vSAN Hyperconverged Infrastructure Software-Defined Storage

Effective Capacity Management with vSAN 7 Update 1

vSAN 7 Update 1 is loaded with significant features and enhancements, focusing on building a developer ready infrastructure, improving scalability, simplifying operations and extending file service use cases. A detailed insight is outined in this blog What’s new in vSAN 7 Update 1.
vSAN requires additional capacity to accommodate transient operations such as policy changes, rebalancing, object resynchronization, and rebuilds. Before this release, the general guideline was to set aside 25-30% reserve capacity for transient operations such as policy changes, resynchronization, and rebuild operations. This fixed ratio was a conservative threshold based on approximation suited for all environments. In certain cases, the reserve capacity usage was underutilized. This led to several internal optimizations and environment specific guidelines that significantly improves capacity utilization.

Increased Effective Capacity

With vSAN 7 Update 1, the calculation for reserve capacity factors in deployment variables such as cluster size, the number of capacity drives, disk groups and features in use. In parallel, vSAN software stack is optimized by refining the architecture, object layout, and tasks requiring transient space. This helps reduce the reserve capacity requirement drastically.

For example, a 48-node cluster can improve capacity savings up to 18%, resulting in greater resource optimization at a lower cost. vSAN sizer tool provides a guided workflow to factor the design and configuration parameters to provide the appropriate reserve capacity for an environment.

Goodbye Slack Space, Hello Capacity Reserve!!!

Starting with vSAN 7 Update 1, the fixed ratio of reserve capacity termed as Slack Space is effectively replaced with Capacity Reserve to reflect the methodical and improved approach to compute reserve capacity. Capacity Reserve comprises of Operations reserve and Host rebuild reserve. Operations reserve focuses on transient operations and Host rebuild reserve aids in reserving capacity to tolerate a single host failure.

Simplified Capacity Management

vSAN 7 Update 1 operationalizes reserve capacity sizing guidelines into UI workflows for a simplified capacity management experience. Administrators can optionally enable Operations Reserve and Host rebuild reserve. This helps monitor the reserve capacity threshold, generates alerts when the threshold is reached, and prevents further provisioning.

vSAN broadly categorizes reserve capacity into Operations reserve and Host rebuild reserve. Administrators can monitor the overall break-up of capacity reserved in terms of Host rebuild reserve, Operations reserve, and Operations usage.

Operations reserve is the free space required for vSAN internal operations, and Host rebuild reserve can be enabled in addition to the Operations reserve. Enabling Host rebuild reserve demarcates one hosts’ worth of capacity in the cluster. Host rebuild reserve works on the principle of N+1. For example, in a 12-node cluster of identical hardware configuration, Host Rebuild reserve would require 8% (1/12th) reserve capacity to ensure sufficient rebuild capacity. In a heterogeneous cluster of hosts with varying capacity, the host with maximum capacity is considered. As the number of nodes in a cluster size increases, the required reserve capacity decreases. Host Rebuild reserve requires at least 4-nodes in the cluster since 2-node deployments and 3-node cluster cannot accommodate host rebuild capacity.

When the reserve capacity threshold is reached, vSAN prevents newer provisioning tasks and invokes a health check. There is no impact to existing VM I/O, power-on tasks, and maintenance mode tasks continue to function as usual.


vSAN 7 Update 1 drastically improves efficiency and consumable capacity through internal optimizations. Each environment is unique, with varying workloads and design requirements. vSAN factors in multiple variables in a systemic approach to calculate reserve capacity tailored to each configuration and design. This helps ease the design and sizing process and effectively reduces the cost of ownership. The introduction of UI based workflows to monitor and manage the reserve capacity threshold brings operational simplicity, mitigates risks, and greater visibility of capacity utilization.


Next Steps

Here is a list of resources to help get familiarized with vSAN 7 Update 1: