Ubuntu 16.04 server with RAID 1 HDD and single SSD

Current Setup

  • The system is live with this and one other site on Drupal 8 in a multi-site setup.
  • The server is a LEMP server running Drupal 8 on Ubuntu 16.04.
  • The OS is on a RAID 1 array, 1TB hdd (/dev/sda and /dev/sdb).
  • The 256GB SSD is currently unformatted (/dev/sdc).

Disk Usage of Previous Web Server

root@hbw2live:/# du -m --one-file-system --max-depth=1|sort -g
0 ./dev
0 ./proc
0 ./run
0 ./sys
1 ./boot
1 ./lib64
1 ./lost+found
1 ./media
1 ./mnt
1 ./opt
1 ./selinux
1 ./srv
1 ./tmp
9 ./bin
9 ./sbin
10 ./etc
26 ./build
3517 ./usr
3856 ./lib
6832 ./home
8525 ./root
26294 ./var
49074 .

Desired Setup and Requirements

  1. Resilience to disk failure (hdd is already taken care of with RAID 1, but SSD is a single drive and I would not even consider using it were it not for the huge speed benefits).
  2. The root filesystem to stay on the hdd as I don't fancy taking a chance on moving it to another disk, nor do I want to risk the system on a single drive like that.
  3. Frequently-accessed directories in the course of serving a web page should be read from ssd for speed, but all writes must be replicated on the hdd for redundancy/failover/resilience.
  4. I was thinking of mounting selected directories on the ssd (etc, usr, lib, var) and syncing them to the hdd regularly - but point 3 above sounds almost like I should be using an automated file caching system that would use the ssd as a cache for frequently used files.