Lightweight transparent compression filesystem?
Solution 1:
I've run both ZFS and BTRFS on Linux in the last two years. My experience is that BTRFS uses less RAM than ZFS for comparable disk usage. Not including RAID5/6, BTRFS has been very stable for me on Ubuntu 14.04 with BTRFS 3.12.
I've been using LZO compression in BTRFS, and it has been just as fast on writes as uncompressed.
Solution 2:
ZFS is fine with low RAM if you don't use the deduplication feature. You can also limit the amount of RAM ZFS utilizes for caching (ARC). The built-in lz4 compression can be very helpful for your data volume.