Questions about zfs-raid10 limiting memory

[luren549]

I have 4 4T hard drives and 8T of memory available after forming zfs-raid10.
I see that the documentation says 1T HDD 1G memory, is it allocated according to the size of the HDDs before the group raid10 or according to the size of the HDDs after the group raid10?

 

[Dunuin]

Actually PVE will just use 50% of your hosts total RAM for its ARC. That 4GB + 1GB RAM per 1 TB of raw storage rule of thumb is just a rough estimation what you could use when manually limiting the ARC size. And that rule of thumb would refer to raw storage, so total capacity of all disks you use with ZFS.
Best you just test different ARC sizes and then compare the output of arc_summary so see where a big drp in hot rates will happen.
With just 4x 4TB HDDs an ARC between 6-20GB RAM should be fine. I personally would try it with 12GB.