Robocopy is 20x slower than drag/droping files between servers

robocopy

I am migrating 20TB of files between two servers. Both systems are Windows 2012R2 with latest updates installed. Both using gigabit network.

Using robocopy on the new server with the command line robocopy /S /ZB /MOVE /J /DCOPY:A /V /FP \\oldserver\shared\folder X:\folder

That copies at about 5-10MB/s.

Drag and drop the files between servers using explorer yields around 110MB/s, which is the speed I expected.

I can't figure out why robocopy is going so slowly, when the file shares and network are clearly capable of faster speeds.

Things I've tried:

  • Connect servers directly with an ethernet cable to eliminate any switch issues. No change.
  • The /MT switch, with values 1, 4, 8 and 16. All copied at around the same speed.
  • Copying instead of moving files. No change.
  • Using a log files instead of printing to the screen. No change.
  • Pushing files from the old server vs pulling from the new server. No change.

Any ideas?

Best Answer

In my case, it turned out to be the /Z switch, for restartable mode.

Removing that switch brought the copy up to 105-110MB/s, just like the explorer copies.

I still don't know why it causes such a massive performance hit.

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