[HPDD-discuss] lustre FID-in-dirent backward compatibility

We are in process of upgrading a lustre FS. We currently have the MDS and OSS servers running 2.1.5 (upgraded from 1.8.9). However our client are still running 1.8.9. I am trying to clarify if the "FID-in-dirent" feature (ie. tunefs -O dirdata) is backwards compatible with 1.8.9 clients ? I have seen a few talks (SC11?) that says it should be. However the 2.x lustre operations manual at hpdd-intel doesn't *explicitly* say it will work with older clients. Is there high risk involved enabling "dirdata" when running old 1.8.9 clients ? Any opinions if there is benefit in using this flag ? thanks, chris hunter