Abstract:
Real system devices (RSD&#39;s) emulate virtual system devices (VSD&#39;s); system elements can use RSD&#39;s and manipulate VSD&#39;s, while users can only use VSD&#39;s. Identifying when RSD&#39;s have erroneous data. Providing a mapping of RSD&#39;s to VSD&#39;s, to recover a maximum set of VSD&#39;s. Providing a mapping of VSD&#39;s to RSD&#39;s, to recover a specified most valuable VSD. Reconstructing VSD&#39;s from RSD&#39;s having erroneous data, and coordinating with a user&#39;s assistance to couple RSD&#39;s in an order allowing data recovery. Metadata is maintained coupling data identifiable on VSD&#39;s with corresponding data on RSD&#39;s; it is possible to trace connections between RSD&#39;s and VSD&#39;s even without complete data.

Description:
INCORPORATED DISCLOSURES 
     This Application describes technologies that can be used with inventions, and other technologies, described in one or more of the following Applications, each assigned to the same assignee: 
     Application 61/909,118, filed Nov. 26, 2013, in the name of Scott Loughmiller and Philip White, titled “Reduplication of de-duplicated files in a fault-tolerant distributed cluster”. 
     Application 61/909,301, filed Nov. 26, 2013, in the name of Aaron Pagel and Nate Hardt, titled “Resource management in a virtual machine cluster”. 
     Application 61/909,336, filed Nov. 26, 2013, in the name of Aaron Pagel and Nate Hardt, titled “Resource management in a virtual machine cluster”. 
     Application 61/909,342, filed Nov. 26, 2013, in the name of Aaron Pagel and Nate Hardt, titled “Resource management in a virtual machine cluster”. 
     Application 61/909,344, filed Nov. 26, 2013, in the name of Nate Hardt, Scott Loughmiller, and Philip White, titled “Unanticipated operational requirements in a reliable distributed computing system”. 
     Application 61/909,365, filed Nov. 26, 2013, in the name of Nate Hardt, Scott Loughmiller, and Philip White, titled “Recovery after data loss in a reliable distributed computing system”. 
     Application 61/909,352, filed Nov. 26, 2013, in the name of Nate Hardt, Scott Loughmiller, and Philip White, titled “Metadata caches in a reliable distributed computing system”. 
     Application 61/909,356, filed Nov. 26, 2013, in the name of Nate Hardt, Scott Loughmiller, and Philip White, titled “Remote access latency in a reliable distributed computing system”. 
     Application 62/080,773, filed Nov. 17, 2014, in the name of Clint McVey and Scott Loughmiller, titled “Zero memory buffer copying in a reliable distributed computing system”. 
     Application 62/080,791, filed Nov. 17, 2014, in the name of Nate Hardt, Scott Loughmiller, and Philip White, titled “Virtual devices in a reliable distributed computing system”. 
     Application No. 62/123,740, filed Nov. 24, 2014, in the name of Scott Loughmiller, Philip White, Aaron Pagel, Nate Hardt, and Clint McVey, titled “Reliable Independent Block Engine and State Machine”. 
     Each and every one of these documents, as well as all documents cited therein, are hereby incorporated by reference as if fully recited herein. 
     This Application claims priority of the Incorporated Documents, and to each and every document cited therein, to the fullest extent possible. 
     No aspect of incorporating these documents by reference is intended to be limiting in any way. The scope and spirit of the invention remains as broad as possible, given the specification and original claims of this Application. These Incorporated Documents are additive in nature, and can be combined with the technical matter described herein, in whole or in part. 
     RELATED APPLICATIONS 
     This Application describes technologies that can be used with inventions, and other technologies, described in one or more of the following Applications, each assigned to the same assignee: 
     U.S. Provisional Application 61/651,391, filed May 24, 2012, in the name of inventors Jason Collier, Scott Loughmiller, and Philip White, titled “Unified hypervisor and distributed fault-tolerant storage”. 
     U.S. application Ser. No. 13/901,325, filed May 23, 2013, in the name of inventors Jason Collier, Scott Loughmiller, and Philip White, titled “Unified hypervisor and distributed fault-tolerant storage”. 
     U.S. application Ser. No. 14/229,748, filed Mar. 28, 2014, in the name of inventors Philip White, and Hank Hsien, titled “Placement engine for a block device”. 
     U.S. application Ser. No. 14/318,414, filed Jun. 27, 2014, in the name of inventors Philip White, titled “Shared reference counters among a plurality of virtual storage devices”. 
     U.S. application Ser. No. 14/320,427, filed Jun. 30, 2014, in the name of inventors Philip White, titled “Efficient migration of virtual storage devices to a remote node using snapshots”. 
     Each and every one of these documents, as well as all documents cited therein, are hereby incorporated by reference as if fully recited herein. 
     This Application claims priority of the Incorporated Documents, and to each and every document cited therein, to the fullest extent possible. 
     No aspect of incorporating these documents by reference is intended to be limiting in any way. The scope and spirit of the invention remains as broad as possible, given the specification and original claims of this Application. These Incorporated Documents are additive in nature, and can be combined with the technical matter described herein, in whole or in part. 
     BACKGROUND 
     Field of the Disclosure 
     This application generally relates to data distribution, data loss, recovery, reliable computing systems, distributed computing systems, and other matters. 
     Background of the Disclosure 
     In storage systems, it can sometimes occur that storage devices can lose data, or metadata, with the effect that data important to the user might be lost. In a storage system used with virtual machines (VM&#39;s), it can be advantageous to separate virtual storage devices (VSD&#39;s) from the devices that emulate them, real storage devices (RSD&#39;s). In such cases, users and their guest operating systems (guest OS&#39;s) perceive storage as unitary data blocks maintained reliably on the VSD&#39;s, while host operating systems (host OS&#39;s) and other system elements perceive both the VSD&#39;s, and also the RSD&#39;s used to emulate those VSD&#39;s. If an RSD loses all or part of a data block, and that discovery is not made until later, it can sometimes occur that reconstructing any particular VSD from RSD&#39;s known to have good data can be infeasible without assistance. 
     Each of these issues, as well as other possible considerations, might cause difficulty in aspects of operating computing systems, particularly in those computing systems in which virtual machines are implemented, or in computing systems making use of distributed resources. 
     BRIEF SUMMARY OF THE DISCLOSURE 
     This application provides techniques by which one or more of: (1) a user can reconstruct as many VSD&#39;s as possible from RSD&#39;s with known good data blocks, or (2) a user can designate a particular VSD and recover as much of that VSD as possible. Metadata is maintained coupling data identifiable on VSD&#39;s with corresponding data on RSD&#39;s, with the effect that it is possible to trace connections back and forth between VSD&#39;s and RSD&#39;s even without complete data. The user, working with assistance, can determine what data is most valuable to recover, and which RSD&#39;s need to be replaced, and in what order, to recover that data. 
     In one embodiment, the system can identify, with respect to those RSD&#39;s with known good data, a relative maximum of data recoverable on VSD&#39;s, and in what order those RSD&#39;s should be coupled to the system to recover that data. This has the effect that the system identifies, from RSD&#39;s with known good data, which VSD&#39;s are recoverable. The system identifies one or more VSD&#39;s whose data is recoverable, identifies the RSD&#39;s that maintain the metadata and data blocks for those VSD&#39;s, and in what order those RSD&#39;s should be coupled to the system to conduct recovery. At those times when data is needed from RSD&#39;s that are not the currently coupled to the system, the system identifies those RSD&#39;s to the user, identifies in which disk slot they are relatively better positioned, and confirms when the user couples those RSD&#39;s to the system. This has the effect that the system and the user can work together to recover a relative maximum of available data. 
     In one embodiment, the user can obtain assistance with respect to one or more particular VSD&#39;s selected by the user as being of relatively higher priority. This has the effect that the system identifies, from VSD&#39;s desired by the user, which RSD&#39;s are needed to recover a relative maximum with respect to those VSD&#39;s. As in one embodiment described above, the system identifies the RSD&#39;s that maintain the metadata and data blocks for those VSD&#39;s, in what order those RSD&#39;s should be coupled to the system to conduct recovery, advises the user at those times when data is needed from not-then-coupled RSD&#39;s, and confirms when the user couples those RSD&#39;s to the system. This has the effect that the user and the system can work together to recover a relative maximum of data that the user most desires. 
     After reading this application, those skilled in the art would recognize that techniques shown in this application are applicable to more than just the specific embodiments shown herein. For a first example, the concepts of RSD&#39;s and VSD&#39;s are intended to be broad, and can include any system in which a set of generally user-accessable data is implemented using a set of generally user-inaccessable data. This is independent of whether such a system involves multiple cooperating devices, or a portion of one or more devices operating to embody RSD&#39;s and having other functions besides, or otherwise. For a second example, the concepts of reconstruction and recovery are intended to be broad, and can include any system in which spread across a medium with at least some redundancy, including one or more of: backup or multiple copies, error-correcting codes, spread-spectrum encodings, or otherwise. 
     While multiple embodiments are disclosed, including variations thereof, still other embodiments of the present application will become apparent to those skilled in the art from the following detailed description, which shows and describes illustrative embodiments of the application. The application is capable of modifications in various aspects, all without departing from its scope or spirit. The drawings and detailed description are illustrative in nature and not restrictive. 
    
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
         FIG. 1  shows a conceptual drawing of a system. 
         FIG. 2  shows a conceptual drawing of a method. 
     
    
    
     DETAILED DESCRIPTION 
     Terms and Phrases 
     The following definitions are intended to be exemplary and illustrative, not necessarily limiting: 
     The phrase “real storage device,” the term “RSD,” and variants thereof, generally refer to any physical device with which the computer system can communicate. 
     The phrase “virtual storage device,” the term “VSD,” and variants thereof, generally refer to any virtual device whose capabilities are emulated by a host operating system or virtual machine. 
     The term “metadata,” and variants thereof, generally refers to any additional information associated or involved with data use in either RSD&#39;s or VSD&#39;s. 
     The terms “reconstruct,” “recover,” and variants thereof, generally refer to any technique associated or involved with obtaining useful data in response to data known to include errors. 
     The term “user,” and variants thereof, generally refers to any entity external to those systems described herein that maintain data, and can include one or more persons acting individually or collectively, or any other entity capable of carrying out the activities ascribed to or associated with the user described herein. 
     Figures and Text 
       FIG. 1  shows a conceptual drawing of a system. 
       FIG. 2  shows a conceptual drawing of a method. 
     System Overview 
       FIG. 1  shows a conceptual drawing of a system. 
     A system  100  includes elements described herein, other elements shown in the figure, and possibly other elements. In one embodiment, the system  100  includes two or more redundant nodes; however, the figure only shows one of those nodes. This one of multiple nodes is also in communication with those other nodes, with the effect of providing a reliable distributed computer system. As the multiple nodes frequently communicate the content of their memory and storage devices, the system  100  can be considered as if it were a distributed computer system with multiple real storage devices (RSD&#39;s) and a shared memory, each accessible to all processors of each individual redundant node. Accordingly, the figure should be considered to represent a distributed computer system with a shared memory and a set of shared RSD&#39;s. 
     Kernel Elements 
     The system  100  can include a kernel  110 , that can include a set of kernel elements, or in addition to or in lieu thereof, a set of elements operating at a kernel level of authorization. The system  100  can also include a userspace  120 , that can include a set of user elements, or in addition to or in lieu thereof, a set of elements operating at a user level of authorization (other than the kernel level of authorization). 
     The kernel  110  can be managed by a Linux kernel, and can include one or more real storage devices (RSD&#39;s)  111 . While this application primarily describes RSD&#39;s  111  as storage devices, it is alternatively possible that they are other types of devices. For a first example, RSD&#39;s  111  could be network interface cards (NIC&#39;s), or any other suitable device, coupled to the kernel  110  using an iSCSI interface. For a second example, RSD&#39;s  111  could be another other device capable of operating as described herein. 
     As described herein, in one embodiment, RSD&#39;s  111  can include disk drives or other mass storage devices, such as SSD drives or otherwise, and can each be associated with a device queue  112 . The device queue  112  can include instructions from a user in the userspace  120  (authorized by user access to the kernel, and sanitized by the kernel), and can include data to be written, locations of where to place data to be read, read/write instructions, responses to read/write instructions, and possibly error messages. As further described herein, the RSD&#39;s  111  and the device queues  112  can be managed by an I/O Manager (as further described herein) that provides user access to the kernel through a kernel access port  113 . 
     The system  100  also includes at least some shared memory  110  (optionally, each individual node can include some non-shared memory). The shared memory  110  is shown in the figure as being on the border between kernel-accessable memory  120  and user-accessable memory  130 , as the shared memory  110  can be accessed by either user resources or kernel resources. The distinction between user resources and kernel resources is further described herein. 
     Virtual Machines 
     The system  100  also can include one or more virtual machines (VM)  160 , each including a guest operating system (guest OS)  161  and a host operating system (host OS), the latter including a set of drivers and indicated as Qemu  162 . Each of the one or more guest OS&#39;s  161  can call on one or more of the drivers associated with its own copy of Qemu  162 . Its Qemu  162  can include device drivers for virtual storage devices (VSD&#39;s)  170 , each of which emulates a real-world storage device, although not necessarily a real-world storage device that is actually manufactured and sold. 
     While this application primarily describes VSD&#39;s  170  as storage devices, it is alternatively possible that they are other types of virtual devices. For a first example, VSD&#39;s  170  could be network interface cards (NIC&#39;s), or any other suitable device, coupled to its Qemu  162  using an iSCSI interface. For a second example, VSD&#39;s  170  could be another other device capable of operating as described herein. 
     Upon receiving an instruction to read/write, from/to, one or more VSD&#39;s  170 , its Qemu  162  sends that instruction to a block driver element in the hypervisor that is managing its Qemu  162 . That block driver element sends the instruction to a libscribe element  171 , that performs the work of reading/writing data from/to the one or more RSD&#39;s  111  associated with those one or more VSD&#39;s  170 . The libscribe element  171  is shown coupled to the RSD&#39;s  111  and their device queues  112 , but in reality, the libscribe element  171  operates using a scribed daemon  180 , and in particular, a libscribe server  181  within that scribed daemon  180 . 
     “Scribed” Server 
     Accordingly, the libscribe element  171  sends the read/write instruction to the scribed daemon  180 , an in particular, to the libscribe server  181  that can be included in the scribed daemon  180 . (The scribed daemon  180  can operate using a scaled daemon  190 , and in particular, a state machine  191  in the scaled daemon  190 .) The libscribe server  181  sends the read/write instructions to an I/O Manager  182 , which can be granted authorization to access selected portions of the kernel element  110 , such as using a port  183 . 
     The I/O Manager  182 , using the port  183 , can call upon the kernel element  110  to send the read/write instructions to the RSD&#39;s  111  and their device queues  112 , where they can be executed and responded to. The I/O Manager  182 , using the port  183 , can obtain responses to the read/write instructions from the RSD&#39;s  111  and their device queues  112 . The I/O Manager  182  can return those responses to the libscribe server  181 , which can return them to the libscribe element  171 , which can return them to the associated Qemu  162 , which can return them to the associated guest OS  161 . 
     This has the effect that the guest OS  161 , which attempts to issue read/write instructions to a VSD  170 , instead has those read/write instructions rewritten and redirected to one or more RSD&#39;s  111 , upon which the data and metadata for that VSD  170  are stored. 
     Method of Operation 
       FIG. 2  shows a conceptual drawing of a method. 
     A method  200  includes flow points and method steps as described herein. These flow points and method steps are, by the nature of the written word, described in a particular order. This description does not limit the method to this particular order. They might be performed in a different order, or concurrently, or partially concurrently, or otherwise in a parallel, pipelined, quasi-parallel, or other manner. They might be performed in part, paused, and returned to for completion. They might be performed as co-routines or otherwise. In the context of the invention, there is no particular reason for any such limitation. 
     One or more portions of the method  200  are sometimes described as being performed by particular elements of the system  100  described with respect to  FIG. 1 , or sometimes by “the method” itself. When a flow point or method step is described as being performed by “the method,” it can be performed by one or more of those elements, by one or more portions of those elements, by an element not described with respect to the figure, by a combination or conjunction thereof, or otherwise. 
     Beginning of Method 
     A flow point  200 A indicates a beginning of the method. A user has encountered an error with an RSD  111  or a VSD  170 , and wishes to recover important data after the error. 
     Identifying Metadata 
     At a step  211 , the system  100  attempts to identify metadata associating RSD&#39;s  111  with each VSD  170 . As part of this step, once the system  100  identifies metadata associating RSD&#39;s  111  with each VSD  170 , the system  100  identifies which data blocks from each VSD  170  can be found at which RSD  111 . 
     After reading this application, those skilled in the art will see that each VSD  170  is associated with at least two RSD&#39;s  111 , as each data block on each VSD  170  is maintained in two separate places, with the effect of preventing any RSD  111  from being a single point of failure. After reading this application, those skilled in the art will also see that each RSD  111  is very likely to be associated with more than one VSD  170 , as otherwise that RSD  111  would be maintaining data for a single VSD  170 ; this both might be risky and might overly concentrate disk operations for that particular VSD  170  on that particular RSD  111 . 
     At a step  212 , the system  100  attempts to identify metadata associating VSD&#39;s  170  with each RSD  111 . As part of this step, once the system  100  identifies metadata associating VSD&#39;s  170  with each RSD  111 , the system  100  identifies which data blocks at each RSD  111  are supporting which VSD&#39;s  170 . 
     At a step  213 , having identified which blocks at which RSD&#39;s  111  are associated with which VSD&#39;s  170 , and which blocks at which VSD&#39;s are associated with which RSD&#39;s  111 , the system  100  determines a direct connection graph between RSD&#39;s  111  and VSD&#39;s  170 . A direct connection graph can include a set of directed links between points (in this case, always between an RSD  111  and a VSD  170 , or the reverse), and can be represented as an RSD×VSD matrix, with a logical “1” for a connection and a logical “0” for a lack of a connection. 
     At a step  214 , the system  100  interacts with a user, such as by means of a user interface, to request whether the user would like to give priority to any one or more particular VSD&#39;s  170 . As part of this step, if the user does wish to give priority to any one or more VSD&#39;s  170 , the system  100  requests in what order the user desires to give priority. In one embodiment, the user&#39;s priority ordering of VSD&#39;s  170  can take the form of grouping VSD&#39;s  170  into equivalence classes of priority: Thus, class (1)&gt;class (2)&gt; . . . class (N), where the user desires to give priority to those VSD&#39;s  170  in class (1), followed by those VSD&#39;s  170  in class (2), and so on. 
     At a step  215 , the system  100  determines what blocks, and in what order, it will need to recover the VSD&#39;s  170  in class (1), followed by class (2), . . . , followed by class (N). It is possible that some of those blocks are corrupted, or that data has otherwise been lost, so the system  100  might have to look for mirror copies of some of those data blocks, and the system  100  might have to accept that some portions of some VSD&#39;s  170  might not be recoverable. It is also possible that some of those blocks are identified as being on RSD&#39;s  111  that are not coupled to the system  100 , that is, those disks have been removed from their disk slots and are either filed somewhere or in use for some other purpose, and the system  100  might have to ask the user to recouple/replace those RSD&#39;s  111 . 
     At a step  216 , the system  100  interacts with the user, such as by means of the user interface, to present the one or more possible routes toward recovering the user&#39;s preferred VSD&#39;s  170 , how much of those VSD&#39;s  170  can be recovered, and how much of each one or more VSD&#39;s  170  are affected by each particular RSD  111 . 
     In a first example, it is possible that the user will know which RSD  111  has lost data, because (say) the user is physically present and holding a smoking burnt-out RSD  111 , or because (say) the user has been able to rescue only a limited number of RSD&#39;s  111  from a catastrophic event, such as a flood. In such cases, the user might wish to know what VSD&#39;s  170 , or portions thereof, have been lost, or can be recovered, given the knowledge the user has about which RSD&#39;s  111  are available. For example, the user might be a field engineer sent to recover as much data as possible after a major data loss. 
     In a second example, it is possible that the user will know which VSD&#39;s  170  are especially desired, because (say) the user has stored important information on those VSD&#39;s  170 . In such cases, the user might wish to know what RSD&#39;s  111 , or portions thereof, need to be found and recoupled/replaced in the system  100  to obtain that important information. For example, the user might be a field engineer sent to recover at least one item of important information, on a known particular VSD  170 , after a major data loss. 
     At a flow point  220 , the user has indicated to the system  100  which of the one or more possible routes toward data recovery the user prefers. 
     At a step  221 , the system  100  begins recovering data according to the plan approved by the user. As part of this step, the system  100  rebuilds the tree-structure for the file system, re-creating and re-inserting metadata at appropriate data blocks, wherein the metadata identifies appropriate data blocks to recover as much of the data desired by the user, in the order of preference expressed by the user. When the system  100  needs to access an RSD  111 , or a portion thereof, that is not then coupled to the system  100 , the system  100  interacts with the user, by means of the user interface, to request that RSD  111 , to indicate which disk slot into which the user should insert it, and to confirm that the user has selected the correct RSD  111 . 
     As part of rebuilding the file system, the system  100  might have to write data blocks out to one or more RSD&#39;s  111 . In so doing, the system  100  selects those data blocks used to emulate those VSD&#39;s  170  that the user has indicated are of least importance or no importance (for example, they could have been empty before starting the process). 
     At periodic intervals, or when requested by the user, or when completing a priority class of data important to the user, the system  100  can return to the steps  212 ,  213 , or  214 , to re-evaluate how best to accomplish the priorities it obtained by interaction with the user. Otherwise, the system  100  continues with the flow point  200 B. 
     End of Method 
     At a flow point  200 B, the method  200  is completed, and halts. 
     Alternative Embodiments 
     While some embodiments are generally described herein with respect to data recovery when real devices emulate virtual devices, and user programs are prevented access to those real devices, in the context of the invention, there is no particular requirement for any such limitation. For example, individual elements or method steps of the described embodiments could be replaced with substitutes that perform similar functions in other contexts. Examples of such contexts can include any system in which information is transferred between processes (or threads) with differing security authorization. 
     While some embodiments are generally described herein with respect to recovery of data storage, in the context of the invention, there is no particular requirement for any such limitation. For example, individual elements or method steps of the described embodiments could be replaced with substitutes that perform similar functions in other contexts. Examples of such contexts can include any system involving data transfers, such as a system that transfers a collection of multiple data packets. 
     Elements of the system are described herein with respect to one or more possible embodiments, and are not intended to be limiting in any way. In the context of the invention, there is the particular requirement for any such limitations as described with respect to any elements of the system. For example, individual elements of the described apparatuses could be replaced with substitutes that perform similar functions. Moreover, as described herein, many individual elements of the described apparatuses are optional, and are not required for operation. 
     Although control elements of the one or more described apparatuses are described herein as being executed as if on a single computing device, in the context of the invention, there is no particular requirement for any such limitation. For example, the control elements of the one or more described apparatuses can include more than one computing device, not necessarily all similar, on which the element&#39;s functions are performed. 
     Certain aspects of the embodiments described in the present disclosure may be provided as a computer program product, or software, that may include, for example, a computer-readable storage medium or a non-transitory machine-readable medium having stored thereon instructions, which may be used to program a computer system (or other electronic devices) to perform a process according to the present disclosure. A non-transitory machine-readable medium includes any mechanism for storing information in a form (e.g., software, processing application) readable by a machine (e.g., a computer). The non-transitory machine-readable medium may take the form of, but is not limited to, a magnetic storage medium (e.g., floppy diskette, video cassette, and so on); optical storage medium (e.g., CD-ROM); magneto-optical storage medium; read only memory (ROM); random access memory (RAM); erasable programmable memory (e.g., EPROM and EEPROM); flash memory; and so on. 
     While the present disclosure has been described with reference to various embodiments, it will be understood that these embodiments are illustrative and that the scope of the disclosure is not limited to them. Many variations, modifications, additions, and improvements are possible. More generally, embodiments in accordance with the present disclosure have been described in the context of particular embodiments. Functionality may be separated or combined in procedures differently in various embodiments of the disclosure or described with different terminology. These and other variations, modifications, additions, and improvements may fall within the scope of the disclosure and the inventive subject matter.