Patent Publication Number: US-11379405-B2

Title: Internet small computer interface systems extension for remote direct memory access (RDMA) for distributed hyper-converged storage systems

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims priority to International Application No. PCT/CN2019/106151, filed Sep. 17, 2019. The content of the application is hereby incorporated 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 or other computing systems (“nodes”) to aggregate local storage devices (e.g., SSD, PCI-based flash storage, SATA, or SAS magnetic disks) located in or attached to each node 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 nodes 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 computing instances (VCIs) (e.g., virtual machines (VMs), containers, etc.) spawned on host computers or physical machines may use the datastore to store data. In one example, virtual machines may use the datastore to store virtual disks that are accessed by the virtual machines during their operation. The virtual disks may be stored in the datastore in the form of objects, which may also be referred to as virtual disk objects. Nodes in the cluster may access virtual disk objects stored in other nodes in the cluster using a protocol referred to as Small Computer Systems Interface (SCSI), which comprises a set of interfaces that allow nodes in the cluster to access storage resource of other nodes in the cluster. 
     In some cases, to make the data, such as virtual disk objects, available to computing systems (e.g., physical or virtual) outside of the cluster of nodes, each node in the cluster may further be configured with the Internet Small Computer Systems Interface (iSCSI). iSCSI, is an Internet Protocol (IP)-based storage networking standard for linking the nodes in the cluster to the nodes or workloads outside of the distributed storage system. Generally, iSCSI is implemented as a protocol layer to interact with the Transmission Control Protocol (TCP) protocol layer in a network stack of a node within the cluster, thereby, enabling the node to exchange SCSI commands with a node outside the cluster over a network, such as a layer-3 network. However, using the TCP protocol layer may result in low input/output (I/O) performance and high central processing unit (CPU) utilization. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  illustrates an example computing environment in which one or more embodiments may be implemented, according to certain embodiments. 
         FIG. 2  illustrates an example hierarchical structure of objects organized within an object store that represent a virtual disk, according to certain embodiments. 
         FIG. 3  illustrates components of a virtual storage area network module implemented in the computing environment of  FIG. 1 , according to certain embodiments. 
         FIG. 4  illustrates an example network-storage protocol stack, according to certain embodiments. 
         FIG. 5  illustrates an example network-storage protocol stack with an iSCSI extension for RDMA (iSER), according to certain embodiments. 
         FIG. 6  illustrates an example connection lifecycle management procedure between an iSER target and an iSER initiator, according to certain embodiments. 
         FIG. 7  illustrates example operations performed by a network-storage stack at an iSER target for processing an incoming I/O request in the form of an iSER packet, from an iSER initiator, according to certain embodiments. 
         FIG. 8  illustrates an example network-storage protocol stack with an iSER, according to certain embodiments. 
         FIG. 9  illustrates operations performed by network-storage stack at an iSER target for processing an incoming I/O write request in the form of an iSER packet, from an iSER initiator. 
     
    
    
     DETAILED DESCRIPTION 
     Embodiments described herein relate to configuring the network-storage stack of two devices (e.g., physical or virtual) communicating together (e.g., an initiator and a target, as defined below) with iSER, which is a protocol designed to utilize RDMA to accelerate iSCSI data transfer. The iSER protocol is implemented as an iSER datamover layer that acts as an interface between an iSCSI layer and an RDMA layer of the network-storage stacks of the two devices. Using iSER in conjunction with RDMA allows for bypassing the existing traditional network protocol layers (e.g., TCP/IP protocol layers) of the devices and permits data to be transferred directly, between the two devices, using certain memory buffers, thereby avoiding memory copies taking place when the existing network protocol layers are used. 
       FIG. 1  illustrates an example computing environment in which one or more embodiments may be implemented. As shown, computing environment  100  is a software-based “virtual storage area network” (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 nodes  111 . The local commodity storage housed in or otherwise directly attached to each node  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. 
     In addition, as further discussed below, each node  111  may include a storage management module (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. For example, because a VM may be initially configured by an administrator to have specific storage requirements for its “virtual disk” depending on its intended use (e.g., capacity, availability, IOPS, etc.), the administrator may define a storage profile or policy for each VM specifying such availability, capacity, IOPS and the like. As further described below, the VSAN module may then create an “object” for the specified virtual disk by backing it with the datastore of the object store based on the defined policy. 
     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 conventional computing hardware, such as one or more central processing units, random access memory, read-only memory, one or more network interface controllers, SSDs  117 , and magnetic disks  118 ). 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  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 one embodiment, VSAN module  114  is implemented as a “VSAN” device driver within hypervisor  113 . 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 specifies a particular file system for the device object (such device objects hereinafter also thus referred to “file system objects”). For example, 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 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. In certain embodiments, file system objects may further be periodically “auto-unmounted” when access to objects in the file system objects cease or are idle for a period of 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 a virtual machine 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 to 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. 
     A file system object, may, itself, provide access to a number of virtual disk descriptor files 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. Each VSAN module  114  (through a cluster level object management or “CLOM” sub-module) 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 document object manager or “DOM” sub-module, in some embodiments 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 that houses the actual physical local storage that backs the portion of the virtual disk that is subject to the I/O operation. 
       FIG. 2  illustrates an example hierarchical structure of objects organized within object store  116  that represent a virtual disk. 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). 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) stored in VSAN  115  that stores a descriptor file  210  for the virtual disk. 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 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 and partitioned in a RAID 0 configuration. Composite object  200  may thus contain references to a number of “leaf” or “component” objects  220   x  corresponding to each data chunk (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., SDD  117 , magnetic disks  118 , etc.) that actually stores the data chunk (as well as the location of the data chunk within such physical resource). The RAID 1/RAID 0 combination is merely an example of how data associated with a composite object  200  may be stored by nodes  111  (e.g., nodes  111   a - 111   f ) of node cluster  110 . In other examples, all data associated with composite object  200  may be stored in one node (e.g., node  111   a ). In yet another example, data associated with composite object  200  may be only mirrored by a RAID 1 operation such that one copy of the data may be stored in one node and another copy may be stored by another node. In other examples, other RAID operations or a combination of a variety of RAID operations (e.g., RAID1/RAID 5) may be used when distributing data associated with composite object  200 . Regardless of how data associated with a VM  112 &#39;s virtual disk is partitioned or copied across nodes, however, the data is still stored as a plurality of data blocks. 
       FIG. 3  illustrates components of VSAN module  114 . 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 some embodiments, 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 some embodiments, 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 ratio, 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, and more particularly, data blocks of component objects, 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 their 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. 
     As previously discussed, during the handling of I/O operations as well as during object creation, DOM sub-module  340  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 that serves as the coordinator for the particular component object (e.g., data chunk, 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 node  111  running the VM would also have to communicate across the network with the DOM sub-module  340  of the coordinator. In certain embodiments, if the VM issuing the I/O operation resides on a 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 need 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 node  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 , for example, to report whether a storage resource is congested. 
       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 some embodiments, the underlying transport is TCP. Alternatively, other transports such as remote direct memory access (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. 
     As described above, objects in objects store  116  may be made available to computing systems outside node cluster  110 . For example, one or more computing systems may communicate with node cluster  110 , for data storage and retrieval, through a network. A computing system, accessing node cluster  110  for data storage and retrieval, may be referred to as an initiator. Node cluster  110  may be referred to as “storage,” and a node  111  within node cluster  110  that is accessed by the initiator may be referred to as a target. In certain aspects, both the initiator and the target are configured with a network-storage protocol stack that allows the initiator and the target to exchange data over an IP network. A network-storage protocol stack, as further illustrated in  FIG. 4 , comprises different network and storage protocol layers (e.g., SCSI layer, iSCSI layer, TCP layer) that facilitate the exchange of information between the initiator and the target. In certain embodiments, the network-storage protocol stack is implemented in the kernel space of the initiator&#39;s or target&#39;s hypervisor. For example, hypervisor  113  of a target node  111  of node cluster  110  may be configured with a network-storage protocol stack to receive and process read and write requests from an initiator. 
     To illustrate this, in one example, a workload executing on the initiator may need access to a file or certain data stored in node cluster  110 . In such an example, the initiator generates a read request, which is converted to a SCSI command by the SCSI layer. The SCSI command is then converted to an iSCSI command by an iSCSI layer. The iSCSI command is next encapsulated by a TCP/IP layer, resulting in an iSCSI command with TCP/IP headers. For example, in the TCP/IP headers, the TCP/IP layer adds the IP address of the initiator as the source IP address and the destination IP address of the target as the destination IP address. A user configures a target to be available to the initiator and also configures the initiator to communicate with the target when requesting access to information from node cluster  110 . The iSCSI command, comprising the read request, is then processed by a network interface card (NIC) driver of a NIC associated with the initiator. Subsequently, the iSCSI command is transmitted by the NIC over the network to a NIC of the target in node cluster  110 . An iSCSI command that is encapsulated, such as described above, and transmitted over the network may be referred to as an iSCSI Protocol Data Unit (PDU). 
     The iSCSI PDU is then received at a target within node cluster  110  that is configured with the same network-storage protocol stack as the initiator.  FIG. 4  illustrates an example network-storage protocol stack  400  at the target. Network-storage protocol stack  400  comprises a backend layer  402 , a SCSI layer  404 , an iSCSI layer  406 , a TCP/IP datamover layer  408 , a TCP/IP layer  410 , and a NIC driver  412 . Backend layer  402  may refer to a set of software instructions that allow SCSI layer  404  to interface with node cluster  110  in order to retrieve or store information from and in the object store that is provided by node cluster  110 . SCSI layer  404  builds/receives SCSI CDBs (Command Descriptor Blocks) and relays/receives them with the remaining command execute parameters to/from iSCSI Layer  406 . iSCSI layer  406  builds/receives iSCSI PDUs and relays/receives them to/from one or more TCP connections that form an initiator-target “session.” TCP datamover layer  408  provides a set of transport primitives or operations (e.g., connection lifecycle management and PDU transport primitives) that allow the target to manage a connection and communicate with the initiator. TCP/IP layer  410  is configured to decapsulate iSCSI PDUs (e.g., remove TCP/IP headers) or encapsulate SCSI PDUs (e.g., append TCP/IP headers) using TCP/IP instructions. 
     When the iSCSI PDU, transmitted by the initiator, arrives at the target&#39;s NIC, it is processed by NIC driver  412 , which comprises a software program for controlling the target&#39;s NIC. Subsequently, TCP/IP layer  410  de-capsulates the iSCSI PDU packet by removing the TCP and IP headers, thereby extracting the iSCSI command. The iSCSI command is then stored in a memory location in the target&#39;s memory resources (e.g., RAM in hardware  119 ). This memory location is accessible by some of the upper layers, including the TCP/IP datamover layer  408 , the iSCSI layer  406 , and the SCSI layer  404 . As such, each of those upper layers is able to further process and/or de-capsulate the iSCSI command by accessing the iSCSI command at the same memory location. For example, iSCSI layer  406  is able to access the iSCSI command at the memory location and retrieve the SCSI command. 
     When SCSI layer  404  accesses the SCSI command, it allocates a scatter gather list (“sglist”) for the retrieval of the information that is requested by the read request associated with the SCSI command. The sglist is a data structure allocated in memory, with a certain starting memory address and an ending memory address. Backend layer  402  then passes the read request to the node cluster  110  (e.g., VSAN module  114  of the target), which processes the read request by retrieving the requested information from object store  116  and then stores the information in the sglist. Once the information is stored in the sglist, backend layer  402  then passes the ownership of the sglist to SCSI layer  404 , converts the information to a SCSI DATA-IN PDU. iSCSI layer  406  then accesses the SCSI DATA-IN PDU in the sglist and converts the SCSI DATA-IN PDU into a iSCSI command. iSCSI layer  406  then allocates another data structure, referred to as an “mbuffer” or “mbuf,” with a starting and an ending memory address and copies the information in the sglist to the mbuf. This is because TCP/IP datamover layer  408  only recognizes the mbuf data structure. TCP/IP datamover layer  408  then provides the memory address of the mbuf to the TCP/IP layer  410 , which is configured to encapsulate the iSCSI command in the mbuf to create an iSCSI PDU. Once an iSCSI PDU is generated, TCP/IP layer  410  may copy the iSCSI PDU from the mbuf to buffers of NIC driver  412 . Buffers of NIC driver  412  act as queues where outgoing PDUs are stored before being transmitted over the network. 
     Because of the two memory copies discussed above, using the TCP-based protocol layers (TCP/IP datamover layer  408  and TCP/IP layer  410 ) may result in latency as well as an inefficient use of compute resources. Latency is increased due to a network bottleneck associated with having to perform memory copies for each one of a large number of read/write requests to the target. In addition, additional compute cycles have to be utilized for performing such memory copies. 
     Although  FIG. 4  shows memory copies associated with a read command received from an initiator, a memory copy also occurs with respect to a write command received from the initiator. For example, initiator may generate a write command, convert it to a SCSI command, convert the SCSI command to an iSCSI command, encapsulate the iSCSI command with TCP/IP headers, and transmit the iSCSI command with TCP/IP headers to the target. When the iSCSI command with TCP/IP headers is received by the target, the target allocates an mbuf for storing the data that will be received from the initiator later on. The target then sends an R2T (ready to transfer) PDU back to the initiator, indicating that the target is prepared for accepting any incoming DAT-OUT PDUs, that refer to PDUs comprising the data that the initiator intends to send to the target. Once the initiator receives the R2T PDU, it transmits DATA-OUT PDUs to the target. Once the target receives a DATA-OUT PDU, TCP/IP datamover layer  408  stores the data therein in the mbuf. The ownership of the mbuf is then passed through the upper layers, until it reaches the backend layer  402 . However, because VSAN module  114  of the target does not accept or recognize the mbuf, a memory copy has to be performed to move the data included in the mbuf to a data structure (e.g., sglist) that is recognized by node cluster  110 . 
     Accordingly certain embodiments described herein relate to using the iSCSI Extension for Remote Direct Memory Access (RDMA) (iSER), which is a protocol designed to utilize RDMA to accelerate iSCSI data transfer. The iSER protocol is implemented as an iSER datamover layer that acts as an interface between the iSCSI layer and an RDMA layer. In other words, iSER provides the RDMA data transfer capability to the iSCSI layer by layering iSCSI on top of an RDMA-Capable Protocol. Using iSER in conjunction with RDMA allows for bypassing the TCP/IP protocol layers and permits data to be transferred directly, between an initiator and a target, using certain memory buffers, thereby avoiding the memory copies described above. 
     RDMA enables low latency transfer of information between the initiator and the target at the memory-to-memory level, without burdening the CPUs at either the initiator or the target. This transfer function is offloaded to the RDMA-enabled NIC (also referred to as “RNIC”) in order to bypass the operating system&#39;s network stack (e.g., TCP/IP protocol layer). With RDMA, RNICs can work directly with the memory of applications, allowing data transfers over the network without the need to involve the CPU, thereby providing a more efficient and faster way to move data between the initiator and the target at lower latency and CPU utilization. 
       FIG. 5  illustrates an example network-storage protocol stack  500  at the target. Network-storage protocol stack  500  includes backend layer  402 , SCSI layer  404 , iSCSI layer  406 , TCP/IP datamover layer  408 , iSER datamover layer  508 , TCP/IP layer  410 , and RDMA layer  510 . iSER datamover layer  508  functions similar to TCP/IP datamover layer  408  and provides the same transport primitives. iSER datamover layer  508  implements the iSER protocol by providing connection lifecycle management and PDU transport primitives to iSCSI layer  404 , thereby allowing the transfer of iSCSI PDUs through the use of RDMA layer  510 . 
     In the example of  FIG. 5 , because network-storage protocol stack  500  is configured with both TCP/IP datamover layer  408  and iSER datamover layer  508 , the target may utilize any one of the two protocols for data communication. For example, a user may configure the target such that the iSCSI layer  406  may utilize the TCP/IP datamover layer  408  to access the TCP/IP layer  410  when the target is configured with a standard NIC (e.g., a non-RDMA-enabled NIC) or, instead, utilize the iSER datamover layer  508  to access the RDMA layer  510  when the target is configured with an RNIC. In the example of  FIG. 5 , the user configures the target with user configuration  514 , which is consumed by datamover engine  516 . Datamover engine  516  refers to a set of instructions that are used to initialize a datamover layer and establish a link between the initialized datamover layer and iSCSI layer  406 . 
       FIG. 6  illustrates an example flow diagram of how connection lifecycle may be managed between an iSER target  604  and an iSER initiator  602  that are both configured with a network-storage protocol stack having an iSER datamover layer  508  and RDMA layer  510 . An example of such a network-storage protocol stack was shown in  FIG. 5 . An iSER target refers to a target that has been configured with the iSER protocol (e.g., includes the iSER datamover layer in its network-storage stack). An iSER initiator refers to an initiator that has been configured with the iSER protocol (e.g., includes the iSER datamover layer in its network-storage stack). 
     At step  612 , iSER initiator  602  transmits a connection request to iSER target  604 . 
     At step  614 , upon receiving the connection request, iSER target  604  sets up an RDMA queue pair for incoming transport requests. Setting up the RDMA queue pair includes allocating a memory region in the memory of the iSER target, with a starting and an ending address, for operations associated with the RDMA communication between iSER initiator  602  and iSER target  604 . The RDMA communication is based on a set of three queues including a send queue, a receive queue, and a completion queue, which are all instantiated in the allocated memory region. The send and receive queues are responsible for scheduling work and are created in pairs, also referred to as the queue pair and may be referred to as work queues. Work queues are allocated in the allocated memory region and hold instructions as to what data (e.g., messages) stored in buffers (e.g., buffers allocated in memory storing outgoing/incoming messages) are to be sent or received. Such instructions are small structs (e.g., composite data types) and are called work requests or work queue elements (WQE). A WQE includes a pointer to a buffer. For example, a WQE placed on the send queue contains a pointer to a buffer address storing a message to be sent. In another example, a pointer in the WQE on the receive queue contains a pointer to a buffer address for a location in the buffer where an incoming message from the network can be placed. The completion queue is configured to generate a notification when the instructions placed in the work queues have been completed. 
     At step  616 , iSER target  604  allocates a login buffer. A login buffer may also be allocated in the memory region and is configured to store information (e.g., credentials) received from iSER initiator  602  for logging in. 
     At step  618 , iSER target  604  accepts the connection request transmitted by iSER initiator  602 . 
     At step  620 , iSER initiator  602  logs in. For example, iSER initiator  602  transmits information to iSER target  604 , which is stored in the login buffer. 
     At step  622 , iSER target  604  then accesses the information to authenticate and negotiate with iSER initiator  602 . In one example, the negotiation includes determining the maximum number of outstanding iSCSI control-type PDUs that iSER target  604  may hold. Note that iSCSI PDUs that cause the SCSI data to be moved between iSER initiator  602  and iSER target  604  may be referred to as “iSCSI data-type PDUs.” All other possible iSCSI PDUs may be referred to as “iSCSI control-type PDUs.” 
     At step  624 , iSER target  604  allocates multiple memory chunks to store the incoming outstanding iSCSI PDUs. For example, iSER target  604  allocates iSCSI control-type PDU receive buffers. 
     At step  626 , iSER target  604  transmits an indication to iSER initiator  602  that indicates to iSER initiator  602  that the login has been successful. Steps  620  through  626  are performed as part of a phase that is referred to as the login phase. Upon the completion of this phase, iSER target  604  is able to fully perform iSCSI functions such as read and write operations. 
     At step  628 , iSER initiator  602  requests a logout. For example, after the completion of a read operation, iSER initiator  602  sends a logout request to iSER target  604 . 
     At step  630  iSER target  604  releases the iSCSI control-type PDU receive buffers. In some embodiments, a logout may be the result of a connection error, in which case, iSER target  604  removes all the outstanding I/O requests and then releases the iSCSI control-type PDU receive buffers. 
       FIG. 7  illustrates operations  700  performed by network-storage stack  500  at an iSER target for processing an incoming I/O read request in the form of an iSER packet, from an iSER initiator. Network-storage stack  500  and the flow path of the incoming I/O request are shown in  FIG. 8 . Although network-storage stack  500  may, in certain embodiments, also comprise a TCP/IP datamover layer as well as a TCP/IP layer, in the example of  FIG. 8 , those layers are not shown. Operations  700  are described by reference to network-storage stack  500  of  FIG. 8 . 
     At block  702 , the network-storage stack of the iSER target receives an iSER packet. For example, network-storage stack  500  receives an incoming iSER packet. An iSER packet, in some embodiments, may include an iSER header that encapsulates an iSCSI PDU. The iSER header may indicate an identifier (referred to as “STag”) of a remote I/O buffer at the iSER initiator with an RNIC. The identifier informs the iSER target that the remote I/O buffer is available at the iSER initiator for RDMA read or RDMA write access by the iSER target. This remote I/O buffer is where the results of a SCSI read operation may be directly stored in. If the iSER packet includes a write command, the remote I/O buffer is where data associated with the iSCSI write operation may be retrieved from. For example, when an iSER initiator transmits a SCSI read command to an iSER target, the iSER target retrieves the requested data (i.e., results of the SCSI read operation) and transmits the requested data to the remote I/O buffer at the iSER initiator. More specifically, the iSER target writes the requested data to the remote I/O buffer using RDMA layer  510  through an RDMA write operation. 
     For a SCSI write operation, the remote I/O buffer identified by the iSER header contains the data that is to be written to the node cluster  110 . For example, when an iSER initiator transmits a SCSI write command to an iSER target, the iSER target accesses the data stored in the remote I/O buffer and retrieves the data that is stored therein. More specifically, the iSER target reads the data stored in the remote I/O buffer using RDMA layer  510  through an RDMA read operation. In the example of operations  700 , the iSER packet comprises a SCSI read command. In such an example, the iSER packet has an iSER header that identifies a remote I/O buffer where the results of the SCSI read operation will be stored at the iSER initiator. 
     At block  704 , the network-storage stack of the iSER target decapsulates the iSER packet to access an iSCSI PDU. For example, when network-storage stack  500  receives the iSER packet, RDMA layer  510  processes the iSER packet and passes it to iSER datamover layer  508 , which processes the iSER header of the iSER packet and decapsulates the iSER packet by removing the iSER header. Upon processing the iSER header, iSER datamover layer  508  identifies the remote I/O buffer as the location for storing the data that is going to be retrieved from node cluster  110  as a result of the SCSI read operation. Decapsulating the iSER packet results in an iSCSI PDU that comprises the SCSI read command. iSER datamover layer  508  passes the iSCSI PDU to iSCSI layer  406 . 
     At block  706 , the network-storage stack of the iSER target decapsulates the iSCSI PDU to access a SCSI command in the iSCSI PDU. For example, iSCSI layer  406  decapsulates the iSCSI PDU received from iSER datamover layer  508  to access a SCSI read command. 
     At block  708 , the network-storage stack of the iSER target generates a SCSI command structure and places the SCSI command structure in the SCSI layer&#39;s outstanding I/O queue. For example, iSCSI layer  406  generates a SCSI command structure based on the SCSI read request and pushes the SCSI command structure to the SCSI layer  404 &#39;s outstanding I/O queue. 
     At block  710 , the network-storage stack of the iSER target translates the SCSI command to an I/O operation and pushes the I/O operation to an I/O queue of the backend layer. For example, SCSI layer  404  translates the SCSI read command to a read operation and pushes the read operation to an I/O queue of backend layer  402 . 
     At block  712 , the network-storage stack of the iSER target allocates memory at the iSER target&#39;s memory to hold data retrieved as a result of the I/O operation. For example, SCSI layer  404  allocates a scatter gather list (sglist) for holding the data. As discussed, scatter-gather is a type of memory addressing used to do direct memory access (DMA) data transfers of data that is written to noncontiguous areas of memory. A sglist is a list of vectors, each of which gives the location and length of one segment in the overall read or write request. 
     At block  714 , the network-storage stack of the iSER target processes the I/O operation and stores the resulting data in the memory location allocated at step  712  (e.g., the sglist). Backend layer  402  has several threads that work to process I/O requests that are placed in the I/O queue of the backend layer  402 . For example, a thread processes the read request pushed by SCSI layer  404  to the I/O queue of backed layer  402 . Another thread may then pass the read request to the VSAN module (VSAN module  114 ) of the iSER target to retrieve data requested by the read request. As described above, VSAN module  114  comprises a DOM sub-module  340  that handles I/O operations. For example, DOM sub-module  340  handles a read request by accessing object store  116  and retrieving the data requested by the read request. SCSI layer  404  also passes the sglist to backend layer  402 , which in turn passes the sglist to VSAN module  114  to store the retrieved data in the sglist. In certain embodiments, passing the sglist to backend layer  402  may include indicating the starting and ending memory addresses of the sglist. In certain embodiments, passing the sglist to backend layer  402  may also include assigning the ownership of the sglist to backend layer  402 . 
     Once the read request is processed, VSAN module  114  stores the resulting data in the sglist. Backend layer  402  then passes the ownership of the sglist, which at this points stores the resulting data, to SCSI layer  404 . SCSI layer  404  then accesses the data in the sglist and creates a SCSI DATA-IN PDU, comprising the data, by, for example, adding any necessary encapsulation data. The SCSI DATA-IN PDU is stored in the sglist. SCSI layer  404  then notifies iSCSI layer about the sglist&#39;s memory location. 
     At block  716 , the network-storage stack of the iSER target generates an iSCSI PDU comprising the data. For example, iSCSI layer  406  accesses the SCSI DATA-IN PDU in the sglist and generates an iSCSI PDU comprising the SCSI DATA-IN PDU, which itself comprises the data resulting from the processing of the read request. The iSCSI layer  406  creates the iSCSI PDU by, for example, adding any necessary encapsulation information to the SCSI DATA-IN PDU that is stored in the sglist. The iSCSI PDU is stored in the sglist. iSCSI layer  406  then notifies iSER layer  406  of the memory location (e.g., starting and ending memory addresses) of sglist. Upon passing over the iSCSI PDU to iSER layer  406 , iSER layer  406  becomes the owner of the iSCSI PDU or the data therein. 
     At block  718 , the network-storage stack of the iSER target generates an iSER packet using the iSCSI PDU. For example, iSER layer  406  encapsulates the iSCSI PDU with an iSER header in the sglist by, for example, adding the iSER header to the iSCSI PDU. The iSER header comprises the identifier of the remote I/O buffer. Subsequently, iSER layer  406  notifies iSER datamover layer  508  of the memory location of the sglist. iSER datamover layer  508  then communicates with RMDA layer  510  to send out the iSER packet as a RDMA write operation. 
     At block  720 , the network-storage stack of the iSER target transmits the iSER packet to the iSER initiator. For example, RDMA layer  510  transmits the iSER packet, including a RDMA write operation, to the RDMA layer of the iSER initiator. The network-storage stack of the iSER initiator receives the iSER packet, accesses the data within the iSER packet, and stores the data in the remote buffer. 
       FIG. 9  illustrates operations  900  performed by network-storage stack  500  at an iSER target for processing an incoming I/O write request in the form of an iSER packet, from an iSER initiator. At block  902 , the network-storage stack of the iSER target receives an iSER packet. Block  902  is similar to block  702  of  FIG. 7 , with the exception that iSER packet in operations  900  comprises an SCSI write command. The iSER header of the iSER packet includes a remote key and a remote I/O buffer, which stores the data that the iSER initiator intends to write to node cluster  110 . 
     At block  904 , the network-storage stack of the iSER target decapsulates the iSER packet to access an iSCSI PDU. For example, when network-storage stack  500  receives the iSER packet, RDMA layer  510  processes the iSER packet and passes it to iSER datamover layer  508 , which processes the iSER header of the iSER packet and decapsulates the iSER packet by removing the iSER header. Upon processing the iSER header, iSER datamover layer  508  identifies the remote I/O buffer offset associated with a remote I/O buffer, which includes data that the initiator intends to write to node cluster  110 . iSER datamover layer  508  also stores the remote key and remote I/O buffer in memory. Decapsulating the iSER packet results in an iSCSI PDU that comprises the SCSI write command. iSER datamover layer  508  passes the iSCSI PDU to iSCSI layer  406 . 
     At block  906 , the network-storage stack of the iSER target decapsulates the iSCSI PDU to access a SCSI command in the iSCSI PDU. For example, iSCSI layer  406  decapsulates the iSCSI PDU received from iSER datamover layer  508  to access a SCSI write command. 
     At block  908 , the network-storage stack of the iSER target allocates a data structure in memory for storing data associated with the SCSI write command and transmits an R2T PDU to the iSER initiator to indicate that the iSER target is ready to receive the data. For example, iSCSI layer  406  decapsulates the iSCSI PDU received from iSER datamover layer  508  to access a SCSI write command. When the SCSI write command reaches SCSI layer  404 , SCSI layer  404  allocates a sglist in memory for storing the data. SCSI layer  404  then indicates to iSCSI layer  406  that the iSER target is now ready to receive the data. ISCSI layer  406  then transmits an R2T PDU to the iSER datamover layer  508 , which iSER datamover layer  508  translates into an RDMA read operation. ISER datamover layer  508  then transmits the R2T PDU to the iSER initiator. ISER datamover layer  508  also feeds the remote key and remote I/O buffer offset to RDMA layer  510 . 
     At block  910 , the network-storage stack of the iSER target performs an RDMA read operation to read data from the iSER initiator and store it in the allocated memory. For example RDMA layer  510  performs an RDMA read operation to read data that is stored in the remote I/O buffer at the iSER initiator using the remote key and the remote I/O buffer offset. The data is then stored by RDMA layer  510  in the sglist. At this time, iSER datamover layer  508  notifies iSCSI layer  404  that the data is stored in the allocated memory and it is ready for a write operation requested by the SCSI write command (ready to be stored in node cluster  110 ). 
     At block  912 , the network-storage stack of the iSER target causes a write operation associated with the SCSI write command to be performed using the data stored in the allocated data structure. For example, iSCSI  404  passes the ownership of sglist, including the data, to backend layer  402 , which in turn passes the ownership of sglist to node cluster  110  (e.g., VSAN module  114  of the iSER target). VSAN module  114  of the iSER target then causes the write operation to be performed by node cluster  110 . Causing the write operation to be performed by node cluster  110  comprises indicating to node cluster  110 , through backend layer  402 , that node cluster  110  has ownership of the sglist, which includes the data for the write operation. Node cluster  110  then performs the write operation by accessing the sglist and using the data. In operations  900 , because a data structure that is recognized by node cluster  110  is allocated and used, no memory copies have to be performed, resulting in a more resource efficient and expeditious write operation. 
     Accordingly, the embodiments described herein provide a technical solution to a technical problem by using iSER in conjunction with RDMA, which allows for bypassing the TCP/IP protocol layers of a target or an initiator and permits data to be transferred directly, between an initiator and a target, using certain memory buffers, thereby avoiding the memory copies associated with the use of the TCP/IP protocol layers. Note that although some aspects of the disclosure are described with respect to a VM accessing a VSAN cluster, aspects can similarly be used for any virtual computing instance (VCI) or physical machine accessing any suitable distributed storage system (e.g., hyper-converged storage). 
     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. 
     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). In the claims, elements and/or steps do not imply any particular order of operation, unless explicitly stated in the claims.