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[FEAT] Subtract ZFS ARC from memory usage #1995
Description
Description
The displayed memory in use should exclude ZFS's ARC, similarly to tools like htop.
Motivation
Implementing this in a shell is non-trivial, and it's a logical extension of existing functionality.
Additional context
Due to Linux kernel limitations, ZFS's in-memory caches aren't included in MemAvailable despite being logically equivalent. fastfetch should subtract the size of ZFS's cache, similarly to other tools like htop. There's already ZFS-specific code in fastfetch for filesystem handling.
This can be implemented by reading /proc/spl/kstat/zfs/arcstats in addition to /proc/meminfo if it's present, and adding size - c_min to MemAvailable.
Activity
CarterLi commentedon Oct 13, 2025
Any additional information? Htop source? Is it required on FreeBSD and Solaris?
leo60228 commentedon Oct 14, 2025
I don't know for sure but I assume not. The root cause of this is how the Linux kernel computes MemAvailable. The ZFS issue for it is here, but it doesn't show any path to being closed: openzfs/zfs#10251
leo60228 commentedon Oct 14, 2025
Here's the relevant code in htop and btop.
CarterLi commentedon Oct 15, 2025
After reading the issue, especially someone running
sudo sync && echo 2 | sudo tee /proc/sys/vm/drop_cachesin crontab, I think it is a serious issue that should be fixed by openzfs themselves.I don't want to introduce special handling for a out-of-tree module in a common module, and doing it may introduce inconsistant behavior to FreeBSD and Solaris which also use zfs by default.
I'd rather not do this unless it can fix some issues like #1988
leo60228 commentedon Oct 15, 2025
I think the person who needed to automate cleaning the ZFS cache was experiencing an effectively unrelated bug. This is a Linux-specific issue relating to how /proc/meminfo computes MemAvailable that ZFS developers seem to think is intractable to fix without upstream kernel changes.
While it's not as obviously incorrect as reporting higher memory in-use than total, I don't think claiming a ~idle system has >20GB of RAM usage is any more helpful for getting an accurate picture of the system state.
Memory (Linux): takes ZFS arc bytes into account
Memory (Linux): takes ZFS arc bytes into account