Bug 226357

I using 11.1-RELEASE + ZFS. Every few days FreeBSD hangs, means does no more respond (also no ssh connection but there is a ping). At the console you can type in the user name and after pressing enter shows the new empty lines (no password asked, no answer). No error message on the screen on /var/log/messages. Sometimes it returns to action after a few / several minutes. Shutdown the system resolve the problem for the next few days. The problem is repeated on many servers with the same configuration. There was no problem on version 10.3
Any ideas?

ctrl+t:
load:  1.56  cmd:  getty 2660 [ vmmaps] 7074936.52r 0.00u 0.00s 0% 1008k
load:  1.70  cmd:  getty 2660 [ vmmaps] 7074994.31r 0.00u 0.00s 0% 992k
load:  1.70  cmd:  getty 2660 [ vmmaps] 7074996.12r 0.00u 0.00s 0% 992k
load:  1.70  cmd:  getty 2660 [ vmmaps] 7074997.24r 0.00u 0.00s 0% 992k
load:  1.64  cmd:  getty 2660 [ vmmaps] 7075073.03r 0.00u 0.00s 0% 968k
load:  1.60  cmd:  getty 2660 [ vmmaps] 7075142.04r 0.00u 0.00s 0% 964k