@Override
        public OptionalLong getTotalCapacityBytes() {
            if (volumeState == null) {
                return OptionalLong.empty();
            } else {
                final Long bytesTotal = volumeState.getBytesTotal();
                return bytesTotal != null
                        ? OptionalLong.of(bytesTotal)
                        : 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