mergerfs versus X

mhddfs

mhddfs had not been maintained for some time and has some known stability and security issues. mergerfs provides a superset of mhddfs' features and should offer the same or better performance.

Below is an example of mhddfs and mergerfs setup to work similarly.

mhddfs -o mlimit=4G,allow_other /mnt/drive1,/mnt/drive2 /mnt/pool

mergerfs -o minfreespace=4G,category.create=ff /mnt/drive1:/mnt/drive2 /mnt/pool

aufs

aufs is mostly abandoned and no longer available in most Linux distros.

While aufs can offer better peak performance mergerfs provides more configurability and is generally easier to use. mergerfs however does not offer the overlay / copy-on-write (CoW) features which aufs has.

unionfs-fuse

unionfs-fuse is more like aufs than mergerfs in that it offers overlay / copy-on-write (CoW) features. If you're just looking to create a union of filesystems and want flexibility in file/directory placement then mergerfs offers that whereas unionfs is more for overlaying read/write filesystems over read-only ones.

overlayfs

overlayfs is similar to aufs and unionfs-fuse in that it also is primarily used to layer a read/write filesystem over one or more read-only filesystems. It does not have the ability to spread files/directories across numerous filesystems.

RAID0, JBOD, drive concatenation, striping

With simple JBOD / drive concatenation / stripping / RAID0 a single drive failure will result in full pool failure. mergerfs performs a similar function without the possibility of catastrophic failure and the difficulties in recovery. Drives may fail but all other filesystems and their data will continue to be accessible.

The main practical difference with mergerfs is the fact you don't actually have contiguous space as large as if you used those other technologies. Meaning you can't create a 2TB file on a pool of 2 1TB filesystems.

When combined with something like SnapRaid and/or an offsite backup solution you can have the flexibility of JBOD without the single point of failure.

UnRAID

UnRAID is a full OS and its storage layer, as I understand, is proprietary and closed source. Users who have experience with both have often said they prefer the flexibility offered by mergerfs and for some the fact it is open source is important.

There are a number of UnRAID users who use mergerfs as well though I'm not entirely familiar with the use case.

For semi-static data mergerfs + SnapRaid provides a similar solution.

ZFS

mergerfs is very different from ZFS. mergerfs is intended to provide flexible pooling of arbitrary filesystems (local or remote), of arbitrary sizes, and arbitrary filesystems. For write once, read many usecases such as bulk media storage. Where data integrity and backup is managed in other ways. In those usecases ZFS can introduce a number of costs and limitations as described here, here, and here.

StableBit's DrivePool

DrivePool works only on Windows so not as common an alternative as other Linux solutions. If you want to use Windows then DrivePool is a good option. Functionally the two projects work a bit differently. DrivePool always writes to the filesystem with the most free space and later rebalances. mergerfs does not offer rebalance but chooses a branch at file/directory create time. DrivePool's rebalancing can be done differently in any directory and has file pattern matching to further customize the behavior. mergerfs, not having rebalancing does not have these features, but similar features are planned for mergerfs v3. DrivePool has builtin file duplication which mergerfs does not natively support (but can be done via an external script.)

There are a lot of misc differences between the two projects but most features in DrivePool can be replicated with external tools in combination with mergerfs.

Additionally, DrivePool is a closed source commercial product vs mergerfs a ISC licensed OSS project.