[VMC on AWS] How to calculate available free space in VMC before an extra host is added to maintain 25% free slack space
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[VMC on AWS] How to calculate available free space in VMC before an extra host is added to maintain 25% free slack space


Article ID: 328699


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VMware requires that 30% unused space (“slack space”) be maintained in the vSAN datastore within the Service Offering, to support operation of the SDDC. Adequate slack space is required for use of the vSAN datastore. If storage free space reaches (or falls below)25%, it is possible that the customer could lose the ability to utilize the SDDC, and the environment could become inoperable. If unused space in an SDDC vSAN datastore drops reaches (or falls below)25%, VMware will automatically add hosts to the SDDC to prevent damage to the SDDC.

vSAN “slack space” is simply free space that is set aside for operations such as host maintenance mode data evacuation, component rebuilds, rebalancing operations, and VM snapshots. Activities such as rebuilds and rebalancing can temporarily consume additional raw capacity. Host maintenance mode temporarily reduces the total amount of raw capacity a cluster has. This is because the local drives on a host that is in maintenance mode do not contribute to vSAN datastore capacity until the host exits maintenance mode.

There are a number of sources that recommend 25-30% slack space when designing and running a vSAN cluster. For example, a vSAN datastore with 20TB of raw capacity should always have 5-6TB of free space available for use as slack space. This recommendation is not exclusive to vSAN. Most other HCI storage solutions follow similar recommendations to allow for fluctuations in capacity utilization. Storage policy changes is a good reason to maintain that slack space.

There are a couple of cases where storage policy changes can temporarily consume more capacity. One scenario is when a new policy that requires a change in component number and/or layout is assigned to a VM. Another scenario is when an existing storage policy that is assigned to one or more VMs is modified. In both cases, vSAN will use additional capacity to make the necessary changes to components to comply with the assigned storage policy.


This is a rough calculation and doesn't account for the VSAN overhead and Dedup /Compression ratio.
However, this process will help you estimate the amount of free space you need to maintain on the cluster to avoid host additions.

It is recommended, that customers set up a monitor to alert when 30% free space is left to avoid unwanted host additions.
VMware will automatically add hosts to the SDDC until slack space reaches back to 25%.

In our example, we will take a 3 Node SDDC on VMware Cloud on AWS.
  • Total Raw Capacity for a 3 Node SDDC = 31.1 TB (Roughly 10 TB for each node)
  • Slack space to be maintained before additional hosts are added = 25% = (.25 x 31.1 TB) = 7.7 TB
  • Usable Raw Capacity after slack space deduction = 23.4 TB
  • Effective free space available will depend on the storage policy applied.
RAID 0 = Effective free space = 23.4 TB (usable raw capacity / 1)
RAID 1 = Effective free space = 11.7 TB (usable raw capacity / 2)
RAID 5 = Effective free space = 17.5 TB (usable raw capacity / 1.33)