# Tooling

## mergerfs.collect-info

A tool included in recent releases of `mergerfs` which collects
details about your system and configuration to help providing
[support](support.md).

```text
$ 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.

```text
$ 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 IO
[passthrough](config/passthrough.md) in FUSE. IO Passthrough 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](config/passthrough.md)
integration.

`/usr/lib/mergerfs/preload.so`

This [preloadable
library](https://man7.org/linux/man-pages/man8/ld.so.8.html#ENVIRONMENT)
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](config/moveonenospc.md), will no longer work.

Also, understand that this will only work on dynamically linked
software (that is 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 edge cases
there could be some yet unaccounted for so please report any oddities.

Thank you to
[nohajc](https://github.com/nohajc/mergerfs-io-passthrough) for
prototyping the idea.


### casual usage

```sh
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
guarantee 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.


```sh
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

```sh
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://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](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
