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Limiting drive spinup

Does mergerfs do anything to prevent drive spinup?

No. In fact it makes it worse.

How can I setup my system to limit drive spinup?

TL;DR: You really can't. Not through mergerfs alone. In fact using mergerfs makes any general attempt to limit spinup more complicated.

mergerfs is a proxy. Despite having some caching behaviors it is not designed to cache much more than metadata. It proxies calls between client software and underlying filesystems. If a client makes a request such as open, readdir, stat, etc. it must translate that into something that makes sense across multiple filesystems. For readdir that means running the same call against all branches and aggregating the output. For open that means finding the file to open and doing so. The only way to find the file to open is to scan across all branches and sort the results and pick one. There is some variability in how much searching is done due to policies but not much. There is no practical way to do otherwise. Especially given so many mergerfs users expect out-of-band changes to "just work."

The best way to limit spinup of drives is to limit their usage at the client level. Meaning keeping software from interacting with the filesystem (and therefore the drive) all together.

What if you assume no out-of-band changes and cache everything?

This would require a significant rewrite of mergerfs. Everything is done on the fly right now and all those calls to underlying filesystems can cause a spinup. To work around that a database of some sort would have to be used to store ALL metadata about the underlying filesystems and on startup everything scanned and stored. From then on it would have to carefully update all the same data the filesystems do. It couldn't be kept in RAM because it would take up too much space so it'd have to be on a SSD or other storage device. If anything changed out of band it would break things in weird ways (and MANY users depend on out of band changes working.) It could rescan on occasion but that would require spinning up everything. Filesystem watches could be used but since mergerfs is just another piece of software interacting with the filesystem ALL mergerfs requests would be echoed back to it causing lots of overhead throwing away a super majority of events it triggered. And something as "simple" as keeping the current available free space on each filesystem is not as easy as one might think given reflinks, snapshots, and other block level dedup technologies distort the meaning of usage values.

Only if ALL metadata and all file locations were cached could mergerfs then do very targeted file operations so as not to trigger a scan across the pool. But that will break out of band interactions. It would require accepting possibly bad behavior by space based policies since space would be imprecisely computed. It would require long scans of the filesystems on startup and a separate location to store all this data.

What if you only query already active drives?

Let's assume that is plausible (it isn't because some drives actually will spin up if you ask if they are spun down... yes... really) you would have to either cache all the metadata on the filesystem or treat it like the filesystem doesn't exist. The former has all the problems mentioned prior and the latter would break a lot of things. Suddenly whole filesystems worth of data would be missing because it spun down.

Is there anything that can be done where mergerfs is involved?

Yes, but whether it works for you depends on your tolerance for the complexity.

  1. Cleanly separate writing, storing, and consuming the data.
    1. Use a SSD or dedicated and limited pool of drives for downloads / torrents.
    2. When downloaded move the files to the primary storage pool.
    3. When setting up software like Plex, Jellyfin, etc. point to the underlying filesystems. Not mergerfs.
  2. Add a bunch of bcache, lvmcache, dm-cache, or similar block level cache to your setup. After a bit of use, assuming sufficient storage space, you can limit the likelihood of the underlying spinning disks from needing to be hit.
  3. Tweak your system's settings around caching:
    • mount all your filesystems with noatime to disable updating "last accessed" time on files.
    • mount all your filesystems with nodiratime to disable updating "last accessed" time on directories.
    • sudo sysctl -w vm.vfs_cache_pressure=10: Prioritizes keeping file metadata cached in RAM.
    • sudo sysctl -w vm.dirty_ratio=40: Allows up to 40% of system memory to be filled with unwritten "dirty" data before forcing a disk flush.
    • sudo sysctl -w vm.dirty_background_ratio=5: Sets a lower threshold (5%) for the kernel to start writing data in the background without blocking applications.
    • sudo sysctl -w vm.dirty_writeback_centisecs=6000: Increases the interval between kernel writeback checks to 60 seconds (default is usually 5).
    • sudo sysctl -w vm.dirty_expire_centisecs=12000: Allows data to stay "dirty" in RAM for up to 120 seconds before it must be written to disk.
    • To make sysctl values permanent add the above to /etc/sysctl.conf.
    • Note: for systems with limited RAM the above sysctl values could cause issues.
    • Note: if you increase the time for which data is stored in RAM before being flushed to disk there is also an increased risk that a sudden powerloss could lead to lost data.
  4. Enable all the appropriate caches in mergerfs:
    • https://trapexit.github.io/mergerfs/latest/config/cache/
    • cache.entry=86400: cache file existence for 24h.
    • cache.negative-entry=86400: cache file non-existence for 24h.
    • cache.attr=86400: cache file metadata for 24h.
    • cache.symlinks=true: enable symlink caching.
    • cache.readdir=true: enable directory listing caching.

References

Tradeoffs

While it may be a trade off you are willing to live with there is decent evidence that spinning down drives puts material increased wear on them and can lead to their death earlier than otherwise. You may end up saving money on electricity but spending more on having to purchase new drives sooner.

Alternatives

Project Comparisons lists a few projects that are similar to mergerfs but offer caching. Please note that these projects are very young and not general purpose like mergerfs. Their compliance to POSIX filesystem rules is probably low enough that certain behaviors may not work or worse lead to data corruption.

I'm not suggesting not to look at those projects but filesystem development is complicated even when it appears to be just a layer on top of another. Just be careful and do your research. That stands for anything touching your data... mergerfs included.

To the degree that mergerfs can add features which help in this situation, so long as it doesn't interfere with other fundamental features, they may be added over time.