So, it's been awhile since this question has been asked, but I find it still useful in 2017.
Namely - I was moving around host around Fault Domains, and every time you do that - a resync gets scheduled. So if you move it around 2-3 times without thinking, you will have 2-3 times of resyncs to occur as they all occur sequentially.
So - if you don't want your whole VSAN to come to a grinding performance halt, you can log into each host via SSH, and run this:
vsish -e set /vmkModules/vsan/dom/MaxNumResyncCopyInFlight 25
Now - note 50 is the default - and VMware support told me NOT to lower this for too long - i.e. use it sparingly and temporarily or larger issues may happen.
I ask them can I stop it, cancel it or pause it, but they said no.
However, and use this at your own risk (i.e. in a lab only) there is another setting there:
Does issuing a 1 (or true) with vsish -e set to this cause it all to stop? Dunno. Will test in a lab when I get a chance.
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Just to clarify some things here:
Adjusting number of resyncing Objects in-flight does not instantly trim the current processes, it only stops adding new Objects to be resynced until it gets down to the value set. Additionally, this must be set on ALL hosts in the cluster to take effect.
Pausing a resync in any ESXi builds below 6.0 U3 or 6.5.0d can cause multiple hosts to simultaneously PSOD, so if on a lower build, do not do this:
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Awesome! Thanks for the info!