Are you from the PolarFS team? This paper looks amazing, I'll be sure to give it a thorough read!

I was curious: could you compare and contrast it with what I imagine are it's competitors? Hdfs, ceph, glusterfs etc.

Have you replaced any of those existing systems internally yet?

HDFS isn't really a filesystem.

The main competitors at this scale would be Lustre & GPFS

Yes I am from the PolarFS team. You can read from the paper that we have compared PolarFS with ceph.