TOOLING

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.) However, there have been several complications which have kept the feature from making it into the mainline Linux kernel. Until that feature is available there are two methods to provide similar functionality. One method is using the LD_PRELOAD feature of the dynamic linker. The other leveraging ptrace to intercept syscalls. Each has their disadvantages. At the moment only a preload based tool is available. A ptrace based tool may be developed later if there is a need.

/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. 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.

general usage

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

Docker 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.

docker run \
  -e LD_PRELOAD=/usr/lib/mergerfs/preload.so \
  -v /usr/lib/mergerfs/preload.so:/usr/lib/mergerfs/preload.so:ro \
  -v /media:/data \
  -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:/data \
  -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
• 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
• 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