Elasticsearch with disks cache on SSD

I am looking for way to optimize read and agregations latency when using kibana over large number of documents.
I have seen using SSD is a good option but out of budget, so I am looking for a way to use some kind of tiering with few SSD caching hot data from HDD.
I have identified two solutions and would like to get opinions from you and if you have any experience using them:

  • use ZFS with L2ARC functionality
  • use Rapiddisk/Rapidcache Linux kernel module
    Any opinion ? Constraints ? REX ?