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80 lines
2.6 KiB
Markdown
80 lines
2.6 KiB
Markdown
# mergerfs - a featureful union filesystem
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**mergerfs** is a
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[FUSE](https://en.wikipedia.org/wiki/Filesystem_in_Userspace) based
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[union filesystem](https://en.wikipedia.org/wiki/Union_mount) geared
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towards simplifying storage and management of files across numerous
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commodity storage devices. It is similar to **mhddfs**, **unionfs**,
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and **aufs**.
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## Features
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* Logically combine numerous filesystems/paths into a single
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mount point
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* Combine paths of the same or different filesystems
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* Ability to add or remove filesystems/paths without impacting the
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rest of the data
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* Unaffected by individual filesystem failure
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* Configurable file selection and creation placement
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* Works with filesystems of any size
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* Works with filesystems of almost any type
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* Ignore read-only filesystems when creating files
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* Hard link copy-on-write / CoW
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* Runtime configurable
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* Support for extended attributes (xattrs)
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* Support for file attributes (chattr)
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* Support for POSIX ACLs
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## Non-features
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* Read/write overlay on top of read-only filesystem like OverlayFS
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* File whiteout
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* RAID like parity calculation
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* Redundancy
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* Splitting of files across branches
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## How it works
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mergerfs logically merges multiple filesystem paths together. It acts
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as a proxy to the underlying filesystem paths. Combining the behaviors
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of some functions and being a selector for others.
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When the contents of a directory are requested mergerfs combines the
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list of files from each directory, deduplicating entries, and returns
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that list.
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When a file or directory is created a policy is first run to determine
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which branch will be selected for the creation.
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For functions which change attributes or remove the file the behavior
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may be applied to all instances found.
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Read more about [policies
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here](config/functions_categories_and_policies.md).
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### Visualization
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```
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A + B = C
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/disk1 /disk2 /merged
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+-- /dir1 +-- /dir1 +-- /dir1
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| +-- file1 | +-- file2 | +-- file1
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| | +-- file3 | +-- file2
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+-- /dir2 | | +-- file3
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| | +-- /dir3 |
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| +-- file4 | +-- /dir2
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| +-- file5 | |
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+-- file6 | +-- file4
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+-- /dir3
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| +-- file5
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+-- file6
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```
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