Applications such as databases, video on demand, medical data and web archival have a need for storage systems which are high performance as well as high capacity. The solution used in most cases is a hierarchy of a disk array and tape library. Due to custom hardware, the cost per megabyte of RAID disk arrays increases with system capacity, unlike raw disks and tape systems. Also, a disk array needs to be connected to a host computer, which becomes a bottleneck for both performance and availability. Its scalability is limited by the number of disks that can be supported by the infrastructure.
Some storage consuming applications like web archival have a fixed growth rate of data.
When such applications reach the capacity limit of their disk array, another array must be added. Adding independent disk arrays also lowers the reliability of the total system and complicates storage management. Tertiary Disk is a storage system architecture which exploits the trends mentioned above to create large disk storage systems that avoid the disadvantages of custom built disk arrays. We use commodity, off the shelf components to develop a scalable, low cost, terabyte capacity disk system. Tertiary Disk uses PCs connected by a switched network to host a large number of disks.