@Override
        public OptionalLong getCapacityLimitBytes() {
            return byteLimit != null
                    ? OptionalLong.of(byteLimit)
                    : OptionalLong.empty();
        }
Essentially a read-only view of some parts of {@link FsVolume}
stroom-core-shared/src/main/java/stroom/data/store/impl/fs/shared/FsVolume.java