"System has insufficient buffers to rebalance the tree." after upgrade to 3.8.1

[Marco Righele]

Greetings,

I have a small system that I recently upgraded from 3.6 to 3.8.1.
After the upgrade, the nightly jobs will spew

     hammer: System has insufficient buffers to rebalance the tree. nbuf 
< 3969

on the log while trying to do hammer rebalance.

If I try hammer rebalance on any pfs from the command line I will get

     Rebalance / failed: No space left on device

even though the system has about 10% of free space (13GB out of 140GB). 
Normal operation
doesn't seem to be affected.

hammer(5) says:
  Rebalancing a HAMMER PFS uses quite a bit of memory and can't be done 
on
      low memory systems.  It has been reported to fail on 512MB systems.
      Rebalancing isn't critical for HAMMER file system operation; it is 
done
      by hammer rebalance, often as part of hammer cleanup.

The machine has 2GB of RAM, so unless the requirement changed it 
shouldn't be a an issue of not having enough memory.

Is it something I should worry about ?

Marco