Optimizing the migration of virtual computers

Abstract

1 IntroductionThis paper shows how to quickly move the state of a running computer across a network, including the state in its disks, memory, CPU registers, and I/O devices. We call this state a capsule. Capsule state is hardware state, so it includes the entire operating system as well as applications and running processes.We have chosen to move x86 computer states because x86 computers are common, cheap, run the software we use, and have tools for migration. Unfortunately, x86 capsules can be large, containing hundreds of megabytes of memory and gigabytes of disk data. We have developed techniques to reduce the amount of data sent over the network: copy-on-write disks track just the updates to capsule disks, "ballooning" zeros unused memory, demand paging fetches only needed blocks, and hashing avoids sending blocks that already exist at the remote end. We demonstrate these optimizations in a prototype system that uses VMware GSX Server virtual, machine monitor to create and run x86 capsules. The system targets networks as slow as 384 kbps.Our experimental results suggest that efficient capsule migration can improve user mobility and system management. Software updates or installations on a set of machines can be accomplished simply by distributing a capsule with the new changes. Assuming the presence of a prior capsule, the amount of traffic incurred is commensurate with the size of the update or installation package itself. Capsule migration makes it possible for machines to start running an application within 20 minutes on a 384 kbps link, without having to first install the application or even the underlying operating system. Furthermore, users' capsules can be migrated during a commute between home and work in even less time.Today's computing environments are hard to maintain and hard to move between machines. These environments encompass much state, including an operating system, installed software applications, a user's individual data and profile, and, if the user is logged in, a set of processes. Most of this state is deeply coupled to the computer hardware. Though a user's data and profile may be mounted from a network file server, the operating system and applications are often installed on storage local to the computer and therefore tied to that computer. Processes are tied even more tightly to the computer; very few systems support process migration. As a result, users cannot move between computers and resume work uninterrupted. System administration is also more difficult. Operating systems and applications are hard to maintain. Machines whose configurations are meant to be the same drift apart as different sets of patches, updates, and installs are applied in different orders.We chose to investigate whether issues including user mobility and system administration can be addressed by encapsulating the state of computing environments as first-class objects that can be named, moved, and otherwise manipulated. We define a capsule for a machine architecture as the data type encapsulating th...

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