[FEAT] Subtract ZFS ARC from memory usage

[leo60228]

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.

[CarterLi]

Any additional information? Htop source? Is it required on FreeBSD and Solaris?

[leo60228]

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]

Here's the relevant code in htop and btop.

[CarterLi]

The ZFS issue for it is here, but it doesn't show any path to being closed: openzfs/zfs#10251

After reading the issue, especially someone running sudo sync && echo 2 | sudo tee /proc/sys/vm/drop_caches in 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]

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.