-w Copy Sometimes uses RAM Cache, Sometimes Doesn't - CLI, Debian Linux

[old-square-eyes]

Sometimes my -w takes 6 mins. Seemingly being cached in RAM... ~250MB/s. I have 192 GB, of which 110 is a RAM disk for MM.

Sometimes my -w takes an hour, obviously because it's bound to the single HDD write speed ~20MB/s.

How can I ensure the cache is used every time?

What copy tech does MM use under the hood?

Keen to learn what the bottleneck is here. I also have a question on the Proxmox forum.