Virtualizacion

Virtualization Aware File Systems: Getting Beyond the Limitations of Virtual Disks
Ben Pfaff, Tal Garfinkel, Mendel Rosenblum {blp,talg,mendel}@cs.stanford.edu Stanford University Department of Computer Science Abstract
Virtual disks are the main form of storage in today’s virtual machine environments. They offer many attractive features, including whole system versioning, isolation, andmobility, that are absent from current file systems. Unfortunately, the low-level interface of virtual disks is very coarse-grained, forcing all-or-nothing whole system rollback, and opaque, offering no practical means of sharing. These problems impose serious limitations on virtual disks’ usability, security, and ease of management. To overcome these limitations, we offer Ventana, a virtualization awarefile system. Ventana combines the filebased storage and sharing benefits of a conventional distributed file system with the versioning, mobility, and access control features that make virtual disks so compelling. Ventana, a virtualization aware file system (VAFS). Ventana extends a conventional distributed file system with versioning, access control, and disconnected operation features resembling thoseavailable from virtual disks. This attains the benefits of virtual disks, without compromising usability, security, or ease of management. Unlike traditional virtual disks whose allocation and composition is relatively static, in Ventana storage is ephemeral and highly composable, being allocated on demand as a view of the file system. This allows virtual machines to be rapidly created, specialized,and discarded, minimizing the storage and management overhead of setting up a new machine. We describe the principles behind virtualization aware file systems. We also present our prototype implementation of Ventana and explore the practical benefits that a VAFS offers to VM users and administrators. We will begin by examining the properties of virtual disks. Section 2 explores their limitations, tomotivate our desire for file system-based virtual machine storage, then Section 3 details virtual disks’ compelling features. Section 4 shows how to integrate these features into a distributed file system by presenting Ventana, a virtualization aware file system. Section 5 focuses on our prototype implementation of Ventana and Section 6 demonstrates a usage scenario. Sections 7 and 8 discuss relatedand future work and Section 9 concludes.

1 Introduction
Virtual disks, the main form of storage in today’s virtual machine environments, have many attractive properties, including a simple, powerful model for versioning, rollback, mobility, and isolation. Virtual disks also allow VMs to be created easily and stored economically, freeing users to configure large numbers of VMs. This enables anew usage model in which VMs are specialized for particular tasks. Unfortunately, virtual disks have serious shortcomings. Their low-level isolation prevents shared access to storage, which hinders delegation of VM management, so users must administer their own growing collections of machines. Rollback and versioning takes place at the granularity of a whole virtual disk, which encouragesmismanagement and reduces security. Finally, virtual disks’ lack of structure obstructs searching or retrieving data in their version histories [34]. Conversely, existing distributed file systems support fine-grained controlled sharing, but not the versioning, isolation, and encapsulation features that make virtual disks so useful. To bridge the gap between these two worlds, we present

2 MotivationVirtual machines are changing the way that users perceive a “machine.” Traditionally, machines were static entities. Users had one or a few, and each machine was treated as general-purpose. The design of virtual machines, and even their name, has largely been driven by this perception. However, virtual machine usage is changing as users discover that a VM can be as temporary as a file. VMs can be...