Hi Gobinda, unfortunately it's long gone, because I went back to an un-cached setup.
It was mostly a trial anyway, I had to re-do the 3-node HCI because it had died rather horribly on me (a repeating issue I have so far had on distinct sets of hardware, that I am still trying to hunt down... separate topic).
And since it was a blank(ed) set of servers, I just decided to try the SSD cache, to see if the Ansible script generation issue had been sorted out as described from upstream. I was rather encouraged to see that the Ansible script now had these changes included, that URS had described as becoming necessary with a new Ansible version.
It doesn't actually make a lot of sense in the setup, because the SSD cache is a single Samsung EVO 860 1TB unit while the storage is a RAID6 out of 7 4TB 2.5" drives (per server): Both have similar bandwidth, IOPS would be very much workload dependent (the 2nd SSD I intended to use as a mirror was unfortunately cut from the budget).
It has space left over because the OS doesn't need that much, but I don't dare use a single SSD as a write-back cache, especially because the RAID controller (HP420i) hides all wear information and doesn't seem to pass TRIM either and for write-through I'm not sure it would do noticeably better than the RAID controller (I configured that not to cache the SSD, too).
So after it failed, I simply went back to no-cache for now. This HCI cluster is using relatively low-power hardware recalled from retirement that will host functional VMs, not high-performance workloads. They are well equipped with RAM and that's always the fastest cache anyway.
I guess you should be able to add and remove the SSD as cache layer at any time during the operation, because it's at a level oVirt doesn't manage and I'd love to see examples as to how it's done. Especially the removal part would be important to know, if your SSD signals unexpected levels of wear and you need to swap them out on the fly.
If I hit across another opportunity to test (most likely a single node), I will update here and make sure to collect a full set of log files including the ansible main config file.
Thank you for your interest and the follow-up, Thomas
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