4b5b102e6f
The pre-lock cooldown check used a stale _last_scale_time_usecs read. If another thread (monitor hill-climb, concurrent enqueue) updated the timestamp between the pre-lock check and the lock acquisition, the function would proceed to acquire _scaling_mutex and contend unnecessarily. The CAS that followed would correctly reject the stale value, so this was not a correctness bug, but it caused avoidable lock contention on the hot enqueue path. Add an authoritative cooldown re-check after acquiring _scaling_mutex with a fresh timestamp read. Replace the CAS with a plain store since the _scaling_mutex already serializes writers. Keep the pre-lock speculative check as a fast-path filter. Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com> |
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mergerfs - a featureful union filesystem
mergerfs is a FUSE based union filesystem geared towards simplifying storage and management of files across numerous commodity storage devices. It is similar to mhddfs, unionfs, aufs, DrivePool, etc.
Features
- Logically combine numerous filesystems/paths into a single mount point (JBOFS: Just a Bunch of FileSystems)
- Combine paths of the same or different filesystems
- Ability to add or remove filesystems/paths without impacting the rest of the data
- Unaffected by individual filesystem failure
- Configurable file selection and creation placement
- File IO passthrough for near native IO performance (where supported)
- Works with filesystems of any size
- Works with filesystems of almost any type
- Ignore read-only filesystems when creating files
- Hard links
- Hard link copy-on-write / CoW
- Runtime configurable
- Support for extended attributes (xattrs)
- Support for file attributes (chattr)
- Support for POSIX ACLs
Non-features
- Read/write overlay on top of read-only filesystem like OverlayFS
- File whiteout
- RAID like parity calculation (see SnapRAID)
- Redundancy
- Splitting of files across branches
How it works
mergerfs logically merges multiple filesystem paths together. Not block devices, not filesystem mounts, just paths. It acts as a proxy to the underlying filesystem paths. Combining the behaviors of some functions and being a selector for others.
When the contents of a directory are requested mergerfs combines the list of files from each directory, deduplicating entries, and returns that list.
When a file or directory is created a policy is first run to determine which branch will be selected for the creation.
For functions which change attributes or remove the file the behavior may be applied to all instances found.
The way in which mergerfs behaves is controlled by the config/options/settings. More specifically by policies.
Visualization
A + B = C
/disk1 /disk2 /merged
| | |
+-- /dir1 +-- /dir1 +-- /dir1
| | | | | |
| +-- file1 | | | +-- file1
| | +-- file2 | +-- file2
| | +-- file3 | +-- file3
| | |
+-- /dir2 | +-- /dir2
| | | | |
| +-- file4 | | +-- file4
| | |
| +-- /dir3 +-- /dir3
| | | | |
| | +-- file5 | +-- file5
| | |
+-- file6 | +-- file6
+-- file7 +-- file7 +-- file7
QuickStart
https://trapexit.github.io/mergerfs/latest/quickstart/
Documentation
https://trapexit.github.io/mergerfs
Support
https://trapexit.github.io/mergerfs/latest/support/
Sponsorship and Donations
https://github.com/trapexit/support
Development and support of a project like mergerfs requires a significant amount of time and effort. The software is released under the very liberal ISC license and is therefore free to use for personal or commercial uses.
If you are a non-commercial user and find mergerfs and its support valuable and would like to support the project financially it would be very much appreciated.
If you are using mergerfs commercially please consider sponsoring the project to ensure it continues to be maintained and receive updates. If custom features are needed feel free to contact me directly.