I have a ZFS formatted USB disk that I would like to share on the home network and have an unused Raspberry-Pi (512MB) that is surely not the fastest computer I have but is also surely the one that consume less.
Wake up on LAN request would be a nice feature and hope the USB drive can be put a sleep if not used for some time.
The question is what distribution to use for IT / how to configure it?
I wouldn't suggest using ZFS on a 32 bit system with 512 MB RAM.
You need at least 1GB, and a 64 bit processor is highly recommended. FreeBSD / ARM doesn't seem to do well with ZFS at this point, so Debian on ARM would be your best best, but even then... I wouldn't trust it with data.
Reference: ZFSTuningGuide