vSAN Caching Requirement

[bilalfuuu]

Hi ,

In hybrid  mode its a requirement of 10% SSD for cache purpose , i want to know that does this rule same applied to All flash vSAN , or All Flash doesn't require 10 % SSD , if using then for what purpose .

kindly i need a clarification .

Thanks

[vpradeep01 · Accepted solution]

Yes this applies only for Hybrid. The flash caching device must provide at least 10 percent of the anticipated storage that virtual machines are expected to consume.

The use of snapshots consumes cache resources. If you plan to use several snapshots, consider dedicating more cache than the conventional 10 percent cache-to-consumed-capacity ratio.

All-flash configurations

In all-flash configurations, vSAN uses the cache layer for write caching only. The write cache must be able to handle high write activities. This approach extends the life of capacity flash that might be less expensive and might have lower write endurance

Source: Design Considerations for Flash Caching Devices in vSAN

[TheBobkin]

Hello bilalfuuu​,

How much cache-tier that is required depends on the IO profile (e.g. 70/30 Read/Write), throughput and size of cluster. Just because we don't say you will need at least 10%, don't expect 1% to perform as well as 10%+ on your workload - always know the specifics of your workload and design accordingly.

This article covers the specifics of this well:

https://blogs.vmware.com/virtualblocks/2017/01/18/designing-vsan-disk-groups-cache-ratio-revisited/

Something to note there is that those sizing advisements are made based on 2 DGs per host - multiple DGs per node is always preferable, go with SAS where possible.

Bob