/* * How to merge the on-disk parity maps when reading them in from the * various components; returns whether they differ. In the case that * they do differ, sets *dst to the union of *dst and *src. * * In theory, it should be safe to take the intersection (or just pick * a single component arbitrarily), but the paranoid approach costs * little. * * Appropriate locking, if any, is the responsibility of the caller. */