mergerfs/mkdocs/docs/faq/configuration_and_policies.md

Configuration and Policies

What settings should I use?

Depends on what features you want. Generally, there are no "good", "bad", "inefficient", or "optimal" settings. Options are almost exclusively functional. Meaning they change the behavior of the software. It is best to read over the available options and choose what fits your use case. If something is not clear from the documentation please reach out and the documentation will be improved.

The settings described in the Quick Start are sufficient for most users.

Filesystems are complex and use cases numerous. There simply is no way to provide a singular setup that works for all situations. Since mergerfs does not impact the underlying filesystems and can be added or removed without any impact it is extremely easy to test and experiment with different settings.

Additional reading:

• Config/Options

What create policy should I use?

If it isn't clear to you then you should use pfrd as described in the quickstart guide.

Why is pfrd the default create policy?

Originally the default was epmfs however it was found to cause significant confusion among new users who assumed mergerfs to behave like other software which always choose a branch with most available space without any filtering or restrictions. The reason for using epmfs was to allow people to use mergerfs without impacting the layout of their data across the branches. Keeping the total entropy of the system in check. This, however, was not understood by users who did not read the documentation and caused significant support burdens.

For mergerfs v2.41.0 it was decided to change the default to pfrd to minimize the support burden.

pfrd was chosen because it prioritizes placement to branches based on free space (percentage wise) without overloading a specific branch as mfs, lus, or other policies could when a singular branch has significantly more free space. Since it is impossible to know the usage and access patterns of the filesystem spreading out files across the branches also spreads the load and therefore good for general use cases making it a good default.

Additional reading:

• functions, categories and policies

How can I ensure files are collocated on the same branch?

Many people like the idea of ensuring related files, such as all the files to a TV show season or songs in an album, are stored on the same storage device. However, most people have no technical need for this behavior.

1. If you backup your data it is extremely likely your backup solution can restore only those files you are missing. There is no need to store data in related units to ensure restoring data is easy.
2. Software such as Sonarr can manage the downloading and post processing of bespoke files / episodes which may be missing. Either by downloading the episode individually if available or by downloading a full season.
3. There is no benefit to keeping files collocated with regard to drive spinup, caching, or other secondary concern. In fact depending on usage patterns placing the data on the same underlying device could harm performance.

The main use case for wanting collocation is where the branch is going to be removed from the pool and used in isolation temporarily and you wish to have all data from some logical set on that device. Such as you intend to take a drive out of the pool to take on a trip and want a whole show or artist's works on the drive. However, even in these situations you typically end up needing to curate the files anyway because it has show A but not show B. In such cases it is more convient to use a separate devices and sync what you wish to it.

All that said you can accomplish collocation to varying degrees using the following methods:

1. Use mergerfs.consolidate when consolidation is needed.
2. Use a msp create policy to do a best effort collocation without needing explicit directory curation.
3. Use epmfs or other ep create policy and manually create paths on the branches directly as you need/desire.
4. Use a ep create policy and pfrd or rand for mkdir.

How can I balance files across the pool?

Similar to collocation there is generally little reason to balance files across the pool branches.

1. Since prediction of a device's/filesystem's death or loss of data is near impossible there is little reason to balance in hopes of limiting data loss.
2. While performance could be impacted by having too much reading or writing happen to singular underlying filesystems balancing won't help unless you have the ability to manage the access patterns to the pool.
3. Over time the churn of adding and removing files will lead to a random distribution of files across the branches which is effectively "balancing" if using create policies which do not favor any particular branch.

All that said if you wish to move files around or balance the pool you can:

1. Use rand or pfrd create policies and just use your system as normal.
2. Write scripts using rsync or similar to move files around as you wish.
3. Use mergerfs.balance. Keep in mind that this tool is really just an example of how to accomplish such a task. The tool does not keep track of links so you may encounter an increase in used space if you rely on links to reduce redundancy. However, you can run a file dedup program like rdfind to restore the links.