Ensuring RAM Cache for file copy (Linux CLI)

After a plot (-w), I am copying plots to my farm - destination is a standard HDD. I have 192GB of RAM, of which 110 is a RAM disk for plotting. System is a Linux Server (Proxmox).

Sometimes the copy is reported to be completed in 5mins (presumably) due to RAM caching, with an average speed of 250MB/s. This is great.

However, quite often the RAM cache is not used and I average 20MB/s. It takes an hour or more.

How can I troubleshoot, or otherwise consistently make use of caching?

Other notes…

  • There is a break after each file copy, so it’s not hitting a bottleneck where a new copy is started before the cache is emptied from the previous copy.
  • I have a couple of CTs on this system but nothing that is eating up the ample RAM I have.
  • The copy is happening on the PVE Host, and the destination is a JBOD, HDD on a SAS card + SAS Expander