Limitations

Under Concentrated Dispersal, the unit of failure that causes loss or unavailability of a slice is moved from the "Server" to the "Drive". That is, in a non-Concentrated Dispersal system, the fault tolerance implied the system could tolerate the loss and total destruction of (Width - Threshold) Slicestor® devices. Under a Concentrated Dispersal system, the fault tolerance implies that the system can tolerate the loss and total destruction of (Width - Threshold) drives. This ensures reliability remains high, as drive failure is the most important determinate of data loss, and because data loss requires just as many drive failures as a non-Concentrated Dispersal system with the same IDA Configuration, a similar level of reliability is achieved.

However, under Concentrated Dispersal, the tolerance for outages of Storage nodes is at as high as the tolerance for disk failure. Therefore, the availability of a Concentrated Dispersal system with the same IDA Configuration is lower than the availability of a non-Concentrated Dispersal system.

These issues are mitigated by the selection of the IDA configurations used in Concentrated Dispersal systems. The pre-configured IDA configurations are all selected to provide high levels of availability and reliability. Each IDA configuration is selected such that the system can tolerate the complete loss or destruction of any one Slicestor device, plus any other drive in the system.