dsync has poor performance on large files

[defaziogiancarlo]

dsync seems to go slowly and show low read/write activity users with very large files. It seems like dsync is not using multiple processes per file because the performance is much higher when I use dcp on a single file. I'm not sure if this is actually what is happening or if dsync is just getting stuck like in #588.

this is running on TOSS 4, which is based on RHEL using the 4.18.0-553.22.1 kernel.
lustre 2.15.5_6.llnl
gcc 10.3-1 mvapich2-2.3.7 mpifileutils 0.11.1