Based on this thread it’s the deduplication that requires a lot of RAM.
See also: https://wiki.freebsd.org/ZFSTuningGuide
Edit: from my understand the pool shouldn’t become inaccessible tho and only get slow. So there might be another issue.
Edit2: here’s a guide to check whether your system is limited by zfs’ memory consumption: https://github.com/openzfs/zfs/issues/10251
There’s a bug in Lemmy v0.19 and v0.19.1 that relates to federation: https://github.com/LemmyNet/lemmy/issues/4288
Gotta wait until it’s fixed and all instances update.