Patent Publication Number: US-2023152992-A1

Title: Force provisioning virtual objects in degraded stretched clusters

Description:
BACKGROUND 
     A data center is a facility that houses servers, data storage devices, and/or other associated components such as backup power supplies, redundant data communications connections, environmental controls such as air conditioning and/or fire suppression, and/or various security systems. A data center may be maintained by an information technology (IT) service provider. An enterprise may purchase data storage and/or data processing services from the provider in order to run applications that handle the enterprises&#39; core business and operational data. The applications may be proprietary and used exclusively by the enterprise or made available through a network for anyone to access and use. 
     A software defined data center (SDDC) can include objects, which may be referred to as virtual objects. Virtual objects, such as virtual computing instances (VCIs), for instance, have been introduced to lower data center capital investment in facilities and operational expenses and reduce energy consumption. A VCI is a software implementation of a computer that executes application software analogously to a physical computer. Virtual objects have the advantage of not being bound to physical resources, which allows virtual objects to be moved around and scaled to meet changing demands of an enterprise without affecting the use of the enterprise&#39;s applications. In a software defined data center, storage resources may be allocated to virtual objects in various ways, such as through network attached storage (NAS), a storage area network (SAN) such as fiber channel and/or Internet small computer system interface (iSCSI), a virtual SAN, and/or raw device mappings, among others. 
     A storage policy can be specified for a virtual object. In the absence of resources sufficient to satisfy a storage policy, previous approaches may force provision the object with the simplest possible layout. Such approaches may leave the object with a low level of protection. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG.  1    is a diagram of a host and a system for force provisioning virtual objects in degraded stretched clusters according to one or more embodiments of the present disclosure. 
         FIG.  2    is a diagram of a stretched cluster according to one or more embodiments of the present disclosure. 
         FIG.  3    is a flow chart associated with force provisioning virtual objects in degraded stretched clusters according to one or more embodiments of the present disclosure. 
         FIG.  4    is a diagram of a system for force provisioning virtual objects in degraded stretched clusters according to one or more embodiments of the present disclosure. 
         FIG.  5    is a diagram of a machine for force provisioning virtual objects in degraded stretched clusters according to one or more embodiments of the present disclosure. 
         FIG.  6    illustrates a method for force provisioning virtual objects in degraded stretched clusters according to one or more embodiments of the present disclosure. 
     
    
    
     DETAILED DESCRIPTION 
     The term “virtual computing instance” (VCI) refers generally to an isolated user space instance, which can be executed within a virtualized environment. Other technologies aside from hardware virtualization can provide isolated user space instances, also referred to as data compute nodes. Data compute nodes may include non-virtualized physical hosts, VCIs, containers that run on top of a host operating system without a hypervisor or separate operating system, and/or hypervisor kernel network interface modules, among others. Hypervisor kernel network interface modules are non-VCI data compute nodes that include a network stack with a hypervisor kernel network interface and receive/transmit threads. 
     VCIs, in some embodiments, operate with their own guest operating systems on a host using resources of the host virtualized by virtualization software (e.g., a hypervisor, virtual machine monitor, etc.). The tenant (i.e., the owner of the VCI) can choose which applications to operate on top of the guest operating system. Some containers, on the other hand, are constructs that run on top of a host operating system without the need for a hypervisor or separate guest operating system. The host operating system can use name spaces to isolate the containers from each other and therefore can provide operating-system level segregation of the different groups of applications that operate within different containers. This segregation is akin to the VCI segregation that may be offered in hypervisor-virtualized environments that virtualize system hardware, and thus can be viewed as a form of virtualization that isolates different groups of applications that operate in different containers. Such containers may be more lightweight than VCIs. 
     Where the present disclosure refers to VCIs, the examples given could be any type of virtual object, including data compute node, including physical hosts, VCIs, non-VCI containers, virtual disks, and hypervisor kernel network interface modules. Embodiments of the present disclosure can include combinations of different types of virtual objects (which may simply be referred to herein as “objects”). As used herein, a container encapsulates an application in a form that&#39;s portable and easy to deploy. Containers can run without changes on any compatible system—in any private cloud or public cloud—and they consume resources efficiently, enabling high density and resource utilization. Although containers can be used with almost any application, they may be frequently associated with microservices, in which multiple containers run separate application components or services. The containers that make up microservices are typically coordinated and managed using a container orchestration platform. 
     As used herein, a “disk” is a representation of storage policy resources that are used by an object. As used herein, “storage policy resource” includes secondary or other storage (e.g., mass storage such as hard drives, solid state drives, removable media, etc., which may include non-volatile memory). The term “disk” implies a single physical memory device being used by an object. 
     A cluster is a collection of hosts and associated virtual machines intended to work together as a unit. A stretched cluster is a cluster spanning more than one site (e.g., geographic location). A stretched cluster can provide a higher level of availability and intersite load balancing. Stretched clusters may typically be deployed in environments where the distance between data centers is limited, such as metropolitan or campus environments. Stretched clusters can be used to manage planned maintenance and avoid disaster scenarios, because maintenance or loss of one site does not affect the overall operation of the cluster. In a stretched cluster configuration, both sites are active sites. If either site fails, the storage on the other site can be used. A stretched cluster can be said to be in a “degraded state” when one or more hosts of that cluster have failed. 
     When an object, such as a container, is created or modified, a storage policy for that VCI may be specified. This specification may be made by a user (e.g., a customer). A storage policy, as referred to herein, is a policy that defines storage requirements for an object. Storage requirements can refer to stripe width and/or mirror quantity, among other requirements. A storage policy can determine how the objects are provisioned and allocated within a datastore to provide a desired level of performance and/or availability. 
     A Redundant Array of Independent (or Inexpensive) Disks (RAID) can employ a flexible stripe width and can allow for flexible quantities of mirrors. RAID is an umbrella term for computer information storage schemes that divide and/or replicate information among multiple memory devices, for instance. The multiple memory devices in a RAID array may appear to a user and the operating system of a computer as a single memory device (e.g., disk). RAID can include striping (e.g., splitting) information so that different portions of the information are stored on different memory devices. The portions of the more than one device that store the split data are collectively referred to as a stripe. In contrast, RAID can also include mirroring, which can include storing duplicate mirrors (e.g., copies) of data on more than one device. 
     A storage policy can specify a stripe width and/or a mirror quantity. A stripe width, which may alternatively be referred to as a number of stripes per object, is a quantity of memory devices (e.g., disks) across which each replica of an object is striped. For example, a stripe width can range from 1 to 12. Stripes may be spread across memory devices of the same or different types, such as magnetic disks, flash devices, or other memory device types, such as those described with respect to the memory resources  510  illustrated in  FIG.  5   . Mirror quantity refers to a quantity of mirrors (e.g., copies) of data. Mirror quantity can refer to a single mirror or to a plurality of mirrors. For instance, RAID 1 can be configured with one or more mirrors of a set of data on one or more separate disks. 
     Whether a storage policy specified for an object can be satisfied by a software defined datacenter depends on the availability of storage policy resources. Storage policy resources can refer to a quantity of disks and/or an amount of disk space. Thus, a storage policy may not be satisfiable if storage policy resources are insufficient (e.g., unavailable). It is noted that storage policy resources may be simply referred to herein as “resources.” 
     If, however, force provisioning is enabled by a policy setting, the object can be force provisioned and deployed. Force provisioning means that the object is provisioned with resources other than those specified in the storage policy for the object. However, consumers may not be provided the ability to create or edit a storage policy, and may not be aware of infrastructure layer failures. Further, in a stretched cluster configuration, previous approaches may only provide two options: failure of deployment, or deployment without any protection when force provisioning is enabled. Stated differently, if a user desires to create an object and selects a storage policy, the provisioning either fails (in the absence of force provisioning), or if there are reasons why the policy cannot be met (e.g., failures in the cluster), the provisioning may succeed but the object created is left with minimal protection. 
     For example, in previous approaches, force provisioning may include force provisioning objects with the simplest possible layout. For example, force provisioning in previous approaches may include a single component RAID 0 configuration with no performance or protection. In previous approaches, if a storage policy specified a RAID 1 mirrored copy of data but there were insufficient resources to satisfy this specification, no attempt to create even a single RAID 1 mirror would be made; rather, a RAID 0 object would be provisioned, which provides no protection. 
     In an example, a stretched cluster includes a first site and a second site, each having four hosts. A storage policy specifies that an object be provisioned with RAID-1 across the two sites and within each site. A failure of a single host at the first site does not affect the provisioning because resources are still sufficient to satisfy the policy. 
     A failure of two hosts at the first site differs, however. If force provisioning is not enabled, the object will not be provisioned as there are insufficient resources to satisfy the policy. Stated differently, if a failure in a stretched cluster would prevent the creation of the fully requested RAID configuration as specified within the policy, then provisioning may be fully blocked. In an environment where there are little to no transient workloads, this may be desirable. However, in an environment with transient workloads, it may be more important that objects get created (rather than not created), as blocking their creation could negatively impact efficiency. An example of transient workloads is a “persistent volume,” which may be requested by developers during the provisioning of containerized applications, for instance. 
     If force provisioning is enabled, the object (under previous approaches) may be provisioned across sites with RAID-0, within the first site with RAID-0, and within the second site with RAID-0. As a result, the object is provisioned but does not adhere to the specified policy in any respect. As a result, until the failure is resolved, the object(s) will run without any resiliency within each of the stretched cluster sites. If the two failed hosts subsequently come back online the RAID configurations for both the first site and the second site may then be rebuilt to RAID-1 in order to adhere to the specified policy. This rebuilding involves costly traffic and stress to the SDDC. 
     Embodiments of the present disclosure include a “smart forced provisioning” for stretched clusters that can allow the provisioning of objects regardless of an infrastructure failure, while providing the highest possible level of protection in this degraded state. For instance, in some embodiments, an object is provisioned when at least one full RAID tree is available in at least one configured site of the stretched cluster, even though the stretched cluster—and subsequently the object—is in a degraded state. 
     It is noted that each object can be deployed as a RAID tree where each branch of the tree is a component. For instance, if a virtual disk is deployed with a stripe width of two, then a RAID-0 stripe would be configured across two, or more, physical devices (e.g., disks) for this virtual disk. The virtual disk would be the object, and each of the stripes of the virtual disk would be a component of that object. In a stretched cluster, a VCI may be configured (via, e.g., a VCI-level storage policy) to be stretched across sites from a storage perspective, such that the VCI&#39;s associated storage objects (e.g., virtual disk (VMDK) objects, namespace object, swap objects, etc.) are replicated via RAID-1 mirroring at each site. In addition, the replica copies maintained at each site may be further protected, within that individual site, via RAID-1 mirroring or RAID-5/6 parity striping. In an example, a VCI with a storage object (e.g., a VMDK) is deployed in a stretched cluster according to a storage policy specifying a “Primary Failures to Tolerate” (PFTT) parameter value of 1 (indicating that the VCI is a stretched VCI and thus its storage objects should be replicated across sites) and a “Secondary Failures to Tolerate” (SFTT) parameter value of 1 (indicating that the VCI&#39;s storage objects should be protected by RAID-1 mirroring within each site). Per the PFTT parameter value of 1, a replica copy of the storage object is stored at each site of the cluster. Further, per the SFTT parameter value of 1, the replica copy at each site is mirrored via RAID-1 within that site. 
     As previously discussed, embodiments herein can provision an object when at least one full RAID tree is available in at least one configured site of the stretched cluster, even though the stretched cluster—and subsequently the object—is in a degraded state. Consider the example above of a stretched cluster including a first site and a second site, each having four hosts, and a storage policy specifying that an object be provisioned with RAID-1 across the two sites and within each site. Embodiments of the present disclosure may create a RAID-0 configuration in the first site as it has two failed hosts, but in the second site with no failed hosts, embodiments herein can create a full RAID-1 configuration. Compared with previous approaches, this provides an extra level of availability because additional failures would be tolerable before the object becomes inaccessible. 
     After smart force provisioning, in some cases, storage policy resources sufficient to satisfy the storage policy specified for the object may become available. In contrast with previous approaches, embodiments herein may include rebuilding only one RAID configuration, as the first site was the only site that was reduced to RAID-0. 
     The figures herein follow a numbering convention in which the first digit or digits correspond to the drawing figure number and the remaining digits identify an element or component in the drawing. Similar elements or components between different figures may be identified by the use of similar digits. For example,  114  may reference element “14” in  FIG.  1   , and a similar element may be referenced as  414  in  FIG.  4   . As will be appreciated, elements shown in the various embodiments herein can be added, exchanged, and/or eliminated so as to provide a number of additional embodiments of the present disclosure. In addition, as will be appreciated, the proportion and the relative scale of the elements provided in the figures are intended to illustrate certain embodiments of the present invention, and should not be taken in a limiting sense. 
       FIG.  1    is a diagram of a host and a system for force provisioning virtual objects in degraded stretched clusters according to one or more embodiments of the present disclosure. The system can include a host  102  with processing resources  108  (e.g., a number of processors), memory resources  110 , and/or a network interface  112 . The host  102  can be included in a software defined data center. A software defined data center can extend virtualization concepts such as abstraction, pooling, and automation to data center resources and services to provide information technology as a service (ITaaS). In a software defined data center, infrastructure, such as networking, processing, and security, can be virtualized and delivered as a service. A software defined data center can include software defined networking and/or software defined storage. In some embodiments, components of a software defined data center can be provisioned, operated, and/or managed through an application programming interface (API). 
     The host  102  can incorporate a hypervisor  104  that can execute a number of virtual computing instances  106 - 1 ,  106 - 2 , . . . ,  106 -N (referred to generally herein as “VCIs  106 ”). The VCIs can be provisioned with processing resources  108  and/or memory resources  110  and can communicate via the network interface  112 . The processing resources  108  and the memory resources  110  provisioned to the VCIs can be local and/or remote to the host  102 . For example, in a software defined data center, the VCIs  106  can be provisioned with resources that are generally available to the software defined data center and not tied to any particular hardware device. By way of example, the memory resources  110  can include volatile and/or non-volatile memory available to the VCIs  106 . The VCIs  106  can be moved to different hosts (not specifically illustrated), such that a different hypervisor manages the VCIs  106 . The host  102  can be in communication with a force provisioning system  114 . An example of the force provisioning system is illustrated and described in more detail below. In some embodiments, the force provisioning system  114  can be a server, such as a web server. 
       FIG.  2    is a diagram of a stretched cluster according to one or more embodiments of the present disclosure. As shown in  FIG.  2   , the cluster includes a first site (Site A) and a second site (Site B). As shown, Site A includes a first plurality of hosts, including a host1  202 - 1 , a host2, 202 - 2 , a host3  202 - 3 , and a host4  202 - 4 . Site B includes a second plurality of hosts, including a host5  202 - 5 , a host6,  202 - 6 , a host7  202 - 7 , and a host8  202 - 8 . Also illustrated in  FIG.  2   , the cluster includes a witness host  216 . The witness host  216  can reside at a third site and can contains the witness components of virtual machine objects. The witness host  216  can contain only metadata, and does not participate in storage operations. The witness host  216  serves as a tiebreaker when a decision must be made regarding availability of datastore components when the network connection between Site A and Site B is lost. In this case, the witness host typically forms a cluster with the “preferred” site (e.g., Site A). However, if the preferred site becomes isolated from the secondary site and the witness host  216 , the witness host  216  forms a cluster using the secondary site. When the preferred site is online again, data is resynchronized to ensure that both sites have the latest copies of all data. 
       FIG.  3    is a flow chart associated with force provisioning virtual objects in degraded stretched clusters according to one or more embodiments of the present disclosure. At  318 , a request to provision (e.g., create) an object during a degraded state is received. Referring back to the example illustrated in  FIG.  2   , a degraded state may be determined by the failure of two hosts (e.g., host3  202 - 3  and host4  202 - 4 ). At  330 , a determination is made regarding whether force provisioning is enabled. If not, the object is not provisioned at  322 . If force provisioning is enabled, a determination is made at  324  regarding whether “smart force provisioning” is enabled. If not, the object is provisioned with RAID-0 in both locations (Site A and Site B). If “smart force provisioning” is enabled, a determination is made at  328  of whether one site or both sites are degraded. It is noted that “smart force provisioning” is automatically enabled, in some embodiments. In other embodiments, “smart force provisioning” may be optional and indicated via an interface (e.g., user activation of “smart force provisioning.” 
     If both sites are determined to be degraded, the object is provisioned, at  330 , with RAID-0 in both locations. If only one site is determined to be degraded (Site A, in this example), then the object can be provisioned, at  332 , with RAID-0 in the degraded site (Site A) and with the requested RAID configuration in the undegraded (e.g., healthy) site (Site B). Stated differently, even though a full RAID tree cannot be configured in Site A, Site A is configured with RAID-0 and, if RAID-1 was specified in the policy, Site B is configured with a full RAID-1 object. 
       FIG.  4    is a diagram of a system  414  for force provisioning virtual objects in degraded stretched clusters according to one or more embodiments of the present disclosure. The system  414  can include a database  434 , a subsystem  436 , and/or a number of engines, for example request engine  438 , insufficiency engine  440 , force provision engine  442 , and can be in communication with the database  434  via a communication link. The system  414  can include additional or fewer engines than illustrated to perform the various functions described herein. The system can represent program instructions and/or hardware of a machine (e.g., machine  544  as referenced in  FIG.  5   , etc.). As used herein, an “engine” can include program instructions and/or hardware, but at least includes hardware. Hardware is a physical component of a machine that enables it to perform a function. Examples of hardware can include a processing resource, a memory resource, a logic gate, an application specific integrated circuit, a field programmable gate array, etc. 
     The number of engines can include a combination of hardware and program instructions that is configured to perform a number of functions described herein. The program instructions (e.g., software, firmware, etc.) can be stored in a memory resource (e.g., machine-readable medium) as well as hard-wired program (e.g., logic). Hard-wired program instructions (e.g., logic) can be considered as both program instructions and hardware. 
     In some embodiments, the request engine  438  can include a combination of hardware and program instructions that is configured to receive, by an SDDC, a request to provision a virtual object by a stretched cluster with a first RAID configuration according to a storage policy specified as part of the request, wherein the stretched cluster includes a first site and a second site. In some embodiments, the insufficiency engine  440  can include a combination of hardware and program instructions that is configured to determine an insufficiency of storage policy resources at the first site to satisfy the RAID configuration specified for the virtual object. In some embodiments, the force provision engine  442  can include a combination of hardware and program instructions that is configured to force provision the virtual object responsive to a determination that storage policy resources at the second site are sufficient to configure a full RAID tree associated with the virtual object. 
       FIG.  5    is a diagram of a machine  544  for force provisioning virtual objects in degraded stretched clusters according to one or more embodiments of the present disclosure. The machine  544  can utilize software, hardware, firmware, and/or logic to perform a number of functions. The machine  544  can be a combination of hardware and program instructions configured to perform a number of functions (e.g., actions). The hardware, for example, can include a number of processing resources  508  and a number of memory resources  510 , such as a machine-readable medium (MRM) or other memory resources  510 . The memory resources  510  can be internal and/or external to the machine  544  (e.g., the machine  544  can include internal memory resources and have access to external memory resources). In some embodiments, the machine  544  can be a VCI. The program instructions (e.g., machine-readable instructions (MRI)) can include instructions stored on the MRM to implement a particular function (e.g., an action such as configuring a full RAID tree associated with a virtual object at a second site, as described herein). The set of MRI can be executable by one or more of the processing resources  508 . The memory resources  510  can be coupled to the machine  544  in a wired and/or wireless manner. For example, the memory resources  510  can be an internal memory, a portable memory, a portable disk, and/or a memory associated with another resource, e.g., enabling MRI to be transferred and/or executed across a network such as the Internet. As used herein, a “module” can include program instructions and/or hardware, but at least includes program instructions. 
     Memory resources  510  can be non-transitory and can include volatile and/or non-volatile memory. Volatile memory can include memory that depends upon power to store information, such as various types of dynamic random access memory (DRAM) among others. Non-volatile memory can include memory that does not depend upon power to store information. Examples of non-volatile memory can include solid state media such as flash memory, electrically erasable programmable read-only memory (EEPROM), phase change memory (PCM),  3 D cross-point, ferroelectric transistor random access memory (FeTRAM), ferroelectric random access memory (FeRAM), magneto random access memory (MRAM), Spin Transfer Torque (STT)-MRAM, conductive bridging RAM (CBRAM), resistive random access memory (RRAM), oxide based RRAM (OxRAM), negative-or (NOR) flash memory, magnetic memory, optical memory, and/or a solid state drive (SSD), etc., as well as other types of machine-readable media. 
     The processing resources  508  can be coupled to the memory resources  510  via a communication path  546 . The communication path  546  can be local or remote to the machine  544 . Examples of a local communication path  546  can include an electronic bus internal to a machine, where the memory resources  510  are in communication with the processing resources  508  via the electronic bus. Examples of such electronic buses can include Industry Standard Architecture (ISA), Peripheral Component Interconnect (PCI), Advanced Technology Attachment (ATA), Small Computer System Interface (SCSI), Universal Serial Bus (USB), among other types of electronic buses and variants thereof. The communication path  546  can be such that the memory resources  510  are remote from the processing resources  508 , such as in a network connection between the memory resources  510  and the processing resources  508 . That is, the communication path  546  can be a network connection. Examples of such a network connection can include a local area network (LAN), wide area network (WAN), personal area network (PAN), and the Internet, among others. 
     As shown in  FIG.  5   , the MRI stored in the memory resources  510  can be segmented into a number of modules  538 ,  540 ,  542  that when executed by the processing resources  508  can perform a number of functions. As used herein a module includes a set of instructions included to perform a particular task or action. The number of modules  538 ,  540 ,  542  can be sub-modules of other modules. For example, the force provision module  542  can be a sub-module of the insufficiency module  540  and/or can be contained within a single module. Furthermore, the number of modules  538 ,  540 ,  542  can comprise individual modules separate and distinct from one another. Examples are not limited to the specific modules  538 ,  540 ,  542  illustrated in  FIG.  5   . 
     Each of the number of modules  538 ,  540 ,  542  can include program instructions and/or a combination of hardware and program instructions that, when executed by a processing resource  508 , can function as a corresponding engine as described with respect to  FIG.  4   . For example, the request module  538  can include program instructions and/or a combination of hardware and program instructions that, when executed by a processing resource  508 , can function as the request engine  438 , though embodiments of the present disclosure are not so limited. 
     The machine  544  can include a request module  538 , which can include instructions to receive, by a SDDC, a request to provision a virtual object by a stretched cluster according to a storage policy specified as part of the request, wherein the stretched cluster includes a first site and a second site. The machine  544  can include an insufficiency module  540 , which can include instructions to determine an insufficiency of storage policy resources at the first site to satisfy the storage policy specified for the virtual object. The machine  544  can include a force provision module  542 , which can include instructions to force provision the virtual object responsive to a determination of storage policy resources sufficient to satisfy the storage policy at the second site. 
     Some embodiments can include instructions to force provision the virtual object responsive to a determination that storage policy resources at the second site are sufficient to configure a full RAID tree associated with the virtual object. The machine  544  can include instructions to configure less than the full RAID tree associated with the virtual object at the first site. The machine  544  can include instructions to configure the full RAID tree associated with the virtual object at the second site. 
       FIG.  6    illustrates a method for force provisioning virtual objects in degraded stretched clusters according to one or more embodiments of the present disclosure. The method can be performed by a server (e.g., the force provisioning system  114 , previously described in connection with  FIG.  1   ), though embodiments of the present disclosure are not so limited. 
     At  648 , the method can include receiving, by a SDDC, a request to provision a virtual object by a stretched cluster according to a storage policy specified as part of the request, wherein the stretched cluster includes a plurality of sites. The storage policy can include a requested mirror quantity, for instance. 
     At  650 , the method can include determining an insufficiency of storage policy resources to satisfy the storage policy specified for the virtual object. In some embodiments, determining an insufficiency can include determining an insufficiency of storage policy resources to satisfy the requested virtual object mirror quantity. 
     At  652 , the method can include force provisioning the virtual object responsive to determining storage policy resources sufficient to satisfy the storage policy at one of the plurality of sites. In some embodiments, force provisioning can include force provisioning the object responsive to determining storage policy resources sufficient to satisfy the requested virtual object mirror quantity at one of the plurality of sites. In some embodiments, force provisioning can include force provisioning the virtual object with a reduced mirror quantity. A reduced mirror quantity (e.g.,  0 ) can be force provisioned responsive to determining storage policy resources insufficient to satisfy the requested virtual object mirror quantity at one of the plurality of sites and determining storage policy resources insufficient to satisfy the requested virtual object mirror quantity at another one of the plurality of sites. 
     The present disclosure is not limited to particular devices or methods, which may vary. The terminology used herein is for the purpose of describing particular embodiments, and is not intended to be limiting. As used herein, the singular forms “a”, “an”, and “the” include singular and plural referents unless the content clearly dictates otherwise. Furthermore, the words “can” and “may” are used throughout this application in a permissive sense (i.e., having the potential to, being able to), not in a mandatory sense (i.e., must). The term “include,” and derivations thereof, mean “including, but not limited to.” 
     Although specific embodiments have been described above, these embodiments are not intended to limit the scope of the present disclosure, even where only a single embodiment is described with respect to a particular feature. Examples of features provided in the disclosure are intended to be illustrative rather than restrictive unless stated otherwise. The above description is intended to cover such alternatives, modifications, and equivalents as would be apparent to a person skilled in the art having the benefit of this disclosure. 
     The scope of the present disclosure includes any feature or combination of features disclosed herein (either explicitly or implicitly), or any generalization thereof, whether or not it mitigates any or all of the problems addressed herein. Various advantages of the present disclosure have been described herein, but embodiments may provide some, all, or none of such advantages, or may provide other advantages. 
     In the foregoing Detailed Description, some features are grouped together in a single embodiment for the purpose of streamlining the disclosure. This method of disclosure is not to be interpreted as reflecting an intention that the disclosed embodiments of the present disclosure have to use more features than are expressly recited in each claim. Rather, as the following claims reflect, inventive subject matter lies in less than all features of a single disclosed embodiment. Thus, the following claims are hereby incorporated into the Detailed Description, with each claim standing on its own as a separate embodiment.