4.7 KiB
Remote Filesystems
Many users ask about compatibility with remote filesystems. This section is to describe any known issues or quirks when using mergerfs with common remote filesystems.
Keep in mind that, like with caching, it is not a good idea to change the contents of the remote filesystem out-of-band. Meaning that you really shouldn't change the contents of the underlying filesystems or mergerfs on the server hosting the remote filesystem. Doing so can lead to weird behavior, inconsistency, errors, and even data corruption should multiple programs try to write or read the same data at the same time. This isn't to say you can't do it or that data corruption is likely but it could happen. It is better to always use the remote filesystem. Even on the machine serving it.
NFS
NFS is a common remote filesystem on Unix/POSIX systems. Due to how NFS works there are some settings which need to be set in order for mergerfs to work with it.
It should be noted that NFS and FUSE (the technology mergerfs uses) do not work perfectly with one another due to certain design choices in FUSE (and mergerfs.) Due to these issues, it is generally recommended to use SMB when possible till situations change. That said mergerfs should generally work as an export of NFS and issues discovered should still be reported.
To ensure compatibility between mergerfs and NFS use the following settings.
mergerfs settings:
- noforget
- inodecalc=path-hash
NFS export settings:
- fsid=UUID
- no_root_squash
noforget is needed because NFS uses the name_to_handle_at and
open_by_handle_at functions which allow a program to keep a
reference to a file without technically having it open in the typical
sense. The problem is that FUSE has no way to know that NFS has a
handle that it will later use to open the file again. As a result, it
is possible for the kernel to tell mergerfs to forget about the node
and should NFS ever ask for that node's details in the future it would
have nothing to respond with. Keeping nodes around forever is not
ideal but at the moment the only way to manage the situation.
inodecalc=path-hash is needed because NFS is sensitive to
out-of-band changes. FUSE doesn't care if a file's inode value changes
but NFS, being stateful, does. So if you used the default inode
calculation algorithm then it is possible that if you changed a file
or updated a directory the file mergerfs will use will be on a
different branch and therefore the inode would change. This isn't an
ideal solution and others are being considered but it works for most
situations.
fsid=UUID is needed because FUSE filesystems don't have different
st_dev values which can cause issues when exporting. The easiest
thing to do is set each mergerfs export fsid to some random
value. An easy way to generate a random value is to use the command
line tool uuid or uuidgen or through a website such as
uuidgenerator.net.
no_root_squash is not strictly necessary but can lead to confusing
permission and ownership issues if root squashing is enabled.
SMB / CIFS
SMB is a protocol most used by Microsoft Windows systems to share file shares, printers, etc. However, due to the popularity of Windows, it is also supported on many other platforms including Linux. The most popular way of supporting SMB on Linux is via the software Samba.
Samba, and other ways of serving Linux filesystems, via SMB should work fine with mergerfs. The services do not tend to use the same technologies which NFS uses and therefore don't have the same issues. There should not be special settings required to use mergerfs with Samba. However, CIFSD and other programs have not been extensively tested. If you use mergerfs with CIFSD or other SMB servers please submit your experiences so these docs can be updated.
SSHFS
SSHFS is a FUSE filesystem leveraging SSH as the connection and transport layer. While often simpler to setup when compared to NFS or Samba the performance can be lacking and the project is very much in maintenance mode.
There are no known issues using sshfs with mergerfs. You may want to use the following arguments to improve performance but your millage may vary.
-o Ciphers=arcfour-o Compression=no
More info can be found here.
Other
There are other remote filesystems but none popularly used to serve mergerfs. If you use something not listed above feel free to reach out and I will add it to the list.