The Case for Massive Arrays of Idle Disks (MAID)

The declining costs of commodity disk drives is rapidly changing the economics of deploying large amounts of on-line storage. Conventional mass storage systems typically use high performance RAID clusters as a disk cache, often with a file system interface. The disk cache is backed by tape libraries which serve as the final repository for data. In mass storage systems where performance is an issue tape may serve only as a deep archive for disaster recovery purposes. In this case all data is stored on the disk farm. If a high availability system is required, the data is often duplicated on a separate system, with a fail-over mechanism controlling access. This work explores an alternative design using massive arrays of idle disks, or MAID. We argue that this storage organization provides storage densities matching or exceeding those of tape libraries with performance similar to disk arrays. Moreover, we show that through a combination of effective power management of individual drives and the use of cache or migration, this performance can be achieved using a very small power envelope. We examine the issues critical to the performance, energy consumption and practicality of several classes of MAID systems. The potential of MAID to save energy costs with a relatively small performance penalty is demonstrated in a comparison with a conventional RAID 0 storage array.