This paper investigates this tension between iso

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This paper investigates this tension between iso- lation and sharing in systems designed for an emerg- ing scenario requiring scalability in the number of concurrently running VMs, especially where these VMs have substantial similarities in their software requirements. Examples of such scenarios include web/db/game hosting centers, utility data centers, and distributed hosting services such as Akamai and PlanetLab. Our study considers two general ap- proaches to virtualization: paravirtualization as ex- 1
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emplified by Xen [1] and paenevirtualization as typ- ified by Linux Vservers [10]. We have considerable experience with Vservers—i.e., a general-purpose Linux OS retrofitted with abstractions providing both name- space and resource isolation—as we use it to support a large number of researchers on Planet- Lab [13, 2]. However, with the steep maturation of Xen from research prototype to production quality solution, there has been considerable momentum to use Xen for all scenarios involving VMs. For this reason, we take a closer look at Xen as an alternative to Vserver. In terms of performance, we find that Xen is a strong competitor to Vservers. However, especially in temrs of network I/O, we find that Vserver has the advantage due to the high CPU overhead required in Xen to manage the network. In terms of scalability, Vservers scale further than Xen for simple tests and have better response time when using a modest number of concurrently running VMs. The paper is organized as follows. Section 2 teases apart the different requirements that users might place on virtual machine technology and characterizes the nature of the isolation properties of virtual machines, independently of how the un- derlying technology actually implements it. Sec- tion 3 compares reproduced performance results from Xen 1.2 with subsequent generations of Xen, and contrasts its performance with that attainable with Linux Vservers. Section 4 then evaluates the scalability of the two systems in the number of con- currently running VMs. Finally, Section 6 offers some concluding remarks. 2 Requirements The purpose of virtual machine technology is to provide isolation between individual VMs. This section teases apart the different requirements that users might place on virtual machines. The discus- sion considers the full breadth of system-level sup- port for virtual machines, from traditional OSs to low-level hypervisors. For clarity, we settle on a single set of terminology, drawn from the recent vir- tual machine literature: we refer to the underlying system as a virtual machine monitor (VMM) rather than an OS, and the isolated containers running on top of it as virtual machines rather than processes or domains. We first outline the usage scenarios of VMs to set the context within which we compare and constrast the different approaches to virtualization. Next, we characterize the nature of the isolation provided by virtual machines, considered independently of how the underlying VMM is actually implemented. Fi-
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  • Spring '12
  • GwangS.Jung
  • virtual machine, VMs, Linux kernel, Xen, namespace isolation

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