parityrebuild
struct buf *parityrebuild(struct plex *, vinum_off_t, int, enum parityop, struct rangelock **, off_t *);
bp = parityrebuild(plex, sd->revived, size, rebuildparity, &lock, NULL); /* do the grunt work */
pbp = parityrebuild(plex, pstripe, size, op, &lock, &errorloc); /* do the grunt work */