App to Span Filesystem Across Storage Pools

by Rian

The home lab community has long assessed the pros and cons of NAS ecosystems and respective accessories for future expansions. Notably, Synology has been offering expansion units as a companion to their DiskStation lineup. For example, if the main unit only has 5 bays but the spec sheet says it supports up to 15 drives, the implication is that the user may expand the configuration with the compatible expansion unit they offer. That being said, an expansion over a single line of cable also presents a single point of failure — a risk most find outweighs the benefit of additional bays.

mergerfs is an open source union filesystem that works on top of the “filesystem” layer. The principle behind it is almost human: the chosen policy determines where a file gets written, and on access mergerfs searches the branches and serves them as one. It’s like finding and returning a book in the library; so long as everyone knows the rule, everyone can find what they need and return it to the right place. In practice, it would allow creating a logical folder that spans two or more independent storage pools.

Mind you, mergerfs does not guarantee availability or redundancy. If a file goes missing or corrupt on one path, it will show up as missing or corrupt on the logical end. A RAID system is still required for zero downtime. I would recommend using mergerfs alongside expansion units offered in different NAS ecosystems; most DAS for home and small office only offer a 5 Gbps or 10 Gbps connection over USB, and the hardware therein is not rigorously vetted. Though they are still priced higher than most DAS, NAS manufacturers’ expansion units (i.e. proprietary DAS) at least offer some level of confidence. The potential single point of failure in this model is the physical connection. Coupled with mergerfs, though, even if the connection drops the expansion, the RAID does not degrade with it.