Faster writes on the storage array

The storage array now uses reconstruct-write.

Previously every write had to read the existing data and the parity back off the other disks before it could write anything — on an array with two parity disks that is a lot of reading for one write. Reconstruct-write instead computes the new parity directly, so the read step disappears.

The trade is that every disk in the array spins up for a write rather than only the ones involved. On a system that is busy in bursts rather than idle for long stretches, that is a good trade.

Applied with the array online — no downtime and no interruption to anything using it.