@Override
        public OptionalLong getCapacityUsedBytes() {
            if (volumeState == null) {
                return OptionalLong.empty();
            } else {
                final Long bytesUsed = volumeState.getBytesUsed();
                return bytesUsed != null
                        ? OptionalLong.of(bytesUsed)
                        : 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