A technique is directed to storing data on a plurality of storage devices of a data storage array. The technique involves, on each storage device of the plurality of storage devices, providing large disk extents and small disk extents for allocation to RAID extents. The technique further involves forming, from the large disk extents, a user-data RAID extent to store user data for the data storage array. The technique further involves forming, from the small disk extents, an internal-metadata RAID extent to store internal metadata for the data storage array. In some arrangements, spare space is reserved on one or more storage devices between large and small disk extents.