Patent Publication Number: US-9430333-B2

Title: Recovery of application from snapshot

Description:
BACKGROUND 
     Computers and computing systems have affected nearly every aspect of modern living. Computers are generally involved in work, recreation, healthcare, transportation, entertainment, household management, etc. Computer functionality is typically the result of computing systems executing software code. The computer uses a volume to store the software code, and also to store data. 
     Occasionally, the volume may become corrupted, or the volume itself may be damaged. In either case, the volume is rendered unusable. In order to prepare for such a failure event, a computing system may cause periodic snapshots of the volume to be taken. If a failure event occurs, the volume may be replaced if damaged, and a recovery module may facilitate recovery of the volume by copying back from the latest valid snapshot into the volume. Thus, despite failure of the volume, the volume is returned to a prior state, allowing computer functionality to return. 
     SUMMARY 
     In accordance with at least one embodiment described herein, the targeted recovery of application-specific data corresponding to an application is described. As an example, the application might be a virtual machine, while the application-specific data might be a virtual hard drive that corresponds to the virtual machine. The recovery of the application-specific data is performed without performing recovery of the entire volume. 
     The recovery is initiated by beginning to copy the prior state of the content of an application-specific data container from a prior snapshot to the application-specific data container in an operation volume accessible by the application. However, while the content of the application-specific data container is still being copied from the snapshot to the application-specific data container, the application is still permitted to perform read and write operations on the application-specific data container. Thus, the application-specific data container appears to the application to be fully accessible even though recovery of the content of the application-specific data container is still continuing in the background. 
     This Summary is not intended to identify key features or essential features of the claimed subject matter, nor is it intended to be used as an aid in determining the scope of the claimed subject matter. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       In order to describe the manner in which the above-recited and other advantages and features can be obtained, a more particular description of various embodiments will be rendered by reference to the appended drawings. Understanding that these drawings depict only sample embodiments and are not therefore to be considered to be limiting of the scope of the invention, the embodiments will be described and explained with additional specificity and detail through the use of the accompanying drawings in which: 
         FIG. 1  abstractly illustrates a computing system in which some embodiments described herein may be employed; 
         FIG. 2  schematically illustrates an example system in which targeted recovery of an application may occur without recovering the entire system in accordance with the principles described herein; 
         FIG. 3  illustrates a flowchart of a method for recovering application specific data corresponding to an application; 
         FIG. 4  illustrates a flowchart of a method for performing a read operation while a recovery operation is still ongoing; and 
         FIG. 5  illustrates a flowchart of a method for performing a write operation while a recovery operation is still ongoing. 
     
    
    
     DETAILED DESCRIPTION 
     In accordance with embodiments described herein, the targeted recovery of application-specific data corresponding to an application without performing recovery of the entire volume is described. The recovery is initiated by beginning to copy the prior state of the content of an application-specific data container from a prior snapshot to the application-specific data container in an operation volume accessible by the application. However, while the content of the application-specific data container is still being copied from the snapshot to the application-specific data container, the application is still permitted to perform read and write operations on the application-specific data container using the principles described further below. Thus, the application-specific data container appears to the application to be fully accessible even though recovery of the content of the application-specific data container is still continuing in the background. 
     Some introductory discussion of a computing system will be described with respect to  FIG. 1 . Then, the targeted recovery will be described in the order provided above with respect to  FIGS. 2 through 5 . 
     Computing systems are now increasingly taking a wide variety of forms. Computing systems may, for example, be handheld devices, appliances, laptop computers, desktop computers, mainframes, distributed computing systems, or even devices that have not conventionally been considered a computing system. In this description and in the claims, the term “computing system” is defined broadly as including any device or system (or combination thereof) that includes at least one physical and tangible processor, and a physical and tangible memory capable of having thereon computer-executable instructions that may be executed by the processor. The memory may take any form and may depend on the nature and form of the computing system. A computing system may be distributed over a network environment and may include multiple constituent computing systems. 
     As illustrated in  FIG. 1 , in its most basic configuration, a computing system  100  includes at least one processing unit  102  and computer-readable media  104 . The computer-readable media  104  may conceptually be thought of as including physical system memory, which may be volatile, non-volatile, or some combination of the two. The computer-readable media  104  also conceptually includes non-volatile mass storage. If the computing system is distributed, the processing, memory and/or storage capability may be distributed as well. 
     As used herein, the term “executable module” or “executable component” can refer to software objects, routings, or methods that may be executed on the computing system. The different components, modules, engines, and services described herein may be implemented as objects or processes that execute on the computing system (e.g., as separate threads). Such executable modules may be managed code in the case of being executed in a managed environment in which type safety is enforced, and in which processes are allocated their own distinct memory objects. Such executable modules may also be unmanaged code in the case of executable modules being authored in native code such as C or C++. 
     In the description that follows, embodiments are described with reference to acts that are performed by one or more computing systems. If such acts are implemented in software, one or more processors of the associated computing system that performs the act direct the operation of the computing system in response to having executed computer-executable instructions. For example, such computer-executable instructions may be embodied on one or more computer-readable media that form a computer program product. An example of such an operation involves the manipulation of data. The computer-executable instructions (and the manipulated data) may be stored in the memory  104  of the computing system  100 . Computing system  100  may also contain communication channels  108  that allow the computing system  100  to communicate with other processors over, for example, network  110 . 
     Embodiments described herein may comprise or utilize a special purpose or general-purpose computer including computer hardware, such as, for example, one or more processors and system memory, as discussed in greater detail below. Embodiments described herein also include physical and other computer-readable media for carrying or storing computer-executable instructions and/or data structures. Such computer-readable media can be any available media that can be accessed by a general purpose or special purpose computer system. Computer-readable media that store computer-executable instructions are physical storage media. Computer-readable media that carry computer-executable instructions are transmission media. Thus, by way of example, and not limitation, embodiments of the invention can comprise at least two distinctly different kinds of computer-readable media: computer storage media and transmission media. 
     Computer storage media includes RAM, ROM, EEPROM, CD-ROM or other optical disk storage, magnetic disk storage or other magnetic storage devices, or any other tangible storage medium which can be used to store desired program code means in the form of computer-executable instructions or data structures and which can be accessed by a general purpose or special purpose computer. 
     A “network” is defined as one or more data links that enable the transport of electronic data between computer systems and/or modules and/or other electronic devices. When information is transferred or provided over a network or another communications connection (either hardwired, wireless, or a combination of hardwired or wireless) to a computer, the computer properly views the connection as a transmission medium. Transmissions media can include a network and/or data links which can be used to carry desired program code means in the form of computer-executable instructions or data structures and which can be accessed by a general purpose or special purpose computer. Combinations of the above should also be included within the scope of computer-readable media. 
     Further, upon reaching various computer system components, program code means in the form of computer-executable instructions or data structures can be transferred automatically from transmission media to computer storage media (or vice versa). For example, computer-executable instructions or data structures received over a network or data link can be buffered in RAM within a network interface controller (e.g., a “NIC”), and then eventually transferred to computer system RAM and/or to less volatile computer storage media at a computer system. Thus, it should be understood that computer storage media can be included in computer system components that also (or even primarily) utilize transmission media. 
     Computer-executable instructions comprise, for example, instructions and data which, when executed at a processor, cause a general purpose computer, special purpose computer, or special purpose processing device to perform a certain function or group of functions. The computer executable instructions may be, for example, binaries, intermediate format instructions such as assembly language, or even source code. Although the subject matter has been described in language specific to structural features and/or methodological acts, it is to be understood that the subject matter defined in the appended claims is not necessarily limited to the described features or acts described above. Rather, the described features and acts are disclosed as example forms of implementing the claims. 
     Those skilled in the art will appreciate that the invention may be practiced in network computing environments with many types of computer system configurations, including, personal computers, desktop computers, laptop computers, message processors, hand-held devices, multi-processor systems, microprocessor-based or programmable consumer electronics, network PCs, minicomputers, mainframe computers, mobile telephones, PDAs, pagers, routers, switches, and the like. The invention may also be practiced in distributed system environments where local and remote computer systems, which are linked (either by hardwired data links, wireless data links, or by a combination of hardwired and wireless data links) through a network, both perform tasks. In a distributed system environment, program modules may be located in both local and remote memory storage devices. 
       FIG. 2  schematically illustrates an example system  200  in which targeted recovery of an application may occur without recovering the entire system in accordance with the principles described herein. The system  200  includes an operational volume  210  accessible to multiple applications  201 , so that the applications  201  may read to and write from the operational volume  210 . For instance, the applications  201  are illustrated as including four applications  201 A,  201 B,  201 C and  201 D, although the ellipses  201 E represent that the principles described herein are not limited at all to a system that operates a particular number of applications. The system  200  may operate a single application  201 A or perhaps thousands of applications that all share the operational volume  210 . A “volume” is a single namespace that is recognized by the file system as including contiguous logical or physical addresses. 
     The operational volume  210  stores an application-specific data container  211 A, although the operational volume  210  may store voluminous amounts of data other than the application-specific data container as represented by the ellipses  211 B. The application-specific data container  211 A is associated with application  201 A as represented by the line  212 . The application-specific data container  211 A may be, for example, an application-specific file. 
     In one example, the applications  201  may each be virtual machines. In that case, the application  201 A may be a virtual machine, and the application-specific data container  211 A might be a virtual hard drive associated with the application  201 A. In that case, the system  200  may be a portion of a host computing system that hosts multiple virtual machines. The host computing system may have multiple volumes, although only the one volume  210  is illustrated in  FIG. 2 . 
     The principles described herein permit for convenient recovery of an application-specific data container without recovering the entire volume in which the application-specific data container is stored. For instance, in  FIG. 2 , the application-specific data container  211 A may be recovered without recovering the entire volume  210 . This is helpful in that sometimes the application-specific data container  211 A may become corrupted or untrusted, while the volume  210  itself remains uncorrupted as a whole. Furthermore, recovery of the entire volume might take much longer than the recovery of just a portion of the volume. 
     Significantly, the system  200  further operates such that the application  201 A may continue to operate on the application-specific data container  211 A while the recovery of the application-specific data container  211 A is occurring in the background. Thus, from the perspective of the application  201 A, the recovery happens quickly, and perhaps in an imperceptible amount of time, whilst the recovery actually occurs in the background over a much longer period of time. 
     In order to facilitate this targeted and rapid recovery, the system  200  includes a number of executable modules including a management module  221 , a snapshot module  222 , and a recovery module  223 . Each of these modules will be further described below. 
     The modules  221  through  223  may each be instantiated and/or operated by a computing system (such as computing system  100 ) in response to the processors (e.g., processor(s)  102 ) of the computing system executing one or more computer-executable instructions stored on a computing-readable storage media comprising a computer program product. Although specific acts are described as being associated with the modules  221  and  223 , such acts need not be performed by the specific modules described. 
     In order to prepare for a possible need for recovery, the snapshot module  222  takes snapshots of at least the application-specific data container  211 A but perhaps a larger segment of the volume  210  or even perhaps the entire volume  210  as a whole. These snapshots may be periodic and/or in response to particular events. In any case,  FIG. 2  illustrates an example in which the snapshot module has taken three snapshots  231 A,  231 B and  231 C of the application-specific data container  211 A and provided the snapshots  231 A,  231 B and  231 C in a snapshot volume  230  as represented by multi-headed arrow  241 . In this example, the snapshot  231 A is the oldest snapshot, the snapshot  231 B is the second snapshot taken, and the snapshot  231 C is the most recent snapshot of the application-specific data container  211 A. 
     The management module  221  determines if and when the application-specific data container is to be subject to recovery from a snapshot within the snapshot volume  230 . In response, the recovery module  223  performs the recovery of the application-specific data container  211 A from the snapshot  231 C in the snapshot volume  230  to repopulate the content of the application-specific data container  221 A as represented by the arrow  242 . While the application-specific data container  211 A might momentarily not be available to the application  201 A while the recovery is initiated, immediately after the recovery is initiated, the management module  221  may permit the application  201 A to perform read and write operations on the application-specific data container  211 A. 
       FIG. 3  illustrates a flowchart of a method  300  for recovering application specific data corresponding to an application. As the method  300  may be performed within the system  200  of  FIG. 2 , the method  300  will now be described with respect to  FIG. 2 . 
     The method  300  is initiated upon determining that a particular application-specific data container of an application is to be subject to recovery from a snapshot (act  301 ). For instance, referring to  FIG. 2 , the management module  221  determines that the application-specific container  211 A of the application  201 A is to be subject to recovery from snapshot  231 C. 
     In response, the method temporarily makes the application-specific data container unavailable to the application by, for example, closing the application-specific data container (act  302 ). For instance, in  FIG. 2 , the management component  221  may close the application-specific data container  211 A. The application-specific data container  211 A will remain closed for but a brief time compared to the total length of the recovery operation. Furthermore, the application-specific data container  211 A is closed without closing the operational volume  210 , thus keeping the operational volume  210  available to at least one other application (e.g., application  201 B) while the application-specific data container  211 A is closed. 
     The recovery operation is then initiated (act  303 ). For instance, in the case of  FIG. 2 , the recovery module  223  initiates recovery by beginning to copy a prior state of content of the application-specific data container  201 A from a prior snapshot  231 C as represented by arrow  242 . The recovery module  223  will proceed to copy from the prior snapshot  231 C one region of the application-specific data container  201 A at a time. Prior to recovering a region, the recovery module  223  will take a range lock for that region. 
     Once the recovery operation is initiated, the application-specific data container is opened (act  305 ) thereby permitting the application to perform read and write operations on the application-specific data container, so long as there are no range locks held by the recovery module on the region that contains the memory location being read from or written to. The application-specific data container is opened even though the recovery operation continues (act  304 ), and in fact, may have only begun moments before. Thus, referring to  FIG. 2 , the recovery module  223  continues the recovery in the background (as represented by arrow  242 ), while the management module  221  continues to allow the application  201 A to perform read and write operations on the application-specific data container  211 A. 
     While the recovery is still in process (act  304 ), the management module  221  operates to respond to read and write requests from the application  201 A in a manner that the read and write requests are honored consistent with the recovery as described herein. 
     For instance, upon detecting that the application  201 A issues a read request for an address region of content from the application-specific data container (“Yes” in decision block  311 ), the management module  221  causes the recovery to momentarily pause for that address region (act  312 ) such that if the recovery approaches close to that address region, or even reaches that address region, the recovery pauses. The read operation is performed (act  313 ), and thereafter the recovery for that address region is resumed such that if the recovery were to approach or arrive at the region, the recovery would not pause (act  314 ). As an example, the read operation may be performed in the manner illustrated and described with respect to  FIG. 4 . The acts  312  through  314  might be performed each time the application  201 A issues a read request to the application-specific data container  211 A. 
     Upon detecting that the application  201 A issues a write request to an address region of the application-specific data container (“Yes” in decision block  321 ), the management module  221  again causes the recovery to momentarily pause for that address region (act  322 ) such that if the recovery approaches close to that address region, or even reaches that address region, the recovery pauses. The write operation is performed (act  323 ), and thereafter the recovery is resumed such that if the recovery were to approach or arrive at the region, the recovery would not pause (act  324 ). As an example, the write operation may be performed in the manner illustrated and described with respect to  FIG. 5 . The acts  312  through  314  might be performed each time the application  201 A issues a read request to the application-specific data container  211 A. 
       FIG. 4  illustrates a method  400  for responding to a read request while the recovery operation is still in process. The method  400  may be performed in order to perform the read operation of act  313  in  FIG. 3 . Thus, the method  400  may be initiated after act  312  in  FIG. 3 . 
     The method  400  determines whether the address region corresponding to the read request is already restored to the application-specific data container in the operational volume (decision block  401 ). This may be accomplished by the recovery module  223  consulting a recovery progress status  251 . For instance, if the recovery module  223  copies the content from the snapshot  231 C sequentially, the recovery progress status  251  might represent a high water mark representing the highest address recovered into the application-specific data container. In addition, this may be accomplished by the recovery module  223  consulting an already written indication  252  that the address region of the application-specific data container has been written to since the beginning of the recovery. For instance, the address region might have been written to in response to the performance of method  500  or in response to act  406  in  FIG. 4  described further below, even though the address region has not yet been reached yet through the normal recovery performed by act  304 . 
     If the address region for the read request is already restored to the application-specific data container (“Yes” in decision block  401 ), the content of the address region is read from the application-specific data container in the operational volume (act  402 ) and is thus made available to the application  201 A in response to the read request (act  403 ). If the address region for the read request is not already restored to the application-specific data container (“No” in decision block  401 ), the content is read instead from the corresponding portion of the snapshot (act  404 ), and the content is provided to the application (act  403 ) whilst the read content is also optionally provided to the application-specific data container (act  405 ). Furthermore, the already written indication is optionally updated (act  406 ) to reflect that a write has occurred to the address region of the application-specific data container since the recovery began. Once the read operation completes, the method  400  returns to act  314  of  FIG. 3 , where the recovery operation resumes. The method  400  may be repeated numerous times for each read request received during the recovery process. Once the recovery process completes (act  306 ), read operations may continue as normal, without intervention from the management module  221 . 
       FIG. 5  illustrates a method  500  for responding to a write request while the recovery operation is still in process. The method  500  may be performed in order to perform the write operation of act  323  in  FIG. 3 . Thus, the method  500  may be initiated after act  322  in  FIG. 3 . 
     The method  500  performs the write operation by writing the associated content of the write request is written to the address region of the application-specific data container in the operation volume (act  501 ). The method  500  also determines whether the address region corresponding to the write request is already restored to the application-specific data container in the operational volume (decision block  502 ). This again may be accomplished by the recovery module  223  consulting a recovery progress status  251 . In addition, this may be accomplished by the recovery module  223  consulting an already-written indication  252  that the address region of the application-specific data container has been written to since the beginning of the recovery. For instance, the address region might have been written to in response to a previous performance of method  500  or in response to act  406  in  FIG. 4 , even though the address region has not yet been reached yet through the normal recovery performed by act  304 . 
     If the address region for the write request is already restored to the application-specific data container (“Yes” in decision block  502 ), this completes the write operation, thereby returning to act  324  in  FIG. 3  to resume the restore operation. If the address region for the write request has not already been restored to the application-specific data container (“No” in decision block  502 ), the already written indicator  252  is updated to reflect that the address region has been written to since the recovery operation began (act  503 ). Thereafter, again thereby returning to act  324  in  FIG. 3  to resume the restore operation. The method  500  may be repeated numerous times for each write request received during the recovery process. Once the recovery process completes (act  306 ), write operations may continue as normal, without intervention from the management module  221 . 
     From the perspective of the recovery module, the recovery module  223  determines that a particular application-specific data container of an application is to be subject to recovery from a snapshot. For instance, the recovery module  223  may have received an instruction to recover from the management module  221  in response to the management module  221  performing act  301  in  FIG. 3 . In response, the recovery module initiates recovery. 
     The recovery module continues the recovery by copying content from the snapshot to the application-specific data container one portion at a time, which is analogous to act  304  in  FIG. 3 . The recovery module does respond to instructions to pause and resume recovery in response to acts  312 ,  314 ,  322  and  324 , but otherwise simply moves forward with recovery. Despite such momentary pauses, the recovery operation will still be said to be continuing as shown in act  304 . 
     The recovery module also tracks at least one address region corresponding to a write request that is made on the application-specific container in the operational volume apart from the recovery. For instance, this may be accomplished using the already-written indicator  252  in  FIG. 2 . When copying one portion at a time from the snapshot, the recovery module skips copying for any addresses that have already been written as indicated within the indicator  252 . This prevents the recovery from overwriting newer data created by allowing the application  201 A to continue processing during the recovery. 
     An example implementation of how collision avoidance may be facilitated will now be described, first from the perspective of the read/write operation, and then from the perspective of the recovery. 
     When the application issues a read or write request, the system takes a range lock for the region addressed by the read or write request. If the recovery is working on the same range, then this read/write operation will be pended to be resumed later. Second, a high water mark of the recovery (an example of the recovery progress status  251 ) is evaluated, along with a dirty map (an example of the already-written indicator  252 ) is evaluated, to determine the appropriate action as described above. For instance, for a read operation, it is determined whether to read from the data container (act  402 ), or whether to read from the snapshot (act  403 ). For a write operation, it is determined whether the dirty map needs to be updated to reflect a write (act  503 ). The appropriate I/O operation is then performed, and then the range lock is released for the region. If a recovery got pended for this region, then the recovery is resumed for the region. 
     The collision avoidance from the recovery perspective occurs one range of addresses at a time. For each region to be recovered, the recovery process takes a range lock for the region to be recovered. If a read/write operation already has a range lock on that region, the recovery on the region is pended to be resumed later on read/write completion. Once the recovery is able to obtain the range lock on that region, recovery is performed based on the dirty map. For instance, recovery will skip copying from the snapshot for all of those addresses that are marked as already having been written to in the dirty map. Once the region is recovered, the recovery process release the range lock. This allows any pended read/write request to complete on that range. 
     Thus, the principles described herein provide an effective and efficient mechanism for targeted recovery of an application-specific data container without requiring recovery of the entire volume, and also while allowing the application to continue operating on the application-specific data container that is in the process of being recovered. 
     The present invention may be embodied in other specific forms without departing from its spirit or essential characteristics. The described embodiments are to be considered in all respects only as illustrative and not restrictive. The scope of the invention is, therefore, indicated by the appended claims rather than by the foregoing description. All changes which come within the meaning and range of equivalency of the claims are to be embraced within their scope.