4 Node Cluster
Each node has 1 SSD and 2 Capacity disk
Had a disk failure in node 4, Did we lose data? or with RAID=1 we have the mirred it to another host? Please help.
If it is FTT=0 then it is not mirrored/replicated in any form - single data component (depending on size), no copies, no witness component. If you lost this then it is gone.
Don't store anything you care about as FTT=0, restore what you lost from back-up, if you don't take adequate back-ups then consider this from now on - regardless of FTT=0 or FTT=1, always have back-ups regardless of the platform if you care about the data and cannot afford to lose it.
What happened to the disk - Is it actually physically failed? You could attempt putting the host in Maintenance-Mode (though if you have a lot of FTT=0 data do consider the implications of this) and rebooting it in case it is a driver/firmware/controller issue, potentially entering MM, powering off and re-seating the drive might breathe some life into it but of course make sure you are affecting the correct drive.
In that case, you should start checking inventory to confirm what you have lost - any VMs that won't power on etc.
Is this a Hybrid cluster or All-Flash cluster?
Depending on what was lost and how important this is, recovery of failed mechanical drives is in some cases possible (at a price and depending on what internal component failed of course), so consider calling Kroll OnTrack or similar if this data was critical (which I assume it was not if being stored as FTT=0 with no back-ups).
Do try the steps I suggested previously though (with the advised cautions), potentially that drive has some life in it .
I am glad I could help clarify the situation - I hope you didn't lose anything that can't be rebuilt.
Just a note though:
If you had/have a large proportion of FTT=0 data that are changing to FTT=1, do this in a measured approach e.g. don't apply to all Objects at once, do it for a few VMs/Objects at first and monitor the effects of the resync (as this can add a lot of IO which can cause contention). If it is having negligible impact then apply to a larger set of VMs/Objects. Be aware of how much free capacity you have in the cluster as changing from all FTT=0 data to FTT-1 data will obviously double your capacity usage.