vSAN Caching Tier not used in a SSD (non flash vSAN) configuration

[johandijkstra]

A customer has a vSAN (based on vSphere 6.5), their controllers are not flash capable (i.e. vSAN says no flash), but the disks are SSD... so we keep the vSAN recommendation.
But, we have 800gb caching tier disks, the problem is however that the caching tier not is being used. Peformance seems OKE, but the caching tier is not being used in vSAN.

I thought there was an article somewhere which describes how to force caching in such configuration.

Anyone an idea where to find this article / enable force cache?

Thanks!!

[TheBobkin]

Hello Johan,

Please clarify, when you say "their controllers are not flash capable" do you mean they are not on the vSAN HCL for All-Flash clusters or just that we say to disable the controller cache?(as we do on all controllers that have these)

"the problem is however that the caching tier not is being used."

Cache-tier isn't used for permanent storage of data so you won't see it contributing to vsanDatastore size - it is used for caching data (100% Write cache in All-Flash, 70/30 Read/Write cache in Hybrid). Trust me, if your cache-tier device was not being used you would have no functional Disk-Group and thus no storage provided by any Capacity-tier device in that Disk-Group.

If you want to look at the Cache-tier metrics (e.g. buffer-fill levels, destage etc.) these are available in vSAN Performance graphs here:

Host > Monitor > Performance > vSAN - Disk Group

Bob

[depping]

I am not sure I understand the problem to be honest. Let's try to break it down:

1. Is this all-flash or hybrid?
2. If they have 800GB caching devices, why would you say it is not used?
3. What type of devices is the capacity tier?

When you create a vSAN Datastore you will always need a Caching device and at least 1 Capacity device per disk group which is contributing storage to the vSAN datastore. So I am not sure I understand what you are saying, without a caching tier it wouldn't work?

[johandijkstra]

Well, it are a little bit older systems, but with SSD’s.

however, vSAN (ehhh, vsphere) marks the SSD’s as non-flash.

vsan complains when we force flash disks

so, we stick at how vsan wants the config. But...this means we are using ssd instead of spinning disks. Performance of ssd is much better than spinning so, (read) cache is presumably not used. But vsan should use the caching tier...

2. When looking in the statistics we see dat 0 of the caching tier is being used

3. Storage tier is ssd, but vsphere marks as non-flash

[depping]

I am not sure I fully understand, but if you are using SSDs as the capacity tier marked as HDD then this is an unsupported configuration and I would urge you to file a support request to get the problem resolved.

[hmaricle]

As said earlier the cache tier does not store any permanent data. So the cache tier disks will say 0 bytes for Used Capacity and the amount of data. All of the data is stored only on the capacity disks.