What is VM Overreserved and Why is it taking so much Space?

Today I saw a question in the VMTN communities and thought is was “blog worthy”. In essence the question was “What is VM Overreserved and Why is it taking so much space?” hence the very original title for this blog…

When we talk about VM overreserved, we are talking in the context of VMs within vSAN, more specifically Objects within vSAN. There are a couple of KB articles explaining that VM overreserved is seen when dedupe & Compression is disabled and we are using Object Space Reservation (OSR = non-zero). To quote the KB “Used – VM Over-reserved: Space wasted due to higher than needed space reservation setting. Reducing object space reservation policy can free up space without the need to delete or move any data”

What if OSR is set to 0%, but I still see a lot of space being consumed???

Remember we are talking about objects. By default, vSAN thin-provisions objects on the back end; however, this does not apply to swap files, and these objects are thick provisioned by default. To take it a step further, these objects (swap) utilize the default policy (FTT=1 default setting). This means that the swap object in vSAN will be “size of memory of VM * 2 (FTT=1)”. If you change the FTT on the default policy, then the amount of space used will increase (FTT=2 & 3).

If you do not wish to have this space “wasted”, you can disable SwapThickProvision advanced setting on the hosts… BUT, Proper planning should be done prior to disabling this. I wrote a blog about this not long ago. https://greatwhitetec.com/2017/03/20/vsan-sparse-swap/

The VM overreserved setting appears when dedupe/compression is disabled. I did a quick test on my lab to demonstrate this.

  • Hosts had SwapThickProvisionDisabled set to 0
  • Used – VM overreserved = 40GB
  • Changed /VSAN/SwapThickProvisionDisabled to 1  (see my blog for how to…)
  • Turned VMs off/back on – remember, swap space is taken during VM boot and released when turned off.
  • Space of VM overreserved now = 0 GB