Taken together these two dimensions of isolation

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Taken together, these two dimensions of isolation provide a simple map of the design space, with dif- ferent VMMs (OSs) corresponding to each possibil- ity: Traditional timesharing systems offer both weak performance isolation and weak name- space isolation. Single-user multimedia systems offer strong resource isolation but weak namespace isola- tion. A traditional timesharing system augmented with support for security contexts, as exempli- fied by BSD jails [8], offer weak resource iso- lation and strong namespace isolation. A VMM designed for application hosting cen- ters typically offer both strong resource isola- tion and strong namespace isolation. It is important to keep in mind that this section puts forward just one possible view of the isola- tion landscape. There are certainly other criteria by which VMM strategies can be evaluated. For exam- ple, some VMMs offer the advantage of supporting more than one OS environment. As another exam- ple, a VMM can be characterized by its ability to migrate VMs between physical machines to avoid scheduled downtimes (e.g., for hardware mainte- nance) or to load-balance VMs. Finally, VMMs can be characterized according to the size of its trusted code base, the premise being that the less code there is to assure, the more secure the system is likely to be. While a VM that compromises the VMM is able to gain access to another VM’s state—and hence, compromise the level of security supported by the VMM—there are many other security threats (e.g., physical access to the machines) that must first be addressed before this particular security issue be- comes critical. Instead, we purely focus on isola- tion as it relates to performance and scalability, and return to these other criteria in Section 6. 2.3 Design Choices Based on the current state of VMM research, it appears that two technologies are best posi- tioned to support good performance, high scalabil- ity, and strong isolation: paravirtualizing systems such as Xen [1] and Denali [20], and paenevirtualiz- ing systems—i.e., a general-purpose OS retrofitted with abstractions providing namespace and resource isolation—such as Linux Vservers [10], Solaris 10 (Zones + PRM), and Virtuozzo for Linux [16]. Figure 1 illustrates the architecture of these vir- tualization approaches at a Birdseye level. Shown on the left, a hypervisor-based VMM runs separate copies of a conventional operating system kernel in- side each VM; paravirtualizing hypervisors achieve greater scalability by modifying the OS running in each VM to make it aware that the hypervisor is beneach it. Shown on the right, a paenevirtualized system supports multiple UNIX VMs using a single OS kernel, with each VM appearing to the user as a separate UNIX system. The homogeneous nature of paenevirtualized VMs simplifies the underlying kernel’s task of managing a large number of such VMs.
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  • Spring '12
  • GwangS.Jung
  • virtual machine, VMs, Linux kernel, Xen, namespace isolation

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