Flash Disk and Used Capacity

[KurtDePauw1]

Hello,

My Flash Disks are all showing 0 (zero) as used capacity in a hybrid environment.

Is this normal ?

[zdickinson]

Good afternoon, I would say that is not normal.  All 6 of my SSDs show some used capacity.  Anywhere from 50% to 75%, give or take.

I am wondering if it's a GUI error.  Can you run this RVC command?  vsan.disks_stats  What's interesting is that the example output from Manage Virtual SAN with RVC – Complete Guide | Virten.net, show 0% usage on the SSD layer.

I suppose there could be no usage if there was no IO.  Thank you, Zach.

[MBrownWFP]

All my flash disks show 0 bytes used capacity as well.

We have not migrated our full workload to VSAN yet but have about 65 VMs running in the cluster right now with no performance or stability issues...

[perthorn]

This is normal, although slightly confusing. This view shows how much capacity is used on each disk. Remember that the flash disks are not used for capacity, only for cache in a hybrid vSAN, so this would always show up as 0. Basically, the purpose of this view (Monitoring > Virtual SAN > Physical Disks) is to get an overview of the utilization of the capacity disks, to see if the cluster capacity is balanced of if you are close to running out of space on any particular disk. This would never happen on the cache disk.

[zdickinson]

I see your point, but my screen shot shows used capacity on the SSD.  Thank you, Zach.

[perthorn]

Used capacity (and reserved capacity) will actually show up on the flash devices when you have the components of VMs with the flash read cache policy on those disk groups. This makes sense as that is the only space that is basically 'used up' on the cache. All other cache space is used all the time for the write buffer and the read cache, but you can't run out. Are you using FRC?.

[zdickinson]

We're using read cache in that it's 70% for read and 30% for write by default.  But we don't have any reservations setup.  Thank you, Zach.

[elerium]

It's normal to have the cache disks show as 0 used capacity in VSAN 6.2. In previous versions 6.1, 6.0, it did show a non-zero number.