Petal: Dispersed Virtual Disks

Abstract

The ideal storage system is universally accessible, always available, provides limitless performance and capacity for a large number of clients, and requires no management.   This paper puts light on the design, implementation, and performance of Petal, a technique that attempts to approximate this ideal in practice through a novel combination of features. Petal consists of a collection of network- attatched servers that cooperatively manage a pool of physical disks. To a Petal client, this collection appears as a highly available block-level storage system that provides large abstract containers called virtual disks. A virtual disk is universally accessible to all Petal clients on the network. A client can create a virtual disk on demand to tap the entire capacity and performance of the underlying physical resources. Furthermore, additional resources, such as servers and disks, can be automatically incorporated into Petal.

We have an initial Petal prototype consisting of four 225 MHz DEC 3000/700 workstations running Digital Unix and connected by a 155 Mbit/s ATM network.   The prototype provides clients with virtual disks that tolerate and recover from disk, server, and network failures. Latency is comparable to a locally attached disk, and throughput scales with the number of servers. The prototype can achieve I/O rates of up to 3150 requests/sec and bandwidth up to 43.1 Mbytes/sec.