this post was submitted on 19 Jan 2026
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19
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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[–] victorz@lemmy.world 1 points 9 hours ago (1 children)

It's just that the system freezes for me when I used to run out of memory when I had only 32 GB of memory. Then I couldn't do anything and had to hard reset the computer with its reset button. Then it would be nice to have a little bit of swap to kill some stuff before literally everything just stops working.

[–] B0rax@feddit.org 1 points 6 hours ago (1 children)

Was that on Linux?

[–] victorz@lemmy.world 2 points 5 hours ago

Yeah, on multiple computers. Linux I feel will just happily hand out memory on loan like a bank rather than from what's actually available. Then when it runs out, the next request for more memory will just freeze the system. ☠️