The RAID Advisory Board has at last got around to publishing the replacements for the long established six Redundant Array of Inexpensive Disk technology levels, which Computergram first got wind of last July (CI No 2,948). The RAB Disk System is described as a new rating guide for disk systems based on practical data availablility properties rather than the technical criteria used in the former RAID levels 0 to 5. As expected, there are now three broad classifications: Failure Resistant Disk Systems, Failure Tolerant Disk Systems and Disaster Tolerant Disk Systems. Each classification also includes a premium plus level, which requires additional protection against environmentally-related and other failures. Starting from the least functional, FDRS Failure Resistant Disk Systems protect against data loss due to any single component failure within the system, and against loss of data access due to a single disk failure. FDRS+ requires additional automatic disk swap and hot swap, protection against data loss due to cache failure and external power failure or temperature. The next level up is FTDS Failure Tolerant Disk Systems, for continuous data availability in the event of any single system component failure, as well as a significant level of data protection against environmental failures. FTDS+ offers data access protection against host and host input-output bus failure and external power failures, meaning that it needs to be able to connect to at least two external hosts through separate buses, and connect a set of disks to clusters of two or more computers. Finally, DTDS Disaster Tolerant Disk Systems must be divisible into two or more zones which cooperate to protect against data loss or access to data, in the event that one system suffers a complete failure. It also protects against massive power outages and cooling system failures. DTDS+ systems maintain full availability during host computer and disk system software and hardware upgrades, and protection against massive equipment failures, fire, flood, and even invasion of premises. Unlike the old RAID levels, the new classifications are hierarchical, meaning that each class of disk system must satisfy the criteria for lower classes. Companies such as Adaptec Inc, Amdahl Corp, Data General Corp, Digital Equipment Corp, EMC Corp, Fujitsu Ltd, Hewlett-Packard Co, IBM Corp, NEC Corp, Seagate Technology Inc, and StorageTek Inc are amongst the supporters. The RAID Advisory Board is based in St Peter, Minnesota.
