Distributed Rebuild and Reduced Overhead
Current methods for implementing storage redundancy involve mirrored pairs of disks or RAID-5 disk groups. Unfortunately, in most other systems, rebuild time takes between 6 and 25 hours, depending on disk size and protection scheme. The disk I/O-consuming rebuild process typically causes severe performance degradation until the rebuild is over.

The XIV system's revolutionary redundancy scheme provides a distributed rebuild mechanism in which all disks participate in a given rebuild, with each disk rebuilding only a small portion. As a result, rebuild process overhead is minimal, and performance levels are unaffected. Rebuild time with IBM XIV is as little as 40 minutes (or less!) for a 1 TB drive.

No Write-through Mode
To achieve redundancy, all storage architectures must store written data in two cache units before the host is acknowledged. Yet, due to their architecture, most systems are unable to maintain cache redundancy upon failure of a cache unit or of the UPS system that maintains the cache data during power outages. These systems switch to "write-through" mode, acknowledging the host only after the write transaction has been committed to the disks. Unfortunately, write-through mode reduces the performance level during work so much as to be practically the equivalent of shutting down the system.

The XIV system never uses write-through mode. Its flexible new architecture enables write requests to be cached in two different modules, even after module or UPS failure.