The best replacement for rdm is vmdk.

The best alternatives to rdm are rated as following:
★★★★★  vmdk
★★★★★  vmfs
★★★  iscsi initiator
★★★  sdm
★★  datastore
★★  vmfs performance
★★  vmdk sql
★★  avb
★★  passthrough
★  mdm
★  vmdk performance
★  maat
★  blu
  vmfs vmware
  npiv

Your alternative for rdm is not listed? Suggest!
Speaking about rdm, one probably shouldn't forget about vmotion.

Other search terms related to rdm and their ratings are:
★★★★★ vmotion
★ srm
You have in mind a companion for rdm and it is not yet here? Propose!



Google Trends

Scale is based on the average worldwide traffic of rdm in all years. Click on graph for more.
rdm
vmdk
vmfs