The above paper lists average checkpointing at 1.47 and 1.75 hours on different platforms, with checkpoints occurring more frequently under heavy load. Checkpoints are infrequent when the cell is not in use, naturally. See the above cited University of Michigan paper. The number of requests it receives. Exact numbers vary per platform, but, in general, objects are small, and softlinks smaller still.
Directories are quite expensive.
You would want a broad, flat namespace. See the CITI report for the answer to all the above. The XDS interface can also be used to access CDS, but you can't do things such as create directories with it. CDS also has the CDS-PI, which is unsupported and undocumented, but easily used if you figure out cdsclerk.h. 4.7. DCE 1.1's dcecp program, particularly with its TCL scripting capability, is quite powerful.