Virtualized Cluster {#guide-virtualizedcluster}

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Overview # {#guide-virtualizedcluster-overview}

A virtualized cluster service is an abstraction of a compute service that corresponds to a platform of physical resources on which Virtual Machine (VM) instances can be created. The virtualized cluster service provides all necessary abstractions to manage VMs and their allocation to physical resources, including creation on specific physical hosts, suspend/resume, migration, etc. Compute jobs submitted to the virtualized cluster service run on previously created VM instances. If a VM that meets a job's requirements cannot be found, the service will throw an exception. In the virtualized cluster service, a VM behaves as a bare-metal service.

The main difference between a cloud service and a virtualized cluster service is that the latter does expose the underlying physical infrastructure (e.g., it is possible to instantiate a VM on a particular physical host, or to migrate a VM between two particular physical hosts).

Creating a virtualized cluster compute service # {#guide-virtualizedcluster-creating}

In WRENCH, a virtualized cluster service represents a cloud service (wrench::CloudComputeService, which itself represents a wrench::ComputeService), which is defined by the wrench::VirtualizedClusterComputeService class. An instantiation of a virtualized cluster service requires the following parameters:

• A hostname on which to start the service (this is the entry point to the service)
• A list (std::vector) of hostnames (all cores and all RAM of each host is available to the virtualized cluster service)
• A mount point (corresponding to a disk attached to the host) for the scratch space, i.e., storage local to the virtualized cluster service (used to store workflow files, as needed, during job executions)
• Maps (std::map) of configurable properties (wrench::VirtualizedClusterComputeServiceProperty) and configurable message payloads (wrench::VirtualizedClusterComputeServiceMessagePayload).

The example below shows how to create an instance of a virtualized cluster service that runs on host "vc_gateway", provides access to 4 execution hosts, and has a scratch space of 1TiB:

auto virtualized_cluster_cs = simulation.add(
          new wrench::VirtualizedClusterComputeService("vc_gateway", {"host1", "host2", "host3", "host4"}, 
                                                "/scratch/",
                                                {{wrench::VirtualizedClusterComputeServiceProperty::SUPPORTS_PILOT_JOBS, "false"}}));

Virtualized cluster service properties {#guide-virtualizedcluster-creating-properties}

All properties for a virtualized cluster service are inherited from wrench::CloudServiceProperty (see the Cloud service properties section).

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Migrating a VM to another execution host # {#guide-virtualizedcluster-migrating}

As expected, a virtualized cluster service provides access to all mechanisms to manage the set of VMs instantiated on the execution hosts. In addition to such capabilities, the virtualized cluster service provides an additional wrench::VirtualizedClusterComputeService::createVM function, which instantiates a VM on a specific execution host; and a wrench::VirtualizedClusterComputeService::migrateVM function, which allows VM migration between execution hosts.

Here is an example of VMs migration on a virtualized cluster service:

// Create a VM with 2 cores and 1 GiB of RAM on host1
auto vm1 = virtualized_cluster_cs->createVM("host1", 2, pow(2,30));
// Create a VM with 4 cores and 2 GiB of RAM on host2
auto vm2 = virtualized_cluster_cs->createVM("host2", 4, pow(2,31));

// Migrating vm1 to host3
virtualized_cluster_cs->migrateVM(vm1, "host3");
// Migrating vm2 to host4
virtualized_cluster_cs->migrateVM(vm2, "host4");

Submitting jobs to a virtualized cluster compute service # {#guide-virtualizedcluster-using}

See the cloud service page for an example of how to submit a job to a VM instance (as both the cloud service and virtualized cluster service operate in the same way).

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