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Limits

What mergerfs cannot do, collected in one place and stated plainly. Most of these are in earlier chapters in context; this is the unvarnished list.

  • No redundancy. No parity, no mirroring, no error correction. A dead drive's files are gone unless a backup or a parity partner (chapter 11) covers them. This is the limit people most often talk themselves out of believing.
  • Files are never split. No striping means no file reads faster than its own drive, and no file can be larger than the free space of a single branch. A pool of five 1 TB drives cannot hold a 1.2 TB file.
  • FUSE overhead is real. Every operation passes through a userspace program. For media serving and general storage this is usually invisible; for workloads of many small files, heavy metadata traffic, or the last percent of disk throughput, measure on your own hardware before committing. (This guide invents no numbers: it did not benchmark, and neither should its claims.)
  • Not for databases. mmap-heavy and locking-sensitive applications — databases above all — meet real FUSE limits. The project's manual says so itself. Keep databases on plain filesystems.
  • A dead mergerfs process is a dead mount. The branches are unharmed, but everything talking to the pool sees errors until it is restarted. It is one daemon; run it like one (chapter 7, and monitor it).
  • Permissions are the branches' permissions. MergerFS does not add an access-control layer of its own; it acts with the credentials of the operations it serves. If the branch folders are a mess, the pool is a neat window onto a mess.
  • Free space is a sum, not a place. The pool can report gigabytes free while being unable to accept a file, when no single branch has room for it whole (chapter 6).
  • Case, charset, and clock quirks are inherited. Branches on filesystems with different filename rules or skewed clocks produce the behaviours you'd predict once you remember the pool is only a view: newest-style policies trust those clocks.
  • It needs FUSE. No /dev/fuse, no pool — in locked-down containers and some virtualised environments that is the end of the conversation, as this guide's own lab found (chapter 3).