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Help with understanding memory usage discrepancy (lemmy.world)

submitted 12 hours ago* (last edited 10 hours ago) by a_fancy_kiwi@lemmy.world to c/selfhosted@lemmy.world

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I recently noticed that htop displays a much lower 'memory in use' number than free -h, top, or fastfetch on my Ubuntu 25.04 server.

I am using ZFS on this server and I've read that ZFS will use a lot of RAM. I also read a forum where someone commented that htop doesn't show caching used by the kernel but I'm not sure how to confirm ZFS is what's causing the discrepancy.

I'm also running a bunch of docker containers and am concerned about stability since I don't know what number I should be looking at. I either have a usable ~22GB of available memory left, ~4GB, or ~1GB depending on what tool I'm using. Is htop the better metric to use when my concern is available memory for new docker containers or are the other tools better?

Server Memory Usage:

htop = 8.35G / 30.6G
free -h =

               total        used        free      shared  buff/cache   available
Mem:            30Gi        26Gi       1.3Gi       730Mi       4.2Gi       4.0Gi

top = MiB Mem : 31317.8 total, 1241.8 free, 27297.2 used, 4355.9 buff/cache
fastfetch = 26.54GiB / 30.6GiB

EDIT:

Answer

My Results

tldr: all the tools are showing correct numbers. Htop seems to be ignoring ZFS cache. For the purposes of ensuring there is enough RAM for more docker containers in the future, htop seems to be the tool that shows the most useful number with my setup.

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[–] non_burglar@lemmy.world 1 points 2 hours ago

Yes, ZFS cache has been contentious for exactly the reason you posted, but it is generally not a functional issue.

ZFS will release cache under memory pressure, however nice values of virtualizing can potentially demand it sooner than ZFS can release it.

There have been many changes to ZFS to improve this, but the legacy of "invisible cache" is still around.