120 . The file or set of files may be generally referred to herein as a virtual disk and, in one embodiment, complies with virtual machine disk format specifications promulgated by VMware (e.g., sometimes referred to as a vmdk files). File system calls received by guest operating system  208  are translated to instructions applicable to particular file in a virtual disk visible to guest operating system  208  (e.g., data block-level instructions for 4 KB data blocks of the virtual disk, etc.) to instructions applicable to a corresponding vmdk file in VMFS (e.g., virtual machine file system data block-level instructions for 1 MB data blocks of the virtual disk) and ultimately to instructions applicable to a DSU exposed by data storage unit  125  that stores the VMFS (e.g., SCSI data sector-level commands). Such translations are performed through a number of component layers of an “IO stack,” beginning at guest operating system  208  (which receives the file system calls from applications  206 ), through host bus emulator  213 , VMFS driver  216 , a logical volume manager  218  which assists VMFS driver  216  with mapping files stored in VMFS with the DSUs exposed by data storage systems networked through SAN  105 , a data access layer  222 , including device drivers, and host bus adapters  226  (which, e.g., issues SCSI commands to data storage system  125  to access LUN  120 ). 
         [0021]    Deltadisk driver  215 , implements snapshots for virtual disk by maintaining change delta in abstractions called delta disk chain, and providing copy-on-write semantics on these delta chains. Hypervisor  214  may take a snapshot of virtual disk  220  at a given point in time to preserve the content within virtual disk  220  at that point in time. Base disk  221  includes the content that was preserved when hypervisor  214  took the snapshot of virtual disk  220 . Each data block in base disk  221  is marked as “copy-on-write.” Deltadisk driver  215  captures subsequent writes received from the guest operating system  208  to virtual disk  220  in one or more delta disk  223  of virtual disk  220  so that the point in time snapshotted content in base disk  221  is not modified. In summary the deltadisk driver  215  implements the copy-on-write logic for delta disks, and is responsible for grouping set of delta disk and presenting it as single virtual disk in the form of a delta chain to the HBA emulator  213 . 
         [0022]    In operation, when deltadisk driver  215  receives a write operation on a data block that is marked as “copy-on-write,” deltadisk driver  215  creates a copy of the data block. Deltadisk driver  215  then performs the write operation on the copy of the data block. A collection of data blocks that were created as a result of write operations associated with data blocks in the base virtual disk is a delta disk  223  of virtual disk  220 . Each delta disk  223  is represented in file system  115  as a separate file having a corresponding file inode. 
         [0023]    Deltadisk driver  215  may subsequently be instructed to create additional virtual disk snapshots at a later time. Each snapshot creates a new delta disk  223  within which subsequent write operations are captured. It is then possible to “revert” a virtual disk to any earlier state (i.e., a state marked by an earlier timestamp) by choosing which delta disk to use. Deltadisk driver  215  serves read operations received for a data block in a virtual disk with delta disks by looking up each delta disk in order of the most recent delta disk to the oldest delta disk. Subsequently, deltadisk driver  214  retrieves the most recent copy of the data block from the delta disks. 
         [0024]    The deltadisk driver  215  is an integral part of the Hypervisor  214  which is specifically responsible for emulating snapshotting for virtual disks. 
         [0025]    It should be recognized that the various terms, layers and categorizations used to describe the virtualization components in  FIG. 2  may be referred to differently without departing from their functionality or the spirit or scope of the invention. For example, virtual machine monitors (VMM)  204  may be considered separate virtualization components between VMs  203  and hypervisor  214  (which, in such a conception, may itself be considered a virtualization “kernel” component) since there exists a separate VMM for each instantiated VM. Alternatively, each VMM may be considered to be a component of its corresponding virtual machine since such VMM includes the hardware emulation components for the virtual machine. In such an alternative conception, for example, the conceptual layer described as virtual hardware platform  210  may be merged with and into VMM  204  such that virtual host bus adapter  212  is removed from  FIG. 2  (i.e., since its functionality is effectuated by host bus adapter emulator  213 ). 
         [0026]      FIG. 3  illustrates a detailed view of base disk  221  of  FIG. 2  and a file inode  306  associated with base disk  221 . As shown, base disk  221  includes multiple data blocks  304 . As previously discussed, one example of base disk  221  is a vmdk file that is stored in, for example, LUN  120 . 
         [0027]    File inode  306  specifies the logical (file space) to physical (disk space) mapping of blocks belonging to a particular file associated with file inode  306 . Data belonging to the file associated with file inode  306  is stored in data blocks  304 , which correspond to physical units of storage, i.e., physical data blocks, managed by file system  115 . In one embodiment, the size of a particular data block  304  can range between 1 MB and 8 MB. The structure of file inode  306  is described below. Persons skilled in the art would recognize that other implementations of an inode are within the scope of this invention. 
         [0028]    File inode  306  includes inode metadata  308  and a set of block references, such as block reference  310  and block reference  312 . Inode metadata  308  stores attributes associated with the file, such as the size of the, the size and the number of data blocks  304  associated with the file, etc. Each non-empty block reference corresponds to a particular portion of the file logical space and includes the address of the particular data block  304  storing that portion of the file. For example, block reference  310  corresponds to portion A of the file and includes the address of block  304 (N−1), which stores portion “A′ of the file. Similarly, block reference  312  corresponds to portion ‘B’ of the file and includes the address of block  304 ( 1 ), which stores portion B of the file. 
         [0029]    When, for example, VMFS driver  216  performs a read or write operation (referred to herein as an “IO operation”) on a portion of a particular file (e.g., the vmdk file for base disk  221 ), VMFS driver  216  first accesses file inode  306  that is stored in LUN  120  to identify the specific data block(s)  304  that store the data belonging to that portion of the file. The identification process typically involves an address resolution operation performed via a block resolution function (not shown). VMFS driver  216  can then perform the IO operation on the data stored within the specific data block(s)  304  associated with the IO operation. 
         [0030]      FIG. 4A  illustrates delta disks generated after a snapshot of virtual disk  220  of  FIG. 220  is taken. Virtual disk  220  is associated with a delta disks  223 , i.e. a chain of delta disks, that includes delta disk  408 , delta disk  418  and delta disk  422 . Delta disk  408  includes data blocks  410 , delta disk  418  includes data blocks  420  and delta disk  422  includes data block  424 . The size of a data block  410  is the same as the size of a data block  420 . 
         [0031]    Delta disk  408  is associated with file inode  402  and corresponds to a snapshot of base disk  221  taken at a given point in time. In operation, hypervisor  214  allocates a data block  410  included in delta disk  408  in response to a write request received for performing a write operation on a corresponding data block  304  in base disk  221 . Hypervisor  214  then services the write request by performing the write operation on the newly allocated data block  410 . In such a manner, hypervisor  214  preserves the state of base disk  221  from the time a snapshot is taken. Once the hypervisor  214  performs the write operation on data block  410 , hypervisor  214  updates block reference  406  in file inode  412  to include the address of data block  410  on which the write operation was performed. 
         [0032]    Delta disk  418  is associated with file inode  412 . Delta disk  418  corresponds to changes made to the virtual disk since the point in time when base disk  221  was frozen as a snapshot. For delta disk  408 , the deltadisk driver  215  allocates a data block  402  in response to a write request received for the original block  304  in the snapshotted base disk  302 . Delta disk driver  215  then services the write request by performing the write operation on the newly allocated data block  420 . Once deltadisk driver  215  performs the write operation data block  420 , deltadisk driver  215  updates block reference  416  in file inode  412  to include the address of data block  420  on which the write operation was performed. 
         [0033]    Similarly, delta disk  422  is associated with inode  412  and corresponds to changes made to the virtual disk since the point in time when contents of delta disk  418  were frozen as a snapshot. Delta disk  422  is the currently active delta disk. 
         [0034]    As discussed above, over time, continued creation of delta disks may create a long chain of delta disk associated with base disk  221 , affecting the performance of IO operations to base disk  221 . More specifically, because the number of delta disks that need to be searched increases, the latency of a read operation also increases. Performance of such IO operations may be increased when redundant delta disks are consolidated to reduce the number of delta disk in a given chain. A technique for efficiently consolidating delta disk, while minimally impacting the performance of read operations during the consolidation, is described below in conjunction with  FIGS. 4B-5 . 
         [0035]    Delta disks  418 ,  420  and  422  are managed in the same storage container. In the described embodiment, a storage container is a file system volume. In alternate embodiments, the consolidation techniques described herein can be extended to other storage containers such as individual instances of storage arrays, volume manager etc. 
         [0036]      FIG. 4B  illustrates the consolidation of the delta disks  408  and  418  shown in  FIG. 4A . In operation, when hypervisor  214  receives a request to consolidate delta disk  408  (“source delta disk  408 ”) with delta disk  418  (“destination delta disk  418 ”), hypervisor  214  first causes file inode  402  to “share” data blocks with file inode  412 . More specifically, each block reference in file inode  402  that includes an address of a data block  410  in source delta disk  408  is updated to reflect the address of a data block  420  in destination delta disk  418  that corresponds to data block  410 . For example, as illustrated in  FIG. 4B , block reference  406  in file inode  402  is updated to include the address of data block  420 ( 2 ) in delta disk  418  instead of an address of a data block  410  in source delta disk  408 . Once all block references in file inode  402  include addresses of data blocks  420  in destination delta disk  418 , data blocks  410  in source delta disk  408  are freed and unallocated (as shown in  FIG. 4B ). 
         [0037]    In one embodiment, the request to consolidate source delta disk  408  with destination delta disk  418  specifies which data block  420  in delta disk  418  corresponds to a particular data block  410  in delta disk  408  that is to be consolidated. 
         [0038]      FIGS. 5A and 5B  set forth a flow diagram of method steps for consolidating two delta disks corresponding to different snapshots of a virtual disk, according to one embodiment. While the example operations are depicted as being performed by the systems illustrated in  FIGS. 1-2 , it is contemplated that embodiments of the invention may be performed by other suitable systems. 
         [0039]    Method  500  begins at step  502 , where hypervisor  214  receives a request to consolidate a particular delta disk (referred to herein as “source delta disk”) of a virtual disk with another delta disk (referred to herein as “destination delta disk”) of the virtual disk. At step  504 , deltadisk driver  215  updates consolidation tracking state information to indicate the start of the consolidation operation. As discussed below, deltadisk driver  215  updates the consolidation tracking information when the consolidation operation completes. Consolidation tracking state information is persisted in LUN  120 . Thus, even if a system shutdown or a crash causes the consolidation operation to be interrupted before completion, the consolidation tracking state information still indicates that the consolidation operation is still in progress. In certain embodiments, once the consolidation operation is initiated, the virtual disk cannot be reverted to the source delta disk. 
         [0040]    Deltadisk driver  215  then identifies a pair of corresponding data blocks in the source delta file and the destination delta file that need to be consolidated. In operation, at step  506 , deltadisk driver  215  sets the pointer, MERGE_BLOCK, to point to a particular data block in the source delta disk. At step  508 , hypervisor  214  sets a variable MERGE_OFFSET to the offset associated with the particular data block in the address space of the source delta disk. At step  510 , deltadisk driver  215  identifies the data block in the destination delta disk located at MERGE_OFFSET and sets the pointer, DELETE_BLOCK, to point to the identified data block in the destination delta disk. After step  510 , MERGE_BLOCK points to the particular data block in the source delta disk and DELETE_BLOCK points to the data block in the destination delta disk corresponding to the particular data block in the source delta file. In one embodiment, there is no data block in destination data block that corresponds to MERGE_BLOCK and DELETE_BLOCK is set to null. 
         [0041]    At step  512 , the deltadisk driver  215 , forwards MERGE_BLOCK and DELETE_BLOCK information to the VMFS driver  216 , to perform the block merge operation. VMFS driver  216  determines whether the data block being pointed to by MERGE_BLOCK is the same as the data block being pointed to by DELETE_BLOCK. In one embodiment, the data block being pointed to by MERGE_BLOCK is the same as the data block being pointed to be DELETE_BLOCK when the offsets associated with each of the data blocks is the same. Data blocks being pointed to by MERGE_BLOCK and DELETE_BLOCK may be the same when a crash occurs during a prior consolidation operation and VMFS driver  216  has already merged certain data blocks of source delta disk with corresponding data blocks of destination delta disk. If MERGE_BLOCK and DELETE_BLOCK are not the same, then the method proceeds to step  514  which perform the crux of the block merge operation. 
         [0042]    At step  514 , VMFS driver  216  within hypervisor  214  updates the reference at MERGE_OFFSET in the file Mode associated with the destination delta disk to point to the data block being pointed to by MERGE_BLOCK, i.e., the particular data block in the source delta disk. At step  516 , VMFS driver  216  unalloacates the data block pointed to by DELETE_BLOCK and frees the data block from the destination delta disk. In one embodiment, the unallocation and freeing of the data block occurs only when DELETE_BLOCK is not null. Steps  514  and  516  are performed atomically. 
         [0043]    At step  518 , deltadisk driver  215  determines whether all the data blocks in the source delta disk have been processed in the manner discussed above. More specifically, all the data blocks in the source delta disk have been processed when the file Mode associated with the destination delta disk points to each data block that was originally in the source delta disk. If all the data blocks in the source delta disk have not been processed, then, at step  520 , deltadisk driver  215  updates MERGE_BLOCK to point to the next data block in the source delta disk. Method  500  then returns to step  508  described above. If, however, all the data block in the source delta disk have been processed, then method  500  proceeds to step  522 . At step  522 , deltadisk driver  215  ask the VMFS driver to deletes the file mode associated with source delta disk. At step  524 , the VM  203   r  updates the delta disk chain associated with the virtual disk to indicate that the source delta disk has been deleted. 
         [0044]    Referring back to step  514 , if the MERGE_BLOCK and DELETE_BLOCK are the same, then the method proceeds to step  518 , previously described herein. 
         [0045]    One advantage of the present technique is that delta disk consolidation is performed without any data movement operations. Specifically, data movement is not involved because delta disks are consolidated by modifying inode references. Modifying inode data block pointers involves less IO operations when compared to actually moving data that is pointed by the block pointers. Therefore, a consolidation operation performed to consolidate two delta disks is quick and results in minimal IO performance degradation of the virtual disk. 
         [0046]    Although the inventive concepts disclosed herein are described with reference to specific implementations, many other variations are possible. For example, although the embodiments described herein refer to data block sizes of 4 KB, 1 MB and 512 KB, it should be recognized that alternative embodiments may utilize any various data block sizes consistent with the teachings herein. Further, although embodiments of processes and methods herein are described in terms of certain steps, it should be recognized that such described steps do not connote any particular ordering of such steps and that alternative embodiments may implement such steps in differing orders. Similarly, the inventive techniques and systems described herein may be used in both a hosted and a non-hosted virtualized computer system, regardless of the degree of virtualization, and in which the virtual machine(s) have any number of physical and/or logical virtualized processors. In addition, the invention may also be implemented directly in a computer&#39;s primary operating system, both where the operating system is designed to support virtual machines and where it is not. Moreover, the invention may even be implemented wholly or partially in hardware, for example in processor architectures intended to provide hardware support for virtual machines. Further, the inventive system may be implemented with the substitution of different data structures and data types, and resource reservation technologies other than the SCSI protocol. Also, numerous programming techniques utilizing various data structures and memory configurations may be utilized to achieve the results of the inventive system described herein. For example, tables, record structures, objects and other data structures may all be implemented in different configurations, redundant, distributed, etc., while still achieving the same results. Further, the invention can be implemented in ANY context that involves consolidating snapshots of a data-set, such contexts are not limited to virtual disk snapshots. 
         [0047]    The inventive features described herein may be applied in non-virtualized embodiments having applications running on top of an operating system and a filter driver implemented on top of a native file system driver of the operating system. The filter driver in such embodiments may be implemented in software or hardware and is configured to expose and manage thinly-provisioned files in a similar manner as the virtual disk in the virtualized embodiments. 
         [0048]    The various embodiments described herein may employ various computer-implemented operations involving data stored in computer systems. For example, these operations may require physical manipulation of physical quantities—usually, though not necessarily, these quantities may take the form of electrical or magnetic signals, where they or representations of them are capable of being stored, transferred, combined, compared, or otherwise manipulated. Further, such manipulations are often referred to in terms, such as producing, identifying, determining, or comparing. Any operations described herein that form part of one or more embodiments of the invention may be useful machine operations. In addition, one or more embodiments of the invention also relate to a device or an apparatus for performing these operations. The apparatus may be specially constructed for specific required purposes, or it may be a general purpose computer selectively activated or configured by a computer program stored in the computer. In particular, various general purpose machines may be used with computer programs written in accordance with the teachings herein, or it may be more convenient to construct a more specialized apparatus to perform the required operations. 
         [0049]    The various embodiments described herein may be practiced with other computer system configurations including hand-held devices, microprocessor systems, microprocessor-based or programmable consumer electronics, minicomputers, mainframe computers, and the like. 
         [0050]    One or more embodiments of the present invention may be implemented as one or more computer programs or as one or more computer program modules embodied in one or more computer readable media. The term computer readable medium refers to any data storage device that can store data which can thereafter be input to a computer system—computer readable media may be based on any existing or subsequently developed technology for embodying computer programs in a manner that enables them to be read by a computer. Examples of a computer readable medium include a hard drive, network attached storage (NAS), read-only memory, random-access memory (e.g., a flash memory device), a CD (Compact Discs)—CD-ROM, a CD-R, or a CD-RW, a DVD (Digital Versatile Disc), a magnetic tape, and other optical and non-optical data storage devices. The computer readable medium can also be distributed over a network coupled computer system so that the computer readable code is stored and executed in a distributed fashion. 
         [0051]    Although one or more embodiments of the present invention have been described in some detail for clarity of understanding, it will be apparent that certain changes and modifications may be made within the scope of the claims. Accordingly, the described embodiments are to be considered as illustrative and not restrictive, and the scope of the claims is not to be limited to details given herein, but may be modified within the scope and equivalents of the claims. In the claims, elements and/or steps do not imply any particular order of operation, unless explicitly stated in the claims. 
         [0052]    Virtualization systems in accordance with the various embodiments, may be implemented as hosted embodiments, non-hosted embodiments or as embodiments that tend to blur distinctions between the two, are all envisioned. Furthermore, various virtualization operations may be wholly or partially implemented in hardware. For example, a hardware implementation may employ a look-up table for modification of storage access requests to secure non-disk data. 
         [0053]    Many variations, modifications, additions, and improvements are possible, regardless the degree of virtualization. The virtualization software can therefore include components of a host, console, or guest operating system that performs virtualization functions. Plural instances may be provided for components, operations or structures described herein as a single instance. Finally, boundaries between various components, operations and data stores are somewhat arbitrary, and particular operations are illustrated in the context of specific illustrative configurations. Other allocations of functionality are envisioned and may fall within the scope of the invention(s). In general, structures and functionality presented as separate components in exemplary configurations may be implemented as a combined structure or component. Similarly, structures and functionality presented as a single component may be implemented as separate components. These and other variations, modifications, additions, and improvements may fall within the scope of the appended claims(s).