New server build, hard drive questions

[epicrean]

I am building a new server and currently have 2 SSDs drive and 2 8 TB drives. The plan is to install TrueNAS on one of the SSDs, use the other SSD as a cache drive a build a pool with the two 8 TB drives.

I want the pool to be expandable. In other words, in a couple months what set up would allow me to add another 8 TB drive?

 

[ChrisRJ]

No, ZFS does not support this right now or in the foreseeable future.

For more details please check the first couple of entries from the "Recommended readings" in my signature.

 

[Etorix]

Cache L2ARC is only applicable with enough memory (64 GB RAM and up). Set up first, measure the ARC hit ratio and see whether L2ARC would help first.
With two drives, the only possible (safe) setup is a mirror, and it is perfectly possible to add a third drive to make a 3-way mirror. But that's likely not what you mean. You need to bring in two drives to add a second mirror vdev.