Testing VMware VSAN - No SSDs for the cache

[BlizzardX]

Hi

I want to do some test with vSphere 6 VSAN but I don't have any SSDs available for the cache. I know you can fool VSAN in to thinking that a spinner is an SSD for the purposes of testing and I was wondering whether doing so would make the performance worse than spinners acting as local storage only without vsan.

I'm not in need of blistering performance at all, I just want to test some things and not spend money on new drives yet, but I don't want to cripple a test VM on the VSAN worse than if it was just running locally.

 

[Lanz]

I'm guessing it'll be the same or worse, as local disks don't have to go over a Ethernet backplane like VSAN. Why don't you try it and come back and tell us?

 

[Yamahahahahaha]

It will slow things down as requests hit relatively few cache disks (1?) to check for hot blocks before being fetched either from there or from the array.

If you have to do it I would recommend making the spinning disk cache as small as possible to minimise the impact. This will help in two ways: 1) shortstroking the drive decreases random access time, 2) the less data in the cache, the more time data will be fetched from the multi-disk array.

Treat the exercise as a 'worst case performance' scenario. Any kind of flash will massively accelerate performance.