13 Virtual Machines
A Powder virtual node is a virtual machine running on top of a regular operating system. Powder virtual nodes are based on the Xen hypervisor, which allows groups of processes to be isolated from each other while running on the same physical machine. Powder virtual nodes provide isolation of the filesystem, process, network, and account namespaces. Thus, each virtual node has its own private filesystem, process hierarchy, network interfaces and IP addresses, and set of users and groups. This level of virtualization allows unmodified applications to run as though they were on a real machine. Virtual network interfaces support an arbitrary number of virtual network links. These links may be individually shaped according to user-specified link parameters, and may be multiplexed over physical links or used to connect to virtual nodes within a single physical node.
There are a few specific differences between virtual and physical nodes. First, Powder physical nodes have a routable, public IPv4 address allowing direct remote access (unless the Powder installation has been configured to use unroutable control network IP addresses, which is very rare). However, virtual nodes are assigned control network IP addresses on a private network (typically the 172.16/12 subnet) and are remotely accessible over ssh via DNAT (destination network-address translation) to the physical host’s public control network IP address, to a high-numbered port. Depending on local configuration, it may be possible to request routable IP addresses for specific virtual nodes to enable direct remote access. Note that virtual nodes are always able to access the public Internet via SNAT (source network-address translation; nearly identical to masquerading).
Second, virtual nodes and their virtual network interfaces are connected by virtual links built atop physical links and physical interfaces. The virtualization of a physical device/link decreases the fidelity of the network emulation. Moreover, several virtual links may share the same physical links via multiplexing. Individual links are isolated at layer 2, but they are not isolated in terms of performance. If you request a specific bandwidth for a given set of links, our resource mapper will ensure that if multiple virtual links are mapped to a single physical link, the sum of the bandwidths of the virtual links will not exceed the capacity of the physical link (unless you also specify that this constraint can be ignored by setting the best_effort link parameter to True). For example, no more than ten 1Gbps virtual links can be mapped to a 10Gbps physical link.
Finally, when you allocate virtual nodes, you can specify the amount of CPU and RAM (and, for Xen VMs, virtual disk space) each node will be allocated. Powder’s resource assigner will not oversubscribe these quantities.
13.1 Xen VMs
These examples show the basics of allocating Xen VMs: a single Xen VM node, two Xen VMs in a LAN, a Xen VM with custom disk size. In the sections below, we discuss advanced Xen VM allocation features.
13.1.1 Controlling CPU and Memory
You can control the number of cores and the amount of memory allocated to each VM by setting the cores and ram instance variables of a XenVM object, as shown in the following example:
"""An example of constructing a profile with a single Xen VM. Instructions: Wait for the profile instance to start, and then log in to the VM via the ssh port specified below. (Note that in this case, you will need to access the VM through a high port on the physical host, since we have not requested a public IP address for the VM itself.) """ import geni.portal as portal import geni.rspec.pg as rspec # Create a Request object to start building the RSpec. request = portal.context.makeRequestRSpec() # Create a XenVM node = request.XenVM("node") # Request a specific number of VCPUs. node.cores = 4 # Request a specific amount of memory (in MB). node.ram = 4096 # Print the RSpec to the enclosing page. portal.context.printRequestRSpec() """An example of constructing a profile with a single Xen VM. Instructions: Wait for the profile instance to start, and then log in to the VM via the ssh port specified below. (Note that in this case, you will need to access the VM through a high port on the physical host, since we have not requested a public IP address for the VM itself.) """ import geni.portal as portal import geni.rspec.pg as rspec # Create a Request object to start building the RSpec. request = portal.context.makeRequestRSpec() # Create a XenVM node = request.XenVM("node") # Request a specific number of VCPUs. node.cores = 4 # Request a specific amount of memory (in MB). node.ram = 4096 # Print the RSpec to the enclosing page. portal.context.printRequestRSpec()
13.1.2 Controlling Disk Space
Each Xen VM is given enough disk space to hold the requested image. Most Powder images are built with a 16 GB root partition, typically with about 25% of the disk space used by the operating system. If the remaining space is not enough for your needs, you can request additional disk space by setting a XEN_EXTRAFS node attribute, as shown in the following example.
"""An example of constructing a profile with a single Xen VM with extra fs space. Instructions: Wait for the profile instance to start, and then log in to the VM via the ssh port specified below. (Note that in this case, you will need to access the VM through a high port on the physical host, since we have not requested a public IP address for the VM itself.) """ import geni.portal as portal import geni.rspec.pg as rspec # Import Emulab-specific extensions so we can set node attributes. import geni.rspec.emulab as emulab # Create a Request object to start building the RSpec. request = portal.context.makeRequestRSpec() # Create a XenVM node = request.XenVM("node") # Set the XEN_EXTRAFS to request 8GB of extra space in the 4th partition. node.Attribute('XEN_EXTRAFS','8') # Print the RSpec to the enclosing page. portal.context.printRequestRSpec() """An example of constructing a profile with a single Xen VM with extra fs space. Instructions: Wait for the profile instance to start, and then log in to the VM via the ssh port specified below. (Note that in this case, you will need to access the VM through a high port on the physical host, since we have not requested a public IP address for the VM itself.) """ import geni.portal as portal import geni.rspec.pg as rspec # Import Emulab-specific extensions so we can set node attributes. import geni.rspec.emulab as emulab # Create a Request object to start building the RSpec. request = portal.context.makeRequestRSpec() # Create a XenVM node = request.XenVM("node") # Set the XEN_EXTRAFS to request 8GB of extra space in the 4th partition. node.Attribute('XEN_EXTRAFS','8') # Print the RSpec to the enclosing page. portal.context.printRequestRSpec()
This attribute’s unit is in GB. As with Powder physical nodes, the extra disk space will appear in the fourth partition of your VM’s disk. You can turn this extra space into a usable file system by logging into your VM and doing:
mynode> sudo mkdir /dirname mynode> sudo /usr/local/etc/emulab/mkextrafs.pl /dirname
where dirname is the directory you want your newly-formatted file system to be mounted.
"""An example of constructing a profile with a single Xen VM. Instructions: Wait for the profile instance to start, and then log in to the VM via the ssh port specified below. (Note that in this case, you will need to access the VM through a high port on the physical host, since we have not requested a public IP address for the VM itself.) """ import geni.portal as portal import geni.rspec.pg as rspec # Create a Request object to start building the RSpec. request = portal.context.makeRequestRSpec() # Create a XenVM node = request.XenVM("node") # Request a specific number of VCPUs. node.cores = 4 # Request a specific amount of memory (in MB). node.ram = 4096 # Print the RSpec to the enclosing page. portal.context.printRequestRSpec() """An example of constructing a profile with a single Xen VM. Instructions: Wait for the profile instance to start, and then log in to the VM via the ssh port specified below. (Note that in this case, you will need to access the VM through a high port on the physical host, since we have not requested a public IP address for the VM itself.) """ import geni.portal as portal import geni.rspec.pg as rspec # Create a Request object to start building the RSpec. request = portal.context.makeRequestRSpec() # Create a XenVM node = request.XenVM("node") # Request a specific number of VCPUs. node.cores = 4 # Request a specific amount of memory (in MB). node.ram = 4096 # Print the RSpec to the enclosing page. portal.context.printRequestRSpec()
13.1.3 Setting HVM Mode
By default, all Xen VMs are paravirtualized. If you need hardware virtualization instead, you must set a XEN_FORCE_HVM node attribute, as shown in this example:
"""An example of constructing a profile with a single Xen VM in HVM mode. Instructions: Wait for the profile instance to start, and then log in to the VM via the ssh port specified below. (Note that in this case, you will need to access the VM through a high port on the physical host, since we have not requested a public IP address for the VM itself.) """ import geni.portal as portal import geni.rspec.pg as rspec # Import Emulab-specific extensions so we can set node attributes. import geni.rspec.emulab as emulab # Create a Request object to start building the RSpec. request = portal.context.makeRequestRSpec() # Create a XenVM node = request.XenVM("node") # Set the XEN_FORCE_HVM custom node attribute to 1 to enable HVM mode: node.Attribute('XEN_FORCE_HVM','1') # Print the RSpec to the enclosing page. portal.context.printRequestRSpec() """An example of constructing a profile with a single Xen VM in HVM mode. Instructions: Wait for the profile instance to start, and then log in to the VM via the ssh port specified below. (Note that in this case, you will need to access the VM through a high port on the physical host, since we have not requested a public IP address for the VM itself.) """ import geni.portal as portal import geni.rspec.pg as rspec # Import Emulab-specific extensions so we can set node attributes. import geni.rspec.emulab as emulab # Create a Request object to start building the RSpec. request = portal.context.makeRequestRSpec() # Create a XenVM node = request.XenVM("node") # Set the XEN_FORCE_HVM custom node attribute to 1 to enable HVM mode: node.Attribute('XEN_FORCE_HVM','1') # Print the RSpec to the enclosing page. portal.context.printRequestRSpec()
You can set this attribute only for dedicated-mode VMs. Shared VMs are available only in paravirtualized mode.
13.1.4 Dedicated and Shared VMs
In Powder, Xen VMs can be created in dedicated or shared mode. In dedicated mode, VMs run on physical nodes that are reserved to a particular experiment, and you have root-level access to the underlying physical machine. In shared mode, VMs run on physical machines that host VMs from potentially many experiments, and users do not have access to the underlying physical machine.
To request that a VM run in dedicated mode, set its exclusive attribute to True; for example node.exclusive = True. To run it in shared mode, set this value to False.