Patent Publication Number: US-10769036-B2

Title: Distributed transaction log

Description:
CROSS-REFERENCE TO RELATED APPLICATION(S) 
     This application is a continuation of U.S. application Ser. No. 14/469,526, filed Aug. 26, 2014, which claims the benefit of U.S. Provisional Application No. 61/870,149, filed Aug. 26, 2013, each of which is incorporated herein by reference in its entirety. 
    
    
     BACKGROUND 
     Distributed systems allow multiple clients in a network to access a pool of shared resources. For example, a distributed storage system allows a cluster of host computers to aggregate local disks (e.g., SSD, PCI-based flash storage, SATA, or SAS magnetic disks) located in or attached to each host computer to create a single and shared pool of storage. This pool of storage (sometimes referred to herein as a “datastore” or “store”) is accessible by all host computers in the cluster and may be presented as a single namespace of storage entities (such as a hierarchical file system namespace in the case of files, a flat namespace of unique identifiers in the case of objects, etc.). Storage clients in turn, such as virtual machines spawned on the host computers may use the datastore, for example, to store virtual disks that are accessed by the virtual machines during their operation. Because the shared local disks that make up the datastore may have different performance characteristics (e.g., capacity, input/output operations per second or IOPS capabilities, etc.), usage of such shared local disks to store virtual disks or portions thereof may be distributed among the virtual machines based on the needs of each given virtual machine. 
     This approach provides enterprises with cost-effective performance. For instance, distributed storage using pooled local disks is inexpensive, highly scalable, and relatively simple to manage. Because such distributed storage can use commodity disks in the cluster, enterprises do not need to invest in additional storage infrastructure. However, one issue with such a distributed system is in failure recovery for nodes that return to the cluster after being offline for a period. For example, if a cluster node goes offline (e.g., due to a power outage), active and visible nodes in the cluster still perform regular transactions as designed, but one consequence of this is that if the offline node returns to the cluster, the node and corresponding resource component objects of the node are not up-to-date with the current state of the cluster and the operations previously performed on the component objects. In that state, the previously offline node is unusable in the cluster, which is ultimately inefficient because the distributed resources system is not using all of the resources available in the cluster. 
     SUMMARY 
     One or more embodiments disclosed herein provide a method for updating a distributed transaction log of a previously offline resource component object in a distributed resources system. The method generally includes retrieving distributed transaction logs from one or more active resource component objects. The method also generally includes sending, in parallel, the distributed transaction logs to the previously offline resource component object. The method also generally includes filtering, from each distributed transaction log of one or more component objects, corresponding data missing from the distributed transaction log of the previously offline resource component object. The method also generally includes merging the corresponding data to the distributed transaction log of the previously offline resource component object. The method also generally includes a mechanism for persisting the fact that a resource component has become stale on a majority of the resource components in the object, before making progress without the offline resource component, and using this information to prevent the stale component from servicing operations on the object until it has been brought up to date by the aforementioned resynchronization methods. In this way a “live set” of resource components with up-to-date data is maintained where components are subtracted from the set when they become stale, and are re-added only after resynchronization. 
     Other embodiments include, without limitation, a computer-readable medium that includes instructions that enable a processing unit to implement one or more aspects of the disclosed methods as well as a system having a processor, memory, and application programs configured to implement one or more aspects of the disclosed methods. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  illustrates an example computing environment, according to one embodiment. 
         FIG. 2  illustrates an example hierarchical structure of objects organized within an object store that represent a virtual disk, according to one embodiment. 
         FIG. 3  illustrates components of a VSAN module, according to one embodiment. 
         FIG. 4  illustrates a method flow diagram for creating a virtual disk object based on a defined storage policy, according to one embodiment, according to one embodiment. 
         FIG. 5  illustrates the handling of an I/O operation originating from a VM, according to one embodiment. 
         FIG. 6  illustrates a method for updating a stale component using distributed transaction logs of live components belong to an adjoining RAID layout, according to one embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     Embodiments disclosed herein provide techniques for data recovery in a distributed resources system. More specifically, the techniques use transaction logs (or journals) of currently active resource component objects of a RAID layout in the system to resynchronize stale component objects (i.e., previously offline components). In one embodiment, host computer nodes forming a cluster in the distributed resources system perform actions on resource objects through distributed transactions. Each cluster node maintains a journal on each resource object component for recording the transactions. In the event that a component (or the node on which the component resides) goes offline and subsequently returns online some time later, the previously missing component may be out-of-sync with the other components (e.g., due to actions performed on the resource object while the component was offline). A distributed storage module resynchronizes the “stale” component by sending the journals of each of the components of the live set (i.e., the set of currently active and visible nodes) to the stale component. The stale component updates the journal hosted on the component using relevant data journals received. Once the update is complete, the module updates the live set of the new cluster status. The aforementioned techniques for data recovery require distributed knowledge of which resource components are stale in order to prevent them from servicing operations with stale data. Up-to-date information about which resource components are stale is obtained by querying a majority of the resource components belonging to an object. In one embodiment, the distributed knowledge is encoded as a configuration data structure containing an entry for each resource component identifying its state, which may be ACTIVE, STALE, or any other state that is useful to the embodiment. In this embodiment, such data is sent over the network to each resource component and persisted as metadata, which can later be retrieved. 
     For instance, the techniques described herein may apply to a distributed storage system where each host computer maintains records of distributed transactions performed on storage resources in a journal. One example of an applicable distributed storage system is a software-based “virtual storage area network” (VSAN) where host servers in a cluster each act as a node that contributes its commodity local storage resources (e.g., hard disk and/or solid state drives, etc.) to provide an aggregate “object” store. Each host server may include a storage management module (also referred to herein as a VSAN module) in order to automate storage management workflows (e.g., create objects in the object store, etc.) and provide access to objects in the object store (e.g., handle I/O operations to objects in the object store, etc.) based on predefined storage policies specified for objects in the object store. In one particular embodiment, the host servers further support the instantiation of virtual machines (VMs) which act as clients to the VSAN object store. In such an embodiment, the “objects” stored in the object store may include, for example, file system objects that may contain VM configuration files and virtual disk descriptor files, virtual disk objects that are accessed by the VMs during runtime and the like. The storage objects may comprise components from multiple disks on different nodes. Further, the VSAN modifies the storage objects using distributed transactions to each component object in the cluster. The VSAN uses distributed transaction journals to record transactions performed as well as persist data. 
     Reference is now made in detail to several embodiments, examples of which are illustrated in the accompanying figures. Note, that wherever practicable, similar or like reference numbers may be used in the figures and may indicate similar or like functionality. The figures depict embodiments for purposes of illustration only. One of skill in the art will readily recognize from the following description that alternative embodiments of the structures and methods illustrated herein may be employed without departing from the principles described herein. 
     In the following, an example of a software-defined storage area network in a virtualized computing environment is used as a reference example of recording distributed transactions in a distributed resources system to logs and using the logs in disk recovery. This reference example is included to provide an understanding of the embodiments described herein. However, it will be apparent to one of skill in the art that embodiments are applicable in other contexts related using distributed transaction logs to perform disk recovery. 
     Similarly, numerous specific details are provided to provide a thorough understanding of the embodiments. One of skill in the art will recognize that the embodiments may be practiced without some of these specific details. In other instances, well known process operations and implementation details have not been described in detail to avoid unnecessary obscuring novel aspects of the disclosure. 
       FIG. 1  illustrates a computing environment  100 , according to one embodiment. As shown, computing environment  100  is a VSAN environment that leverages the commodity local storage housed in or directly attached (hereinafter, use of the term “housed” or “housed in” may be used to encompass both housed in or otherwise directly attached) to host servers or nodes  111  of a cluster  110  to provide an aggregate object store  116  to virtual machines (VMs)  112  running on the nodes. The local commodity storage housed in or otherwise directly attached to the nodes  111  may include combinations of solid state drives (SSDs)  117  and/or magnetic or spinning disks  118 . In certain embodiments, SSDs  117  serve as a read cache and/or write buffer in front of magnetic disks  118  to increase I/O performance. 
     A virtualization management platform  105  is associated with cluster  110  of nodes  111 . Virtualization management platform  105  enables an administrator to manage the configuration and spawning of VMs on the various nodes  111 . As depicted in the embodiment of  FIG. 1 , each node  111  includes a virtualization layer or hypervisor  113 , a VSAN module  114 , and hardware  119  (which includes the SSDs  117  and magnetic disks  118  of a node  111 ). Through hypervisor  113 , a node  111  is able to launch and run multiple VMs  112 . Hypervisor  113 , in part, manages hardware  119  to properly allocate computing resources (e.g., processing power, random access memory, etc.) for each VM  112 . Furthermore, as described further below, each hypervisor  113 , through its corresponding VSAN module  114 , provides access to storage resources located in hardware  119  (e.g., SSDs  117  and magnetic disks  118 ) for use as storage for virtual disks (or portions thereof) and other related files that may be accessed by any VM  112  residing in any of nodes  111  in cluster  110 . In a particular embodiment, vSphere Hypervisor from VMware, Inc. (VMware) may be installed on nodes  111  as hypervisor  113  and vCenter Server from VMware may be used as virtualization management platform  105 . 
     In one embodiment, VSAN module  114  is implemented as a “VSAN” device driver within hypervisor  113 . In such an embodiment, VSAN module  114  provides access to a conceptual “VSAN”  115  through which an administrator can create a number of top-level “device” or namespace objects that are backed by object store  116 . In one common scenario, during creation of a device object, the administrator may specify a particular file system for the device object (such device objects hereinafter also thus referred to “file system objects”). For example, in one embodiment, each hypervisor  113  in each node  111  may, during a boot process, discover a /vsan/ root node for a conceptual global namespace that is exposed by VSAN module  114 . By, for example, accessing APIs exposed by VSAN module  114 , hypervisor  113  can then determine all the top-level file system objects (or other types of top-level device objects) currently residing in VSAN  115 . When a VM (or other client) attempts to access one of the file system objects, hypervisor  113  may dynamically “auto-mount” the file system object at that time. A file system object (e.g., /vsan/fs_name1, etc.) that is accessible through VSAN  115  may, for example, be implemented to emulate the semantics of a particular file system such as VMware&#39;s distributed or clustered file system, VMFS, which is designed to provide concurrency control among simultaneously accessing VMs. Because VSAN  115  supports multiple file system objects, it is able provide storage resources through object store  116  without being confined by limitations of any particular clustered file system. For example, many clustered file systems (e.g., VMFS, etc.) can only scale to support a certain amount of nodes  111 . By providing multiple top-level file system object support, VSAN  115  overcomes the scalability limitations of such clustered file systems. 
     As described in further detail in the context of  FIG. 2  below, a file system object, may, itself, provide access to a number of virtual disk descriptor files (e.g., .vmdk files in a vSphere environment, etc.) accessible by VMs  112  running in cluster  110 . These virtual disk descriptor files contain references to virtual disk “objects” that contain the actual data for the virtual disk and are separately backed by object store  116 . A virtual disk object may itself be a hierarchical or “composite” object that, as described further below, is further composed of “component” objects (again separately backed by object store  116 ) that reflect the storage requirements (e.g., capacity, availability, IOPs, etc.) of a corresponding storage profile or policy generated by the administrator when initially creating the virtual disk. As further discussed below, each VSAN module  114  (through a cluster level object management or “CLOM” sub-module, in embodiments as further described below) communicates with other VSAN modules  114  of other nodes  111  to create and maintain an in-memory metadata database (e.g., maintained separately but in synchronized fashion in the memory of each node  111 ) that contains metadata describing the locations, configurations, policies and relationships among the various objects stored in object store  116 . This in-memory metadata database is utilized by a VSAN module  114  on a node  111 , for example, when an administrator first creates a virtual disk for a VM as well as when the VM is running and performing I/O operations (e.g., read or write) on the virtual disk. As further discussed below in the context of  FIG. 3 , VSAN module  114  (through a distributed object manager or “DOM” sub-module, in one embodiment as further described below) traverses a hierarchy of objects using the metadata in the in-memory database in order to properly route an I/O operation request to the node (or nodes) that houses (house) the actual physical local storage that backs the portion of the virtual disk that is subject to the I/O operation. 
     In one embodiment, the currently active and visible nodes  111  in cluster  110  is a live set. Further, although the interaction between nodes  111  is largely peer-based, one node  111  in cluster  110  is designated as a “master” node. The master node is responsible for disseminating updates to an in-memory database maintained by the VSAN module  114  of each node  111  (described in further detail below). The in-memory database serves as a cluster membership directory and stores information regarding each node  111 , such as inventory, resources, and object configurations. If any one node  111  makes an update to the in-memory database within the given node  111 , node  111  also forwards the update information to the “master” node, upon which the “master” node  111  propagates the update information to other nodes  111  in cluster  110 . For example, if a node  111  goes offline, the “master” node  111  designates the node and hosted components as unhealthy. The distributed object manager for the composite object will then mark the component object as stale. If node  111  comes back, the stale component object on node  111  is unable to rejoin the live set until it is updated with any missing data from when node  111  was offline. 
       FIG. 2  illustrates an example hierarchical structure of objects organized within object store  116  that represent a virtual disk, according to one embodiment. As previously discussed above, a VM  112  running on one of nodes  111  may perform I/O operations on a virtual disk that is stored as a hierarchical or composite object  200  in object store  116 . Hypervisor  113  provides VM  112  access to the virtual disk by interfacing with the abstraction of VSAN  115  through VSAN module  114  (e.g., by auto-mounting the top-level file system object corresponding to the virtual disk object, as previously discussed, in one embodiment). For example, VSAN module  114 , by querying its local copy of the in-memory metadata database, is able to identify a particular file system object  205  (e.g., a VMFS file system object in one embodiment, etc.) stored in VSAN  115  that stores a descriptor file  210  for the virtual disk (e.g., a .vmdk file, etc.). It should be recognized that the file system object  205  may store a variety of other files consistent with its purpose, such as virtual machine configuration files (e.g., .vmx files in a vSphere environment, etc.) and the like when supporting a virtualization environment. In certain embodiments, each file system object may be configured to support only those virtual disks corresponding to a particular VM (e.g., a “per-VM” file system object). 
     Descriptor file  210  includes a reference to composite object  200  that is separately stored in object store  116  and conceptually represents the virtual disk (and thus may also be sometimes referenced herein as a virtual disk object). Composite object  200  stores metadata describing a storage organization or configuration for the virtual disk (sometimes referred to herein as a virtual disk “blueprint”) that suits the storage requirements or service level agreements (SLAs) in a corresponding storage profile or policy (e.g., capacity, availability, IOPs, etc.) generated by an administrator when creating the virtual disk. For example, in the embodiment of  FIG. 2 , composite object  200  includes a virtual disk blueprint  215  that describes a RAID 1 configuration where two mirrored copies of the virtual disk (e.g., mirrors) are each further striped in a RAID 0 configuration. Composite object  225  may thus contain references to a number of “leaf” or “component” objects  220   x  corresponding to each stripe (e.g., data partition of the virtual disk) in each of the virtual disk mirrors. The metadata accessible by VSAN module  114  in the in-memory metadata database for each component object  220  (e.g., for each stripe) provides a mapping to or otherwise identifies a particular node  111   x  in cluster  110  that houses the physical storage resources (e.g., magnetic disks  118 , etc.) that actually store the stripe (as well as the location of the stripe within such physical resource). 
     Further, an “owner” node of composite object  225  (designated by an election protocol in a directory service of cluster  110 ) coordinates transactions to corresponding component objects  220   x . Further, the “owner” node serves as a commit coordinator for the transaction. VSAN module  114  sends a request to prepare a change to each participating component. VSAN module  114  returns a completion as soon as all of the prepare requests have been completed. If the prepare request fails, VSAN module  114  aborts the transaction. 
     In one embodiment, each component object  220  includes a journal that acts as a distributed transactions log on component object  220 . That is, the VSAN module  114  on each node  111  modifies component objects  220  using distributed transactions. Whenever the VSAN module performs a distributed transaction on a particular component object  220 , VSAN module  114  records entries in the corresponding journal describing the transaction. Each of the entries includes a sequence identifier that increments with each additional transaction added to the journal. VSAN module  114  uses the sequence identifiers to reconcile the entries on different journals. For example, if the node  111  which owns an object goes offline (e.g., due to a power failure), and loses its knowledge of in-flight transactions, the VSAN module  114  on another node  111  compares the content of the journals using the sequence identifiers. Continuing the example, if the journal on node  111   C  includes a write operation performed on node  111   B  that should have also been performed on node  111   C  (i.e., only node  111   B  was sent the write operation before the power failure), node  111   B , through its VSAN module  114 , delivers the copy of the write operation to node  111   B . 
     Another case arises where a single node  111  goes offline and returns some time later. For example, assume that node  111   B  reboots and returns after five minutes. In this case, component objects  220   C  and  220   D , for example, may have journals that are not up-to-date. VSAN module  114  marks component objects  220   x  on node  111   B  as “stale,” and no longer part of the live set of nodes component objects  220  (i.e., the component objects that are currently active and visible). As a result, before node  111   B  is able to perform any further operations as a part of the live set, the node  111   B  updates the hosted component objects  220   x  with journal information from the live set of nodes that have also have a copy of component object  220   X . Upon completely updating the journals on node  111   B , it returns to the live set and is subsequently able to perform operations on component objects  220 . 
       FIG. 3  illustrates components of a VSAN module  114 , according to one embodiment. As previously described, in certain embodiments, VSAN module  114  may execute as a device driver exposing an abstraction of a VSAN  115  to hypervisor  113 . Various sub-modules of VSAN module  114  handle different responsibilities and may operate within either user space  315  or kernel space  320  depending on such responsibilities. As depicted in the embodiment of  FIG. 3 , VSAN module  114  includes a cluster level object management (CLOM) sub-module  325  that operates in user space  315 . CLOM sub-module  325  generates virtual disk blueprints during creation of a virtual disk by an administrator and ensures that objects created for such virtual disk blueprints are configured to meet storage profile or policy requirements set by the administrator. In addition to being accessed during object creation (e.g., for virtual disks), CLOM sub-module  325  may also be accessed (e.g., to dynamically revise or otherwise update a virtual disk blueprint or the mappings of the virtual disk blueprint to actual physical storage in object store  116 ) on a change made by an administrator to the storage profile or policy relating to an object or when changes to the cluster or workload result in an object being out of compliance with a current storage profile or policy. 
     In one embodiment, if an administrator creates a storage profile or policy for a composite object such as virtual disk object  200 , CLOM sub-module  325  applies a variety of heuristics and/or distributed algorithms to generate virtual disk blueprint  215  that describes a configuration in cluster  110  that meets or otherwise suits the storage policy (e.g., RAID configuration to achieve desired redundancy through mirroring and access performance through striping, which nodes&#39; local storage should store certain portions/partitions/stripes of the virtual disk to achieve load balancing, etc.). For example, CLOM sub-module  325 , in one embodiment, is responsible for generating blueprint  215  describing the RAID 1/RAID 0 configuration for virtual disk object  200  in  FIG. 2  when the virtual disk was first created by the administrator. As previously discussed, a storage policy may specify requirements for capacity, IOPS, availability, and reliability. Storage policies may also specify a workload characterization (e.g., random or sequential access, I/O request size, cache size, expected cache hit ration, etc.). Additionally, the administrator may also specify an affinity to VSAN module  114  to preferentially use certain nodes  111  (or the local disks housed therein). For example, when provisioning a new virtual disk for a VM, an administrator may generate a storage policy or profile for the virtual disk specifying that the virtual disk have a reserve capacity of 400 GB, a reservation of 150 read IOPS, a reservation of 300 write IOPS, and a desired availability of 99.99%. Upon receipt of the generated storage policy, CLOM sub-module  325  consults the in-memory metadata database maintained by its VSAN module  114  to determine the current state of cluster  110  in order generate a virtual disk blueprint for a composite object (e.g., the virtual disk object) that suits the generated storage policy. As further discussed below, CLOM sub-module  325  may then communicate the blueprint to its corresponding distributed object manager (DOM) sub-module  340  which interacts with object space  116  to implement the blueprint by, for example, allocating or otherwise mapping component objects (e.g., stripes) of the composite object to physical storage locations within various nodes  111  of cluster  110 . 
     In addition to CLOM sub-module  325  and DOM sub-module  340 , as further depicted in  FIG. 3 , VSAN module  114  may also include a cluster monitoring, membership, and directory services (CMMDS) sub-module  335  that maintains the previously discussed in-memory metadata database to provide information on the state of cluster  110  to other sub-modules of VSAN module  114  and also tracks the general “health” of cluster  110  by monitoring the status, accessibility, and visibility of each node  111  in cluster  110 . The in-memory metadata database serves as a directory service that maintains a physical inventory of the VSAN environment, such as the various nodes  111 , the storage resources in the nodes  111  (SSD, magnetic disks, etc.) housed therein and the characteristics/capabilities thereof, the current state of the nodes  111  and there corresponding storage resources, network paths among the nodes  111 , and the like. As previously discussed, in addition to maintaining a physical inventory, the in-memory metadata database further provides a catalog of metadata for objects stored in object store  116  (e.g., what composite and component objects exist, what component objects belong to what composite objects, which nodes serve as “coordinators” or “owners” that control access to which objects, quality of service requirements for each object, object configurations, the mapping of objects to physical storage locations, etc.). As previously discussed, other sub-modules within VSAN module  114  may access CMMDS sub-module  335  (represented by the connecting lines in  FIG. 3 ) for updates to learn of changes in cluster topology and object configurations. For example, as previously discussed, during virtual disk creation, CLOM sub-module  325  accesses the in-memory metadata database to generate a virtual disk blueprint, and in order to handle an I/O operation from a running VM  112 , DOM sub-module  340  accesses the in-memory metadata database to determine the nodes  111  that store the component objects (e.g., stripes) of a corresponding composite object (e.g., virtual disk object) and the paths by which those nodes are reachable in order to satisfy the I/O operation. 
     In addition, CMMDS sub-module  335  includes a protocol for electing a “master” node  111  within cluster  110 . Upon creation of the cluster, CMMDS sub-module  335  elects a “master” node  111 . “Master” node  111  is responsible for making distributed updates to the directory services of other nodes  111  in cluster  110  and assigning owner nodes  111  to composite objects  200 . The elected “master” node  111  also appoints a backup node  111  to become the “master” node  111  if the current master node  111  fails. 
     As previously discussed, DOM sub-module  340 , during the handling of I/O operations as well as during object creation, controls access to and handles operations on those component objects in object store  116  that are stored in the local storage of the particular node  111  in which DOM sub-module  340  runs as well as certain other composite objects for which its node  111  has been currently designated as the “coordinator” or “owner.” For example, when handling an I/O operation from a VM, due to the hierarchical nature of composite objects in certain embodiments, a DOM sub-module  340  that serves as the coordinator for the target composite object (e.g., the virtual disk object that is subject to the I/O operation) may need to further communicate across the network with a different DOM sub-module  340  in a second node  111  (or nodes) that serves as the coordinator for the particular component object (e.g., stripe, etc.) of the virtual disk object that is stored in the local storage of the second node  111  and which is the portion of the virtual disk that is subject to the I/O operation. If the VM issuing the I/O operation resides on a node  111  that is also different from the coordinator of the virtual disk object, the DOM sub-module  340  of the node running the VM would also have to communicate across the network with the DOM sub-module  340  of the coordinator. In owner mode, DOM sub-module  340  coordinates all transactions performed on a component object  220 , serving as a commit coordinator for each transaction. DOM sub-module  340  assigns a sequence number for every distributed operation so that the transaction entries in the journals of various component objects can be collated during recovery. In certain embodiments, if the VM issuing the I/O operation resides on node that is different from the coordinator of the virtual disk object subject to the I/O operation, the two DOM sub-modules  340  of the two nodes may to communicate to change the role of the coordinator of the virtual disk object to the node running the VM (e.g., thereby reducing the amount of network communication needed to coordinate I/O operations between the node running the VM and the node serving as the coordinator for the virtual disk object). 
     DOM sub-modules  340  also similarly communicate amongst one another during object creation. For example, a virtual disk blueprint generated by CLOM module  325  during creation of a virtual disk may include information that designates which nodes  111  should serve as the coordinators for the virtual disk object as well as its corresponding component objects (stripes, etc.). Each of the DOM sub-modules  340  for such designated nodes is issued requests (e.g., by the DOM sub-module  340  designated as the coordinator for the virtual disk object or by the DOM sub-module  340  of the node generating the virtual disk blueprint, etc. depending on embodiments) to create their respective objects, allocate local storage to such objects (if needed), and advertise their objects to their corresponding CMMDS sub-module  335  in order to update the in-memory metadata database with metadata regarding the object. In order to perform such requests, DOM sub-module  340  interacts with a log structured object manager (LSOM) sub-module  350  that serves as the component in VSAN module  114  that actually drives communication with the local SSDs and magnetic disks of its node  111 . In addition to allocating local storage for component objects (as well as to store other metadata such a policies and configurations for composite objects for which its node serves as coordinator, etc.), LSOM sub-module  350  additionally monitors the flow of I/O operations to the local storage of its node  111 . 
     Further, LSOM sub-module  350  maintains the journals of the component objects  220   X . More specifically, LSOM sub-module  350  maintains a journal on each disk that incorporates cluster membership sequence numbers and object update configuration identifiers to allow updates to distributed objects composed of two or more LSOM components to be reconciled after a failure or partition. LSOM sub-module  350  labels storage objects with UUIDs so that the disks can be rejoined to the cluster, even if the storage objects are relocated to other nodes  111  (e.g., in event of a node failure). 
       FIG. 3  also depicts a reliable datagram transport (RDT) sub-module  345  that delivers datagrams of arbitrary size between logical endpoints (e.g., nodes, objects, etc.), where the endpoints may potentially be over multiple paths. In one embodiment, the underlying transport is TCP. Alternatively, other transports such as RDMA may be used. RDT sub-module  345  is used, for example, when DOM sub-modules  340  communicate with one another, as previously discussed above to create objects or to handle I/O operations. In certain embodiments, RDT module  345  interacts with CMMDS module  335  to resolve the address of logical endpoints dynamically in order to maintain up-to-date location information in the in-memory metadata database as well as to create, remove, or reestablish connections based on link health status. For example, if CMMDS module  335  reports a link as unhealthy, RDT sub-module  345  may drop the connection in favor of a link in better condition. 
       FIG. 4  illustrates a method flow diagram for creating a virtual disk object based on a defined storage policy, according to one embodiment. For example, in step  400 , an administrator may interact with a user interface of virtual management platform  105  to create a virtual disk having capacity, availability and IOPS requirements (e.g., the defined storage policy). In one embodiment, virtual management platform  105  may then request a “master” node  111  to create an object for the virtual disk in step  405 . In step  410 , such a master node  111  may generate a virtual disk blueprint through its CLOM sub-module  325  in VSAN module. As previously discussed, CLOM sub-module  35  generates a virtual disk blueprint for the creation of a virtual disk object (e.g., a composite object) based on the status of cluster  110  as determined by consulting the in-memory metadata database of CMMDS sub-module  335 . In step  415 , the DOM sub-module  340  of the master node  111  may the request the DOM sub-module  340  of the identified node to create the virtual disk object. In step  420 , the DOM sub-module  340  of the identified node receives the request and creates the virtual disk object, by, for example, communicating with its corresponding the LSOM sub-module  350  to persistently store metadata describing the virtual disk object in its local storage. In step  425 , the DOM sub-module  340 , based on the virtual disk object blueprint, identifies other nodes in cluster  110  in the virtual disk blueprint. The DOM sub-module  340  communicates (e.g., using its RDT sub-module  345 ) with the DOM sub-modules  340  of the other nodes that will serve as coordinators for the component objects and store the data backing such component objects in their local storage. When such DOM sub-modules  340  receive a request from the DOM sub-module  340  of the coordinator of the virtual disk object to create their respective component objects, they, in turn in step  430 , communicate with their respective LSOM modules  350  to allocate local storage for the component object (and its related metadata). Once such component objects have been created, their DOM sub-modules  340  advertise the creation of the components to the in-memory metadata database of its CMMDS sub-module  335  in step  435 . In step  440 , in turn, the DOM sub-module  340  for the coordinator of the virtual disk object also advertises its creation to its CMMDS sub-module  335  to update the in-memory metadata database and ultimately transmits an acknowledgement to the administrator (e.g., via the master node communications back to virtual management platform  105 ). 
       FIG. 5  illustrates the handling of an I/O operation originating from a VM, according to one embodiment. When a VM running on a particular node performs I/O operations to its virtual disk, the VM&#39;s guest operating system, in step  500 , transmits an I/O operation request intended for its virtual disk (through a device driver of the guest operating system) which, in step  505 , is received by hypervisor  113  and ultimately transmitted and transformed through various layers of an I/O stack in hypervisor  113  to DOM sub-module  340  of VSAN module  114 . In step  510 , the I/O request received by DOM sub-module  340  includes a unique identifier for an object representing the virtual disk that DOM sub-module  340  uses to identify the coordinator node of the virtual disk object by accessing the in-memory metadata database of CMMS sub-module  335  (in certain embodiments, accessing the in-memory metadata database to look up a mapping of the identity of the coordinator node to the unique identifier occurs only when the virtual disk object is initially accessed, with such mapping persisting for future I/O operations such that subsequent lookups are not needed). Upon identifying the coordinator node for the virtual disk object, the DOM sub-module  340  of the node running the VM communicates (e.g., using its RDT sub-module  345 ) with the DOM sub-module  340  of the coordinator node to request that it perform the I/O operation in step  515 . As previously discussed, in certain embodiments, if the node running the VM and the node serving as coordinator of the virtual disk object are different, the two DOM sub-modules will communicate to update the role of the coordinator of the virtual disk object to be the node of the running VM. Upon the coordinator&#39;s receipt of the I/O request, in step  520 , its DOM sub-module identifies (e.g., by again referencing the in-memory metadata database, in certain embodiments) those coordinator nodes for the particular component objects (e.g., stripes) of the virtual disk object that are subject to the I/O operation. For example, if the I/O operation spans multiple stripes (e.g., multiple component objects) of a RAID 0 configuration, DOM sub-module  340  may split the I/O operation and appropriately transmit correspond I/O requests to the respective coordinate nodes for the relevant component objects that correspond to the two stripes. In step  525 , the DOM sub-module of the coordinator node for the virtual disk object requests that the DOM sub-modules for the coordinator nodes of the identified component objects perform the I/O operation request and, in step  530 , the DOM sub-modules of such coordinator nodes for the identified component objects interact with their corresponding LSOM sub-modules to perform the I/O operation in the local storage resource where the component object is stored. 
     In certain situations, it should be recognized that multiple VMs may simultaneously send requests to perform I/O operations on a particular local storage resource located in a particular node at any given time. For example, the component objects (e.g., stripes, etc.) of different virtual disk objects corresponding to different VMs may be backed by the same local storage on the same node. Upon receiving an I/O operation, the VSAN module  114  of such a node may place the I/O operation into a storage resource queue for processing. 
       FIG. 6  illustrates a method for updating a distributed transaction log of each stale component object in a stale node using distributed transaction logs of component objects hosted on other nodes using a RAID layout as a source, according to one embodiment. As stated, stale component objects are unable to rejoin the configuration and perform operations until the transaction entries in the component object journals are up-to-date. 
     The method begins at step  605 , where the VSAN module  114  of the node  111  corresponding to the owner DOM sub-module  340  sends the journals from the live nodes  111  that have data corresponding to any stale component object to the given stale component object  220 . To do this, the node  111  (through DOM sub-module  340 ) communicates with LSOM sub-modules  350  of corresponding nodes  111  to retrieve the journals. For example, assume that the RAID-1 configuration of a particular composite object includes a five-way RAID-0 in one arm and a four-way RAID-0 in the other. Suppose that one component from the four-way RAID-0 goes offline for a brief period and returns. In this case, the now-stale component object may not have received any updates during the downtime. However, the node  111  hosting the stale component object  220  can use the distributed transaction logs in each of the five components of the five-way RAID-0 configuration as a source to recover the stale component. 
     At step  610 , VSAN module  114  applies any missing changes to each stale component object  220 . Continuing the previous example, it follows that not all of the transactions of the component logs are applicable to the stale component object. Therefore, VSAN module  114  scans the journals in parallel while filtering relevant source bits by address range. At step  615 , VSAN module  114  merges the results with the transaction log of the stale component. The owner DOM sub-module  340  communicates with the LSOM sub-modules  350  of the nodes  111  in the mirrored RAID configuration and copies the respective blocks from the corresponding component objects  220 . Once the journal has been updated, in step  615 , VSAN module  114  propagates the update to the cluster. The “master” node  111  updates the live set, and the returning component objects on node  111  rejoin the current live set configuration. 
     In sum, embodiments of the present disclosure provide techniques for recovery of previously offline component objects using distributed transaction logs on separate RAID layouts. Advantageously, the techniques provide further continuous data protection of object components using mirrored configurations. Additionally, using the live set model assures a single chain of authoritative memberships from which to resolve distributed transactions given partitions and other failures. 
     Generally speaking, 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 may be useful machine operations. In addition, one or more embodiments 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. 
     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. 
     One or more embodiments 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. 
     Although one or more embodiments 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. 
     In addition, while described virtualization methods have generally assumed that virtual machines present interfaces consistent with a particular hardware system, the methods described may be used in conjunction with virtualizations that do not correspond directly to any particular hardware system. Virtualization systems in accordance with the various embodiments, 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. 
     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 one or more embodiments. 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).