An EW-Array resembles a conventional striped mirror in how data is distributed and reads are performed. However, the two systems differ in how writes are satisfied: instead of performing a write to one of many fixed locations, a EW-Array chooses a disk whose head is closest to a free block among candidates to perform the foreground write. In cases where a higher degree of reliability is desired, the two disk heads that are closest to their free blocks are chosen to perform the foreground writes. The remaining (or ) writes are buffered in the delayed write queues of the remaining disks to be performed in the background, also in an eager-writing fashion.
In an EW-Array, reads enjoy good latency and throughput just as they do in a conventional striped mirror. Foreground write latency is improved greatly due to eager-writing. This latency can be even lower when there are more disk heads to choose from. Unlike a striped mirror, copy propagation is no longer the limiting problem because the writes are sufficiently efficient that they are easily masked even when idle time is scarce. As a result, an EW-Array can sustain higher I/O throughput. The low cost of replica propagation also makes it possible to raise the degree of replication for even lower read latency or to increase the fraction of foreground writes for higher reliability.