Tooling

mergerfs.collect-info

A tool included in recent releases of mergerfs which collects details about your system and configuration to help providing support.

$ mergerfs.collect-info
* Please have mergerfs mounted before running this tool.
* Upload the following file to your GitHub ticket or put on https://pastebin.com when requesting support.
* /tmp/mergerfs.info.txt

fsck.mergerfs

A tool to help diagnose and solve mergerfs pool issues. Primarily related to mismatched permissions and ownership.

$ fsck.mergerfs --help
fsck.mergerfs: A tool to help diagnose and solve mergerfs pool issues


USAGE: fsck.mergerfs [OPTIONS] path


POSITIONALS:
  path TEXT:DIR REQUIRED      mergerfs path

OPTIONS:
  -h,     --help              Print this help message and exit
          --fix TEXT:{none,manual,newest,largest} [none]
                              Will attempt to 'fix' the problem by chown+chmod or copying files
                              based on a selected file.
                              * none: Do nothing. Just print details.
                              * manual: User selects source file.
                              * newest: Use file with most recent mtime.
                              * largest: Use file with largest size.
          --check-size BOOLEAN [false]
                              Considers file size in calculating differences
          --copy-file BOOLEAN [false]
                              Copy file rather than chown/chmod to fix

preload.so

EXPERIMENTAL

For some time there has been work to enable passthrough IO in FUSE. Passthrough IO would allow for near native performance with regards to reads and writes (at the expense of certain mergerfs features.) In Linux v6.9 that feature made its way into the kernel however in a somewhat limited form which is incompatible with aspects of how mergerfs functions and took longer to implement as a result. This preloadable library was created as an alternative to the FUSE passthrough integration.

/usr/lib/mergerfs/preload.so

This preloadable library overrides the creation and opening of files in order to simulate passthrough file IO. It catches the open/creat/fopen calls, has mergerfs do the call, queries mergerfs for the branch the file exists on, reopens the file on the underlying filesystem and returns that instead. Meaning that you will get native read/write performance because mergerfs is no longer part of the workflow. Keep in mind that this also means certain mergerfs features that work by interrupting the read/write workflow, such as moveonenospc, will no longer work.

Also, understand that this will only work on dynamically linked software (a that dynamically linked with the same general libc version as the software being used with it.) Anything statically compiled will not work. Many GoLang and Rust apps are statically compiled.

The library will not interfere with non-mergerfs filesystems. The library is written to always fallback to returning the mergerfs opened file on error.

While the library was written to account for a number of edgecases there could be some yet accounted for so please report any oddities.

Thank you to nohajc for prototyping the idea.

casual usage

LD_PRELOAD=/usr/lib/mergerfs/preload.so touch /mnt/mergerfs/filename

Or run export LD_PRELOAD=/usr/lib/mergerfs/preload.so in your shell or place it in your shell config file to have it be picked up by all software ran from your shell. For instance, in bash, add the export to .bashrc.

Docker and Podman usage

Assume /mnt/fs0 and /mnt/fs1 are pooled with mergerfs at /media.

All mergerfs branch paths must be bind mounted into the container at the same path as found on the host so the preload library can see them.

NOTE: Since a container can have its own OS setup there is no guarentee that preload.so from the host install will be compatible with the loader found in the container. If that is true it simply won't work and shouldn't cause any issues.

docker run \
  -e LD_PRELOAD=/usr/lib/mergerfs/preload.so \
  -v /usr/lib/mergerfs/preload.so:/usr/lib/mergerfs/preload.so:ro \
  -v /media:/media \
  -v /mnt:/mnt \
  ubuntu:latest \
  bash

or more explicitly

docker run \
  -e LD_PRELOAD=/usr/lib/mergerfs/preload.so \
  -v /usr/lib/mergerfs/preload.so:/usr/lib/mergerfs/preload.so:ro \
  -v /media:/media \
  -v /mnt/fs0:/mnt/fs0 \
  -v /mnt/fs1:/mnt/fs1 \
  ubuntu:latest \
  bash

systemd unit

Use the Environment option to set the LD_PRELOAD variable.

• https://www.freedesktop.org/software/systemd/man/latest/systemd.service.html#Command%20lines
• https://serverfault.com/questions/413397/how-to-set-environment-variable-in-systemd-service

[Service]
Environment=LD_PRELOAD=/usr/lib/mergerfs/preload.so

Misc

• https://github.com/trapexit/mergerfs-tools
  • Keep in mind these tools are more to provide examples of custom behaviors that can be build on top of mergerfs. They may not have all the features you are looking for.
  • mergerfs.ctl: A tool to make it easier to query and configure mergerfs at runtime
  • mergerfs.fsck: Provides permissions and ownership auditing and the ability to fix them (should use fsck.mergerfs instead)
  • mergerfs.dedup: Will help identify and optionally remove duplicate files
  • mergerfs.dup: Ensure there are at least N copies of a file across the pool
  • mergerfs.balance: Rebalance files across filesystems by moving them from the most filled to the least filled
  • mergerfs.consolidate: move files within a single mergerfs directory to the filesystem with most free space
• https://github.com/trapexit/scorch
  • scorch: A tool to help discover silent corruption of files and keep track of files
• https://github.com/trapexit/bbf
  • bbf (bad block finder): a tool to scan for and 'fix' hard drive bad blocks and find the files using those blocks