Hello Ron,
A hearty welcome to HCI.
"One of the things I've gleaned from research and vmworld is to keep plenty of white space in the vSAN datastore for maintenance and re-balancing."
You don't just want to keep space free for resilience (e.g. rebuild data back to FTT=1), maintenance (e.g. maintaining FTT=1/2 during maintenance if required by the business), but also for Storage Policy changes and if some/most of your data is Thin-provisioned then to allow for this growth (unless you are just adding more storage/nodes/clusters as it grows).
"My understanding is that once you reach about 20% free space remaining vSAN rebalancing operations can affect storage performance."
When any individual capacity-drive reaches 80% used (default value - can be changed, do so with care) vSAN starts 'Reactive' rebalancing data off these drives to spread the data more uniformly across available capacity-drives (e.g. *moves* the data to less utilised drives). This is akin to resync traffic (as opposed to intentionally very slow 'Proactive rebalance' traffic) and as such when added to the current load of the cluster could of course result in contention (depending on how loaded the cluster is already and how much is being rebalanced):
https://docs.vmware.com/en/VMware-vSphere/6.5/com.vmware.vsphere.virtualsan.doc/GUID-2EC7054E-FBCC-4...
"I'd like to be warned well in advance of that via a vCenter alarm or otherwise but I can't seem to find anything specific to vSAN datastore usage."
The vSAN Health puts warning/alerts on the cluster when certain thresholds are reached, noted here:
https://kb.vmware.com/s/article/2108907
There is a configurable vCenter alarm associated with this - 'vSAN health alarm 'Disk capacity'.
"As a side note is anybody aware of some good guides on operationalizing vSAN clusters for those of us that have been doing VMware with external storage arrays for years?"
depping & CHogan book is a great in-depth start and is available online:
https://www.vsan-essentials.com/
Their blogs (+ lamw ) also cover a lot of ground in great detail.
Launch HOL, make it, play with it, break it, fix it :smileygrin:
http://labs.hol.vmware.com/HOL/catalogs/lab/4687
RVC is still ever-useful and the vsan elements are formatted nicely here (also spbm. commands are useful):
https://www.virten.net/2017/05/vsan-6-6-rvc-guide-part-1-basic-configuration/
Tons of info here and most of the whitepapers/technical articles updated and formatted for utility:
https://storagehub.vmware.com/t/vmware-vsan/
Read Communities posts in vSAN sub-community - we have answered a ton of queries, troubleshooted a lot of scenarios and provided a lot of general info in the last few years here.
Bob
