Opening black boxes: Using semantic information to combat virtual machine image sprawl

Abstract

Virtual-machine images are currently distributed as disk-image files, which are files that mirror the content of physical disks. This format is convenient for the virtual machine monitors that execute these images. However, it is not well-suited for administering images because storing images as disk-image files forces administrators to maintain the software on images with the same tools that they use to maintain the software on physical machines. Already, these tools cannot cope with "physical server sprawl"; in the future, because images can be snapshotted and cloned easily, enterprises that migrate from physical machines to images will need tools that scale to cope with the larger problem of "virtual-machine image sprawl. To address this problem, this paper proposes the Mirage image format (MIF), a new storage format that exposes the rich semantic information currently buried in disk-image files. Disk-image files contain a mapping from file name to file content (and file metadata). MIF decouples this mapping into a manifest that maps file names to content descriptors (and file metadata) and a store that holds the content. Each image has its own manifest and a store may contain content for many images. As with disk-image files, images in MIF fully encapsulate application state including all software dependences. In addition, conversion between MIF and traditional disk-image formats is easy. This paper shows, through examples, that MIF makes some typical software management tasks - inventory control, customized deployment, and image update - faster and easier. The general technique is to operate on manifests instead of on content whenever possible. These tasks can be performed without starting images and, because manifests are simpler and orders of magnitude smaller than disk-image files, without accessing large amounts of data.Copyright © 2008 ACM.