Written by Christian Ahmer | 11/08/2023

MergerFS is a union filesystem geared towards simplifying storage and management of files across numerous commodity storage devices. It is a FUSE-based filesystem, meaning it runs in user-space, leveraging the Filesystem in Userspace (FUSE) Linux kernel module. MergerFS logically amalgamates multiple directories and drives into a single cohesive file system, presenting multiple physical storage devices as one virtual system.

The principle of operation behind MergerFS is one of unification and pooling. Individual storage units, which can be individual directories or mounted drives, are pooled together. This pooling is transparent: to the user, the file system appears as a continuous space, even though it spans across multiple drives. Users can interact with this virtual file system just as they would with any other traditional file system, with the added benefit that it abstracts away the complexity of managing multiple mounts and directories.

MergerFS is particularly adept at handling and optimizing scenarios involving large amounts of data that outgrow the capacity of a single drive. In such situations, MergerFS can facilitate the seamless addition of more storage units to the pool, providing scalability that is not as easily achievable with traditional partitioning and mounting methods.

One of the distinguishing features of MergerFS is its policy-driven approach to file operations. The software allows users to configure policies that dictate how different actions—like file creation, file reading, and file movement—are handled within the pool. For instance, a user could set a policy to ensure that new files are written to the drive with the most free space available or to balance the distribution of files to maintain even usage across all drives.

This policy framework provides a high degree of control and flexibility. Policies can be defined for various operations such as file creation, file deletion, and file renaming, and can be tailored to specific use cases or performance requirements. For example, a policy could prioritize read speeds, write speeds, or redundancy.

In terms of fault tolerance and redundancy, MergerFS does not inherently provide these features since it is a method of unifying storage space rather than replicating data. However, it can be used in conjunction with software RAID, LVM, or filesystems like ZFS and Btrfs, which do provide such features, to create a more robust storage solution.

Moreover, MergerFS supports POSIX standards, which means that it works with most software without any special accommodation. It also offers extended attributes and POSIX ACLs, ensuring compatibility with a wide range of file attributes and permission settings.

Installation and management of MergerFS are straightforward. The system can be installed from source or via package managers on most Linux distributions. Once installed, the configuration is handled through the `/etc/fstab` file or through runtime options provided when the file system is mounted using the `mount` command. The management of the file system can be done through any standard file management tools and commands, making it accessible to users familiar with UNIX-like environments.

The user-space nature of MergerFS, while providing flexibility and ease of use, implies that it might not match the performance of kernel-based file systems under all conditions. However, for many applications, particularly home servers, small business file servers, and media storage, the performance is more than adequate.

In conclusion, MergerFS is a valuable tool for users looking to simplify the management of multiple storage devices without sacrificing flexibility or control. It abstracts the complexities of individual file systems, presenting a unified interface to the user, and provides the means to scale storage infrastructure efficiently and economically. Its policy-driven operation and compatibility with existing Linux systems make it a practical solution for a wide range of storage needs.