OS will not mount the VM partition. Virtual memory must be forced to work Virtual memory was broken on my Mac (cMP 5.1) for several months, with Sierra, High Sierra, and Mojave. Apple support was useless. 

I finally figured out that I was missing all the invisible partitions on the boot drive (vm, recovery, etc.). Probably a casualty of a Carbon Copy Cloner mistake. But CCC allowed me to restore the partitions from a backup.

Still, the system doesn't use the vm partition. I can get it to work manually with sudo diskutil mount -mountPoint /private/var/vm disk1s4, but it doesn't remember this after a restart.

Any ideas on a permanent fix?
Solved. Cobbled together a workaround with information from interwebs.

Created this plist named “local.mountdisk1s4.plist” containing:





             Label
             local.mountdisk1s4.plist
             ProgramArguments
             
                  /sbin/mount_apfs
                  disk1s4
                  /private/var/vm
             
             KeepAlive
             
            SuccessfulExit
            
             



Placed it in /Library/launchdaemons

Swapping now works as it should. Still not sure how this all broke in the first place, or if there's a more elegant solution.