Patent Publication Number: US-2021191823-A1

Title: Snapshot creation with synchronous replication

Description:
RELATED APPLICATIONS 
     This application claims priority to and is a continuation of U.S. application Ser. No. 14/980,428, filed on Dec. 28, 2015, now allowed, titled “SNAPSHOT CREATION WITH SYNCHRONOUS REPLICATION,” which is incorporated herein by reference. 
    
    
     BACKGROUND 
     Many storage networks may implement data replication and/or other redundancy data access techniques for data loss protection and non-disruptive client access. For example, a first storage cluster may comprise a first storage controller configured to provide clients with primary access to data stored within a first storage device and/or other storage devices. A second storage cluster may comprise a second storage controller configured to provide clients with primary access to data stored within a second storage device and/or other storage devices. The first storage controller and the second storage controller may be configured according to a disaster recovery relationship, such that the second storage controller may provide failover access to replicated data that was replicated from the first storage device to a secondary storage device, owned by the first storage controller, but accessible to the second storage controller (e.g., a switchover operation may be performed where the second storage controller assumes ownership of the secondary storage device and/or other storage devices previously owned by the first storage controller so that the second storage controller may provide clients with failover access to replicated data within such storage devices). 
     In an example, the second storage cluster may be located at a remote site to the first storage cluster (e.g., storage clusters may be located in different buildings, cities, thousands of kilometers from one another, etc.). Thus, if a disaster occurs at a site of a storage cluster, then a surviving storage cluster may remain unaffected by the disaster (e.g., a power outage of a building hosting the first storage cluster may not affect a second building hosting the second storage cluster in a different city). 
     In an example, two storage controllers within a storage cluster may be configured according to a high availability configuration, such as where the two storage controllers are locally connected to one another and/or to the same storage devices. In this way, when a storage controller fails, then a high availability partner storage controller can quickly takeover for the failed storage controller due to the local connectivity. Thus, the high availability partner storage controller may provide clients with access to data previously accessible through the failed storage controller. 
     Various replication and synchronization techniques may be used to replicate data (e.g., client data), configuration data (e.g., a size of a volume, a name of a volume, etc.), and/or write caching data (e.g., cached write operations) between storage controllers and/or storage devices. In an example of synchronization, a synchronous replication relationship may be implemented between the first storage controller and the second storage controller, such that an incoming write operation to the first storage controller is locally implemented upon a first storage object (e.g., a file, a logical unit number (LUN), a LUN spanning multiple volumes, or any other type of object) by the first storage controller and remotely implemented upon a second storage object (e.g., maintained as a backup replication of the first storage object) by the second storage controller before an acknowledgement is provided back to a client that sent the incoming write operation. In an example of replication, snapshots of the first storage object may be used to replicate the first storage object to the second storage object. For example, a base snapshot of the first storage object (e.g., a volume comprising the first storage object) may be used to initially create the second storage object. A current incremental snapshot of the first storage object (e.g., the volume) may be used to replicate changes made to the first storage object since the base snapshot or since a last incremental snapshot. Snapshots may also be periodically created and used to recover from operational failures or corruption. Unfortunately, snapshot creation may be disruptive to client access to the first storage object (e.g., client write requests may be blocked during snapshot creation) and/or may be disruptive to the synchronous replication relationship (e.g., if client write operations are not blocked and are implemented upon the first storage object while a snapshot of the second storage object is being created, then data divergence between the first storage object and the second storage object can occur). For example, client write requests to the first storage object may be rejected during snapshot creation, thus increasing latency and client data access disruption. 
    
    
     
       DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a component block diagram illustrating an example clustered network in accordance with one or more of the provisions set forth herein. 
         FIG. 2  is a component block diagram illustrating an example data storage system in accordance with one or more of the provisions set forth herein. 
         FIG. 3  is a flow chart illustrating an exemplary method of snapshot creation. 
         FIG. 4A  is a component block diagram illustrating an exemplary computing device for snapshot creation, where a write operation is locally implemented by a first storage controller and remotely implemented by a second storage controller based upon a synchronous replication relationship. 
         FIG. 4B  is a component block diagram illustrating an exemplary computing device for snapshot creation, where a request to create a snapshot is received. 
         FIG. 4C  is a component block diagram illustrating an exemplary computing device for snapshot creation, where a splitter drains current write operations and queues incoming write operations. 
         FIG. 4D  is a component block diagram illustrating an exemplary computing device for snapshot creation, where a first snapshot request and a second snapshot request are issued. 
         FIG. 4E  is a component block diagram illustrating an exemplary computing device for snapshot creation, where a splitter is instructed to resume processing write operations based upon the creation of a first snapshot and a second snapshot. 
         FIG. 5  is an example of a computer readable medium in accordance with one or more of the provisions set forth herein. 
     
    
    
     DETAILED DESCRIPTION 
     Some examples of the claimed subject matter are now described with reference to the drawings, where like reference numerals are generally used to refer to like elements throughout. In the following description, for purposes of explanation, numerous specific details are set forth in order to provide an understanding of the claimed subject matter. It may be evident, however, that the claimed subject matter may be practiced without these specific details. Nothing in this detailed description is admitted as prior art. 
     One or more techniques and/or computing devices for snapshot creation are provided herein. For example, a synchronous replication relationship may exist between a first storage object hosted by a first storage controller and a second storage object hosted by a second storage controller (e.g., a file, a LUN, a LUN stored across multiple volumes, a volume, a subdirectory, or any other object may be synchronously replicated between the storage controllers, such that modifications to the first storage object are replicated to the second storage object before acknowledgements are provided back to clients requesting such modifications). A request to create a snapshot of the first storage object may be locally implemented by the first storage controller upon the first storage object to create a first snapshot available to the first storage controller, and remotely implemented by the second storage controller upon the second storage object (e.g., while the second storage object mirrors the first storage object) to create a second snapshot available to the second storage controller. The snapshots may be created while the synchronous replication relationship is in a synchronous state, which may mitigate resources and/or client data access disruption otherwise caused by having to resync the storage objects. If the first storage controller encounters a failure or disaster, then the second snapshot is readily available for the second storage controller to use to bring applications, at a secondary site hosting the second storage controller, into an up-to-date and operational state with a similar level of functionality that was possible at a primary site hosting the first storage controller before the failure or disaster. 
     To provide context for snapshot creation,  FIG. 1  illustrates an embodiment of a clustered network environment  100  or a network storage environment. It may be appreciated, however, that the techniques, etc. described herein may be implemented within the clustered network environment  100 , a non-cluster network environment, and/or a variety of other computing environments, such as a desktop computing environment. That is, the instant disclosure, including the scope of the appended claims, is not meant to be limited to the examples provided herein. It will be appreciated that where the same or similar components, elements, features, items, modules, etc. are illustrated in later figures but were previously discussed with regard to prior figures, that a similar (e.g., redundant) discussion of the same may be omitted when describing the subsequent figures (e.g., for purposes of simplicity and ease of understanding). 
       FIG. 1  is a block diagram illustrating the clustered network environment  100  that may implement at least some embodiments of the techniques and/or systems described herein. The clustered network environment  100  comprises data storage systems  102  and  104  that are coupled over a cluster fabric  106 , such as a computing network embodied as a private Infiniband, Fibre Channel (FC), or Ethernet network facilitating communication between the data storage systems  102  and  104  (and one or more modules, component, etc. therein, such as, nodes  116  and  118 , for example). It will be appreciated that while two data storage systems  102  and  104  and two nodes  116  and  118  are illustrated in  FIG. 1 , that any suitable number of such components is contemplated. In an example, nodes  116 ,  118  comprise storage controllers (e.g., node  116  may comprise a primary or local storage controller and node  118  may comprise a secondary or remote storage controller) that provide client devices, such as host devices  108 ,  110 , with access to data stored within data storage devices  128 ,  130 . Similarly, unless specifically provided otherwise herein, the same is true for other modules, elements, features, items, etc. referenced herein and/or illustrated in the accompanying drawings. That is, a particular number of components, modules, elements, features, items, etc. disclosed herein is not meant to be interpreted in a limiting manner. 
     It will be further appreciated that clustered networks are not limited to any particular geographic areas and can be clustered locally and/or remotely. Thus, in one embodiment a clustered network can be distributed over a plurality of storage systems and/or nodes located in a plurality of geographic locations; while in another embodiment a clustered network can include data storage systems (e.g.,  102 ,  104 ) residing in a same geographic location (e.g., in a single onsite rack of data storage devices). 
     In the illustrated example, one or more host devices  108 ,  110  which may comprise, for example, client devices, personal computers (PCs), computing devices used for storage (e.g., storage servers), and other computers or peripheral devices (e.g., printers), are coupled to the respective data storage systems  102 ,  104  by storage network connections  112 ,  114 . Network connection may comprise a local area network (LAN) or wide area network (WAN), for example, that utilizes Network Attached Storage (NAS) protocols, such as a Common Internet File System (CIFS) protocol or a Network File System (NFS) protocol to exchange data packets. Illustratively, the host devices  108 ,  110  may be general-purpose computers running applications, and may interact with the data storage systems  102 ,  104  using a client/server model for exchange of information. That is, the host device may request data from the data storage system (e.g., data on a storage device managed by a network storage control configured to process I/O commands issued by the host device for the storage device), and the data storage system may return results of the request to the host device via one or more storage network connections  112 ,  114 . 
     The nodes  116 ,  118  on clustered data storage systems  102 ,  104  can comprise network or host nodes that are interconnected as a cluster to provide data storage and management services, such as to an enterprise having remote locations, cloud storage (e.g., a storage endpoint may be stored within a data cloud), etc., for example. Such a node in the clustered network environment  100  can be a device attached to the network as a connection point, redistribution point or communication endpoint, for example. A node may be capable of sending, receiving, and/or forwarding information over a network communications channel, and could comprise any device that meets any or all of these criteria. One example of a node may be a data storage and management server attached to a network, where the server can comprise a general purpose computer or a computing device particularly configured to operate as a server in a data storage and management system. 
     In an example, a first cluster of nodes such as the nodes  116 ,  118  (e.g., a first set of storage controllers configured to provide access to a first storage aggregate comprising a first logical grouping of one or more storage devices) may be located on a first storage site. A second cluster of nodes, not illustrated, may be located at a second storage site (e.g., a second set of storage controllers configured to provide access to a second storage aggregate comprising a second logical grouping of one or more storage devices). The first cluster of nodes and the second cluster of nodes may be configured according to a disaster recovery configuration where a surviving cluster of nodes provides switchover access to storage devices of a disaster cluster of nodes in the event a disaster occurs at a disaster storage site comprising the disaster cluster of nodes (e.g., the first cluster of nodes provides client devices with switchover data access to storage devices of the second storage aggregate in the event a disaster occurs at the second storage site). 
     As illustrated in the clustered network environment  100 , nodes  116 ,  118  can comprise various functional components that coordinate to provide distributed storage architecture for the cluster. For example, the nodes can comprise network modules  120 ,  122  and data modules  124 ,  126 . Network modules  120 ,  122  can be configured to allow the nodes  116 ,  118  (e.g., network storage controllers) to connect with host devices  108 ,  110  over the storage network connections  112 ,  114 , for example, allowing the host devices  108 ,  110  to access data stored in the distributed storage system. Further, the network modules  120 ,  122  can provide connections with one or more other components through the cluster fabric  106 . For example, in  FIG. 1 , the network module  120  of node  116  can access a second data storage device  130  by sending a request through the data module  126  of a second node  118 . 
     Data modules  124 ,  126  can be configured to connect one or more data storage devices  128 ,  130 , such as disks or arrays of disks, flash memory, or some other form of data storage, to the nodes  116 ,  118 . The nodes  116 ,  118  can be interconnected by the cluster fabric  106 , for example, allowing respective nodes in the cluster to access data on data storage devices  128 ,  130  connected to different nodes in the cluster. Often, data modules  124 ,  126  communicate with the data storage devices  128 ,  130  according to a storage area network (SAN) protocol, such as Small Computer System Interface (SCSI) or Fiber Channel Protocol (FCP), for example. Thus, as seen from an operating system on nodes  116 ,  118 , the data storage devices  128 ,  130  can appear as locally attached to the operating system. In this manner, different nodes  116 ,  118 , etc. may access data blocks through the operating system, rather than expressly requesting abstract files. 
     It should be appreciated that, while the clustered network environment  100  illustrates an equal number of network and data modules, other embodiments may comprise a differing number of these modules. For example, there may be a plurality of network and data modules interconnected in a cluster that does not have a one-to-one correspondence between the network and data modules. That is, different nodes can have a different number of network and data modules, and the same node can have a different number of network modules than data modules. 
     Further, a host device  108 ,  110  can be networked with the nodes  116 ,  118  in the cluster, over the storage networking connections  112 ,  114 . As an example, respective host devices  108 ,  110  that are networked to a cluster may request services (e.g., exchanging of information in the form of data packets) of nodes  116 ,  118  in the cluster, and the nodes  116 ,  118  can return results of the requested services to the host devices  108 ,  110 . In one embodiment, the host devices  108 ,  110  can exchange information with the network modules  120 ,  122  residing in the nodes  116 ,  118  (e.g., network hosts) in the data storage systems  102 ,  104 . 
     In one embodiment, the data storage devices  128 ,  130  comprise volumes  132 , which is an implementation of storage of information onto disk drives or disk arrays or other storage (e.g., flash) as a file-system for data, for example. Volumes can span a portion of a disk, a collection of disks, or portions of disks, for example, and typically define an overall logical arrangement of file storage on disk space in the storage system. In one embodiment a volume can comprise stored data as one or more files that reside in a hierarchical directory structure within the volume. 
     Volumes are typically configured in formats that may be associated with particular storage systems, and respective volume formats typically comprise features that provide functionality to the volumes, such as providing an ability for volumes to form clusters. For example, where a first storage system may utilize a first format for their volumes, a second storage system may utilize a second format for their volumes. 
     In the clustered network environment  100 , the host devices  108 ,  110  can utilize the data storage systems  102 ,  104  to store and retrieve data from the volumes  132 . In this embodiment, for example, the host device  108  can send data packets to the network module  120  in the node  116  within data storage system  102 . The node  116  can forward the data to the data storage device  128  using the data module  124 , where the data storage device  128  comprises volume  132 A. In this way, in this example, the host device can access the volume  132 A, to store and/or retrieve data, using the data storage system  102  connected by the network connection  112 . Further, in this embodiment, the host device  110  can exchange data with the network module  122  in the host  118  within the data storage system  104  (e.g., which may be remote from the data storage system  102 ). The host  118  can forward the data to the data storage device  130  using the data module  126 , thereby accessing volume  1328  associated with the data storage device  130 . 
     It may be appreciated that snapshot creation may be implemented within the clustered network environment  100 . In an example, a synchronous replication relationship may be established between the volume  132 A (e.g., or a subset of the volume  132 A, such as a file, LUN, subdirectory, or any other type of storage object) of node  116  (e.g., a first storage controller) and the volume  132 B (e.g., or a subset of the volume  1328 , such as a file, LUN, subdirectory, or any other type of storage object) of the node  118  (e.g., a second storage controller). Responsive to receiving a request to create a snapshot of the volume  132 A (e.g., a snapshot of a first storage object within the volume  132 A), a first snapshot of the volume  132 A may be created by the node  116  and a second snapshot of the volume  1328  may be created by the node  118 . It may be appreciated that snapshot creation may be implemented for and/or between any type of computing environment, and may be transferrable between physical devices (e.g., node  116 , node  118 , a desktop computer, a tablet, a laptop, a wearable device, a mobile device, a storage device, a server, etc.) and/or a cloud computing environment (e.g., remote to the clustered network environment  100 ). 
       FIG. 2  is an illustrative example of a data storage system  200  (e.g.,  102 ,  104  in  FIG. 1 ), providing further detail of an embodiment of components that may implement one or more of the techniques and/or systems described herein. The data storage system  200  comprises a node  202  (e.g., host nodes  116 ,  118  in  FIG. 1 ), and a data storage device  234  (e.g., data storage devices  128 ,  130  in  FIG. 1 ). The node  202  may be a general purpose computer, for example, or some other computing device particularly configured to operate as a storage server. A host device  205  (e.g.,  108 ,  110  in  FIG. 1 ) can be connected to the node  202  over a network  216 , for example, to provides access to files and/or other data stored on the data storage device  234 . In an example, the node  202  comprises a storage controller that provides client devices, such as the host device  205 , with access to data stored within data storage device  234 . 
     The data storage device  234  can comprise mass storage devices, such as disks  224 ,  226 ,  228  of a disk array  218 ,  220 ,  222 . It will be appreciated that the techniques and systems, described herein, are not limited by the example embodiment. For example, disks  224 ,  226 ,  228  may comprise any type of mass storage devices, including but not limited to magnetic disk drives, flash memory, and any other similar media adapted to store information, including, for example, data (D) and/or parity (P) information. 
     The node  202  comprises one or more processors  204 , a memory  206 , a network adapter  210 , a cluster access adapter  212 , and a storage adapter  214  interconnected by a system bus  242 . The data storage system  200  also includes an operating system  208  installed in the memory  206  of the node  202  that can, for example, implement a Redundant Array of Independent (or Inexpensive) Disks (RAID) optimization technique to optimize a reconstruction process of data of a failed disk in an array. 
     The operating system  208  can also manage communications for the data storage system, and communications between other data storage systems that may be in a clustered network, such as attached to a cluster fabric  215  (e.g.,  106  in  FIG. 1 ). Thus, the node  202 , such as a network storage controller, can respond to host device requests to manage data on the data storage device  234  (e.g., or additional clustered devices) in accordance with these host device requests. The operating system  208  can often establish one or more file systems on the data storage system  200 , where a file system can include software code and data structures that implement a persistent hierarchical namespace of files and directories, for example. As an example, when a new data storage device (not shown) is added to a clustered network system, the operating system  208  is informed where, in an existing directory tree, new files associated with the new data storage device are to be stored. This is often referred to as “mounting” a file system. 
     In the example data storage system  200 , memory  206  can include storage locations that are addressable by the processors  204  and network adapters  210 ,  212 ,  214  for storing related software application code and data structures. The processors  204  and network adapters  210 ,  212 ,  214  may, for example, include processing elements and/or logic circuitry configured to execute the software code and manipulate the data structures. The operating system  208 , portions of which are typically resident in the memory  206  and executed by the processing elements, functionally organizes the storage system by, among other things, invoking storage operations in support of a file service implemented by the storage system. It will be apparent to those skilled in the art that other processing and memory mechanisms, including various computer readable media, may be used for storing and/or executing application instructions pertaining to the techniques described herein. For example, the operating system can also utilize one or more control files (not shown) to aid in the provisioning of virtual machines. 
     The network adapter  210  includes the mechanical, electrical and signaling circuitry needed to connect the data storage system  200  to a host device  205  over a network  216 , which may comprise, among other things, a point-to-point connection or a shared medium, such as a local area network. The host device  205  (e.g.,  108 ,  110  of  FIG. 1 ) may be a general-purpose computer configured to execute applications. As described above, the host device  205  may interact with the data storage system  200  in accordance with a client/host model of information delivery. 
     The storage adapter  214  cooperates with the operating system  208  executing on the node  202  to access information requested by the host device  205  (e.g., access data on a storage device managed by a network storage controller). The information may be stored on any type of attached array of writeable media such as magnetic disk drives, flash memory, and/or any other similar media adapted to store information. In the example data storage system  200 , the information can be stored in data blocks on the disks  224 ,  226 ,  228 . The storage adapter  214  can include input/output (I/O) interface circuitry that couples to the disks over an I/O interconnect arrangement, such as a storage area network (SAN) protocol (e.g., Small Computer System Interface (SCSI), iSCSI, hyperSCSI, Fiber Channel Protocol (FCP)). The information is retrieved by the storage adapter  214  and, if necessary, processed by the one or more processors  204  (or the storage adapter  214  itself) prior to being forwarded over the system bus  242  to the network adapter  210  (and/or the cluster access adapter  212  if sending to another node in the cluster) where the information is formatted into a data packet and returned to the host device  205  over the network  216  (and/or returned to another node attached to the cluster over the cluster fabric  215 ). 
     In one embodiment, storage of information on disk arrays  218 ,  220 ,  222  can be implemented as one or more storage volumes  230 ,  232  that are comprised of a cluster of disks  224 ,  226 ,  228  defining an overall logical arrangement of disk space. The disks  224 ,  226 ,  228  that comprise one or more volumes are typically organized as one or more groups of RAIDs. As an example, volume  230  comprises an aggregate of disk arrays  218  and  220 , which comprise the cluster of disks  224  and  226 . 
     In one embodiment, to facilitate access to disks  224 ,  226 ,  228 , the operating system  208  may implement a file system (e.g., write anywhere file system) that logically organizes the information as a hierarchical structure of directories and files on the disks. In this embodiment, respective files may be implemented as a set of disk blocks configured to store information, whereas directories may be implemented as specially formatted files in which information about other files and directories are stored. 
     Whatever the underlying physical configuration within this data storage system  200 , data can be stored as files within physical and/or virtual volumes, which can be associated with respective volume identifiers, such as file system identifiers (FSIDs), which can be 32-bits in length in one example. 
     A physical volume corresponds to at least a portion of physical storage devices whose address, addressable space, location, etc. doesn&#39;t change, such as at least some of one or more data storage devices  234  (e.g., a Redundant Array of Independent (or Inexpensive) Disks (RAID system)). Typically the location of the physical volume doesn&#39;t change in that the (range of) address(es) used to access it generally remains constant. 
     A virtual volume, in contrast, is stored over an aggregate of disparate portions of different physical storage devices. The virtual volume may be a collection of different available portions of different physical storage device locations, such as some available space from each of the disks  224 ,  226 , and/or  228 . It will be appreciated that since a virtual volume is not “tied” to any one particular storage device, a virtual volume can be said to include a layer of abstraction or virtualization, which allows it to be resized and/or flexible in some regards. 
     Further, a virtual volume can include one or more logical unit numbers (LUNs)  238 , directories  236 , Qtrees  235 , and files  240 . Among other things, these features, but more particularly LUNS, allow the disparate memory locations within which data is stored to be identified, for example, and grouped as data storage unit. As such, the LUNs  238  may be characterized as constituting a virtual disk or drive upon which data within the virtual volume is stored within the aggregate. For example, LUNs are often referred to as virtual drives, such that they emulate a hard drive from a general purpose computer, while they actually comprise data blocks stored in various parts of a volume. 
     In one embodiment, one or more data storage devices  234  can have one or more physical ports, wherein each physical port can be assigned a target address (e.g., SCSI target address). To represent respective volumes stored on a data storage device, a target address on the data storage device can be used to identify one or more LUNs  238 . Thus, for example, when the node  202  connects to a volume  230 ,  232  through the storage adapter  214 , a connection between the node  202  and the one or more LUNs  238  underlying the volume is created. 
     In one embodiment, respective target addresses can identify multiple LUNs, such that a target address can represent multiple volumes. The I/O interface, which can be implemented as circuitry and/or software in the storage adapter  214  or as executable code residing in memory  206  and executed by the processors  204 , for example, can connect to volume  230  by using one or more addresses that identify the one or more LUNs  238 . 
     It may be appreciated that snapshot creation may be implemented for the data storage system  200 . In an example, a synchronous replication relationship may be established between the volume  230  (e.g., or a subset of the volume  230 , such as file  240 , LUN  238 , subdirectory, or any other type of storage object) of the node  202  (e.g., a first storage controller) and a second volume (e.g., or a subset of the second volume, such as a file, LUN, subdirectory, or any other type of storage object) of a second node (e.g., a second storage controller). Responsive to receiving a request to create a snapshot of the volume  230  (e.g., a snapshot of a first storage object within the volume  230 ), a first snapshot of the volume  230  may be created by the node  116  and a second snapshot of the second volume may be created by the second node. It may be appreciated that snapshot creation may be implemented for and/or between any type of computing environment, and may be transferrable between physical devices (e.g., node  202 , host device  205 , a desktop computer, a tablet, a laptop, a wearable device, a mobile device, a storage device, a server, etc.) and/or a cloud computing environment (e.g., remote to the node  202  and/or the host device  205 ). 
     One embodiment of snapshot creation is illustrated by an exemplary method  300  of  FIG. 3 . In an example, a first storage controller may host a first storage object, such as a file, a LUN, a LUN spanning across multiple volumes, a volume, a subdirectory within the volume, or any other type of object used to store data. A second storage controller may host a second storage object that is a backup replication of the first storage object. A synchronous replication relationship may be established between the first storage object and the second storage object, such that a modification to the first storage object (e.g., a write operation from a client) is replicated to the second storage object before an acknowledgement is provided back to the client, thus preserving data consistency between the first storage object and the second storage object. 
     In an example of the synchronous replication relationship, a modification request (e.g., a write operation) that is received by a data module of the first storage controller is intercepted and checked to see if the modification request is to a file or LUN (e.g., the first storage object) that is part of the synchronous replication relationship. If the modification request is to the file or LUN that is part of the synchronous replication relationship, then the modification request is intercepted and a first copy of the modification request is sent to a primary file system of the first storage controller for implementation, and a second copy of the modification request is sent to the second storage controller for implementation (e.g., implementation upon the second storage object). Once the modification request is completed by both the first storage controller and the second storage controller, then the modification request is acknowledged back to a client that sent the modification request. If the modification request succeeds at the first storage controller but fails at the second storage controller, then the acknowledgment is sent to the client and the synchronous replication relationship is transitioned into an out-of-sync state. Synchronous replication may be provided for a consistency group of files and/or LUNs using multiple splitters, such that a single splitter is used to split modification requests to a particular file or LUN and the splitter tracks a status of ongoing modification requests. 
     At  302 , a request to create a snapshot of the first storage object may be received, such as by the first storage controller hosting the first storage object. A splitter, for the first storage object, is configured to split write operations to target both the first storage object and the second storage object. For example, the splitter may receive a write operation targeting the first storage object. The splitter may split the write operation into a replication write operation. The splitter may instruct the first storage controller to locally implement the write operation upon the first storage object and instruct the second storage controller to remotely implement the replication write operation upon the second storage object. 
     At  304 , a coordinator workflow may be initialized to issue a drain and hold request to the splitter. The drain and hold request may trigger the splitter to stop processing and start queuing incoming write operations, and to drain current write operations (e.g., complete pending write operations received before the drain and hold request was issued). Once the splitter has stopped processing incoming write operations and has drained current write operations, the second storage object may mirror the first storage object (e.g., the storage objects comprise the same data, and thus have data consistency) and a first snapshot request may be sent to (e.g., issued to) the first storage controller to create a first snapshot of the first storage object and a second snapshot request may be sent to the second storage controller to create a second snapshot of the second storage object, at  306 . In an example, the first snapshot request and the second snapshot request may be sent in parallel. 
     In an example, a timer may be started before the start of a drain with hold phase triggered by the drain with hold operation so that the drain with hold phase is not allowed to continue indefinitely. The timer may be used so that incoming write operations are not held up indefinitely in the event an issue occurs without successfully completing the snapshot requests. In an example, a timeout setting of the timer may be configurable through a policy (e.g., an administrator may set the timeout setting). While the timer is active, incoming write operations are queued by the splitter. In an example where the timer times out before the second snapshot is created, the second snapshot request is abandoned. In an example where the timer times out before creation of at least one of the first snapshot or the second snapshot, the second snapshot request is abandoned. In an example where the timer times out before the first snapshot and/or the second snapshot are created, the queued incoming write operations are de-queued and processed upon the first storage object, and the synchronous replication relationship is transitioned into an out-of-sync state. In an example, an administrator may set a policy to have the synchronous replication relationship stay in-sync in case a timeout is encountered. 
     The first snapshot and the second snapshot may be created while the synchronous replication relationship is in a synchronous state. The first snapshot and the second snapshot may correspond to a common snapshot of a data consistent state between the first storage object and the second storage object (e.g., when the storage objects comprise the same data). Responsive to the first snapshot and the second snapshot being created, the splitter may be instructed to process queued incoming write operations and to resume splitting new incoming write operations. 
     Various snapshot commands may be synchronously implemented for the first snapshot and the second snapshot. In an example, responsive to receiving a delete snapshot command for the first snapshot, the delete snapshot command, used to delete the first snapshot, and a second delete snapshot command, used to delete the second snapshot, may be synchronously implemented. In an example, responsive to receiving a rename snapshot command for the first snapshot, the rename snapshot command, used to rename the first snapshot, and a second rename snapshot command, used to rename the second snapshot, may be synchronously implemented. In an example, the delete snapshot command and/or the rename snapshot command are synchronous implemented for snapshots at the first storage controller and the second storage controller without performing a drain with hold operation. 
     Snapshots may be created for a consistency group comprising a plurality of storage objects, such as a plurality of files and/or LUNs. For example, a request may be received to create a snapshot of a first consistency group comprising a first plurality of storage objects, hosted by the first storage controller, having a synchronous replication relationship with a second consistency group comprising a second plurality of storage objects hosted by the second storage controller. A coordinator workflow is initialized to issue drain with hold requests to splitters configured to split write operations to target both storage objects within the first plurality of storage objects and storage objects within the second plurality of storage objects. In an example, each storage object may be associated with its own splitter. For example, a splitter is configured to split a write operation, targeting a first storage object of the first consistency group, into a replication write operation targeting a second storage object, of the second consistency group, that is a backup replication of the first storage object. 
     Responsive to receiving indications that the splitters (e.g., all of the splitters) have stopped processing and started queuing incoming write operations and have drained current write operations based upon the drain with hold requests, a first snapshot request is sent to (e.g., issued to) the first storage controller to create a first snapshot of the first consistency group and a second snapshot request is sent to the second storage controller to create a second snapshot of the second consistency group. Responsive to the first snapshot and the second snapshot being successfully created, the splitters may be instructed to process queued incoming write operations and to resume splitting new incoming write operations. 
       FIGS. 4A-4E  illustrate examples of a system  400  for snapshot creation.  FIG. 4A  illustrates a first storage controller  404  having a synchronous replication relationship  418  with a second storage controller  410 . The first storage controller  404  and the second storage controller  410  may be configured to communicate with one another over a network  408 . In an example, the synchronous replication relationship  418  may be specified between a first storage object  406  (e.g., a file or LUN within a volume or spanning multiple volumes hosted by the first storage controller  404 ) and a second storage object  412  (e.g., a file or LUN within a volume or spanning multiple volumes hosted by the second storage controller  410 ) that is a backup replication of the first storage object  406 . Accordingly, a write operation  402 , targeting the first storage object  406 , may be intercepted. A splitter  414  for the first storage object  406  may locally implement  416  the write operation  402  upon the first storage object  406 , and may send a copy of the write operation  402  to the second storage controller  410  for remote implementation  420  upon the second storage object  412 . Responsive to both the local implementation  416  and the remote implementation  420  successfully completing, an acknowledgement may be provided back to a client that sent the write operation  402 . 
       FIG. 4B  illustrates a request  430 , to create a snapshot of the first storage object  406 , being received. Accordingly, a coordinator workflow  432  may be initialized to issue a drain with hold request  434  to the splitter  414 . The drain with hold request  434  may instruct the splitter  414  to stop processing and queue  442  incoming write operations and to drain (e.g., complete) current write operations  440  to the first storage object  406  and replicated to the second storage object  412 , as illustrated in  FIG. 4C . In an example, a timer  441  may be started, such as before a drain with hold phase. If the timer  441  times out during draining, then the synchronous replication relationship  418  may be transitioned to an out-of-sync state or may stay in-sync with snapshot creation on the first storage object  406  only. 
       FIG. 4D  illustrates the splitter  414  providing an indication  450  that current write operations have been drained and that incoming write operations are being queued for subsequent processing. Accordingly, a first snapshot request  452  may be sent to (e.g., issued to) the first storage controller  404  to create a first snapshot of the first storage object  406 , and a second snapshot request  454  may be sent to the second storage controller  410  to create a second snapshot of the second storage object  412 . If the timer  441  times out before the first snapshot and/or the second snapshot are created, then the second snapshot request  454  may be abandoned so that incoming write operations are not held up indefinitely. The replication relationship may be transitioned to an out-of-sync state or stay in-sync, depending on a policy setting. 
       FIG. 4E  illustrates the first snapshot  460  being successfully created and available to the first storage controller  404 , and the second snapshot  462  being successfully created and available to the second storage controller  410 . In an example, the first snapshot  460  and the second snapshot  462  may correspond to a common snapshot of a data consistent state between the first storage object  406  and the second storage object  412 , and thus the second storage controller  410  can use the second snapshot  462  for disaster recovery purposes in the event the first storage controller  404  fails (e.g., responsive to a failover from a first site, comprising the first storage controller  404 , to a second site, comprising the second storage controller  410 , due to a disaster at the first site, the second storage controller  410  can bring an application into an operational state with a similar level of functionality as was possible on the first site before the failover). Responsive to the first snapshot  460  and the second snapshot  462  being successfully created, the splitter  414  may be instructed  464  to process queued incoming write operations and to resume splitting new incoming write operations. 
     Still another embodiment involves a computer-readable medium comprising processor-executable instructions configured to implement one or more of the techniques presented herein. An example embodiment of a computer-readable medium or a computer-readable device that is devised in these ways is illustrated in  FIG. 5 , wherein the implementation  500  comprises a computer-readable medium  508 , such as a CD-R, DVD-R, flash drive, a platter of a hard disk drive, etc., on which is encoded computer-readable data  506 . This computer-readable data  506 , such as binary data comprising at least one of a zero or a one, in turn comprises a processor-executable computer instructions  504  configured to operate according to one or more of the principles set forth herein. In some embodiments, the processor-executable computer instructions  504  are configured to perform a method  502 , such as at least some of the exemplary method  300  of  FIG. 3 , for example. In some embodiments, the processor-executable computer instructions  504  are configured to implement a system, such as at least some of the exemplary system  400  of  FIGS. 4A-4E , for example. Many such computer-readable media are contemplated to operate in accordance with the techniques presented herein. 
     It will be appreciated that processes, architectures and/or procedures described herein can be implemented in hardware, firmware and/or software. It will also be appreciated that the provisions set forth herein may apply to any type of special-purpose computer (e.g., file host, storage server and/or storage serving appliance) and/or general-purpose computer, including a standalone computer or portion thereof, embodied as or including a storage system. Moreover, the teachings herein can be configured to a variety of storage system architectures including, but not limited to, a network-attached storage environment and/or a storage area network and disk assembly directly attached to a client or host computer. Storage system should therefore be taken broadly to include such arrangements in addition to any subsystems configured to perform a storage function and associated with other equipment or systems. 
     In some embodiments, methods described and/or illustrated in this disclosure may be realized in whole or in part on computer-readable media. Computer readable media can include processor-executable instructions configured to implement one or more of the methods presented herein, and may include any mechanism for storing this data that can be thereafter read by a computer system. Examples of computer readable media include (hard) drives (e.g., accessible via network attached storage (NAS)), Storage Area Networks (SAN), volatile and non-volatile memory, such as read-only memory (ROM), random-access memory (RAM), EEPROM and/or flash memory, CD-ROMs, CD-Rs, CD-RWs, DVDs, cassettes, magnetic tape, magnetic disk storage, optical or non-optical data storage devices and/or any other medium which can be used to store data. 
     Although the subject matter has been described in language specific to structural features or methodological acts, it is to be understood that the subject matter defined in the appended claims is not necessarily limited to the specific features or acts described above. Rather, the specific features and acts described above are disclosed as example forms of implementing at least some of the claims. 
     Various operations of embodiments are provided herein. The order in which some or all of the operations are described should not be construed to imply that these operations are necessarily order dependent. Alternative ordering will be appreciated given the benefit of this description. Further, it will be understood that not all operations are necessarily present in each embodiment provided herein. Also, it will be understood that not all operations are necessary in some embodiments. 
     Furthermore, the claimed subject matter is implemented as a method, apparatus, or article of manufacture using standard application or engineering techniques to produce software, firmware, hardware, or any combination thereof to control a computer to implement the disclosed subject matter. The term “article of manufacture” as used herein is intended to encompass a computer application accessible from any computer-readable device, carrier, or media. Of course, many modifications may be made to this configuration without departing from the scope or spirit of the claimed subject matter. 
     As used in this application, the terms “component”, “module,” “system”, “interface”, and the like are generally intended to refer to a computer-related entity, either hardware, a combination of hardware and software, software, or software in execution. For example, a component includes a process running on a processor, a processor, an object, an executable, a thread of execution, an application, or a computer. By way of illustration, both an application running on a controller and the controller can be a component. One or more components residing within a process or thread of execution and a component may be localized on one computer or distributed between two or more computers. 
     Moreover, “exemplary” is used herein to mean serving as an example, instance, illustration, etc., and not necessarily as advantageous. As used in this application, “or” is intended to mean an inclusive “or” rather than an exclusive “or”. In addition, “a” and “an” as used in this application are generally be construed to mean “one or more” unless specified otherwise or clear from context to be directed to a singular form. Also, at least one of A and B and/or the like generally means A or B and/or both A and B. Furthermore, to the extent that “includes”, “having”, “has”, “with”, or variants thereof are used, such terms are intended to be inclusive in a manner similar to the term “comprising”. 
     Many modifications may be made to the instant disclosure without departing from the scope or spirit of the claimed subject matter. Unless specified otherwise, “first,” “second,” or the like are not intended to imply a temporal aspect, a spatial aspect, an ordering, etc. Rather, such terms are merely used as identifiers, names, etc. for features, elements, items, etc. For example, a first set of information and a second set of information generally correspond to set of information A and set of information B or two different or two identical sets of information or the same set of information. 
     Also, although the disclosure has been shown and described with respect to one or more implementations, equivalent alterations and modifications will occur to others skilled in the art based upon a reading and understanding of this specification and the annexed drawings. The disclosure includes all such modifications and alterations and is limited only by the scope of the following claims. In particular regard to the various functions performed by the above described components (e.g., elements, resources, etc.), the terms used to describe such components are intended to correspond, unless otherwise indicated, to any component which performs the specified function of the described component (e.g., that is functionally equivalent), even though not structurally equivalent to the disclosed structure. In addition, while a particular feature of the disclosure may have been disclosed with respect to only one of several implementations, such feature may be combined with one or more other features of the other implementations as may be desired and advantageous for any given or particular application.