[canadaduane]

The "Why?" is hard to find, but I think this answer to "Why use mergerfs over unionfs" is informative:

> UnionFS is more like aufs than mergerfs in that it offers overlay / CoW features. If you're just looking to create a union of drives and want flexibility in file/directory placement, then mergerfs offers that, whereas unionfs is more for overlaying RW filesystems over RO ones.

Also:

> What should mergerfs NOT be used for?

1. databases: Even if the database stored data in separate files (mergerfs wouldn't offer much otherwise) the higher latency of the indirection will kill performance. If it is a lightly used SQLITE database then it may be fine but you'll need to test.

2. VM images: For the same reasons as databases. VM images are accessed very aggressively and mergerfs will introduce too much latency (if it works at all).

3. As replacement for RAID: mergerfs is just for pooling branches. If you need that kind of device performance aggregation or high availability you should stick with RAID.