In my experience such behavior I saw when we had so many writes to the disk, and write daemon queue only grown up.
In first I would launch mgstat tool from InterSystems. But it should be started before the freezing system, and for quite long time, with at least 5 seconds interval, better if it would be 1 second. Then you should look at some columns such as WDphase, PhyWrs, WDQsz.
What you should look for.
- WDphase - should not be all the time in the 8th phase.
- WDQsz - from time to time should fall to zero
- PhyWrs - may help to calculate a "real" speed of writing to the disk. And compare with expected speed.
- WDPass - time to time should be increased, if WD still at the same cycle so long time, it may cause a freezing system.