VSAN Free Storage Catches

VSAN is a hot topic nowadays. Once it is set up, it’s easy to management and use. No more creating LUN and zoning.

We recently experienced some catches about its free available storage - at least we didn’t think about or were told before; or maybe our expectation to VSAN was too positive.

Our VSAN hardware disk configuration:

• 3 x Dell PowerEdge R730 nodes
• 2 x 400 GB SDD per node (372.61 GB is shown in VSAN Disk Management)
• 14 x 1 TB SATA per node (931.51 GB is shown in VSAN Disk Management)
• Two disk groups (7 SATA + 1 SSD) per node

Calculation of each node storage capacity (RAW):

931.51 x 14 = 13,041.14 GB = 12.73549 TB

Total storage capacity (RAW)

931.51 x 14 x 3 = 39,123.42 GB = 38.20646 TB

This calculation matches the storage capacity shown in the VSAN Cluster’s Summary.

We are adding more VMs to the VSAN. Once the free storage drops below about 12 TB (about one node’s RAW capacity), the VSAN health check starts showing critical alert “Limits Health - After 1 additional host failure” (KB2108743).

And the component resyncing starts more frequently.

My take away:

• I understand there is an overhead for VSAN (or any storage product) to offer the redundancy. But the way VSAN displaying the free storage is quite difference than the traditional SAN storage and it can be confused. The free storage shown in VSAN does not mean you should use it. Otherwise, the VMs may be down when a host is down or taken down for maintenance.
• The used storage in the Summary tab is the previsioned storage, not the actual space in use.
• The frequent resyncing component can potentially impact the overall VSAN storage performance.