The Neighbours
MergerFS is one answer among many to "my files are spread across drives." These are the neighbours, and what each trades. Nothing here is a ranking; the right tool is the one whose failure mode you can live with.
| Tool | Shape | Choose it when |
|---|---|---|
| MergerFS | Userspace (FUSE) union of plain folders; files whole on one drive | Your drives differ, arrive one at a time, and must stay readable on their own |
| RAID 0 | Kernel striping across drives | You want one fast volume and accept that one dead drive takes everything |
| RAID 1 / 5 / 6 (mdadm) | Mirroring or parity in the kernel | You want redundancy and one volume, with matched drives and rebuilds you understand |
| LVM | Volume manager: pools physical drives into logical volumes, striping or linear | You want resizing and snapshots more than per-file placement, on drives you'll manage as a set |
| ZFS | Filesystem and volume manager in one; pools (vdevs) with checksums and parity | You are building the storage properly from the start and can give it the drives (and RAM) it asks for |
| SnapRAID | Scheduled parity over standalone drives | Your data changes slowly and you want drive-level protection; the classic partner under mergerfs, not a rival |
| UnionFS / OverlayFS | Kernel union mounts, built for container image layers | You are stacking a writable layer over read-only ones — images, live systems — not pooling data drives |
| Windows: Storage Spaces / DrivePool | The platform equivalents | Your drives live on a Windows box; DrivePool in particular is mergerfs's closest cousin |
| Just mount the drives and keep a list | No software at all | Honestly, for two drives and a tidy mind — until the third drive arrives |
MergerFS's own trade is visible in the table: it is the only entry that leaves every drive independently readable and demands nothing of the drives — no matching sizes, no formatting, no commitment — and the only cost it refuses to hide is that it protects nothing. RAID and ZFS make the opposite trade: real redundancy, real lock-in, drives that mean nothing alone.
Two honest notes. Moving between these is mercifully easy in one direction: leaving mergerfs means unmounting it and owning some folders — an afternoon, not a migration. Moving to it from a striped array is the usual backup-restore project. And the question every row of the table must answer is chapter 10's: when a drive dies, what exactly do you lose, and what exactly brings it back?