That's probably the ARC, the in-memory caching structure, which gives up memory when applications need it. The weird thing on Linux (and maybe the others) is th

That's probably the ARC, the in-memory caching structure, which gives up memory when applications need it. The weird thing on Linux (and maybe the others) is that the kernel marks the memory as "used" and not "cache", so tools like top/htop show high memory usage, even though ZFS will shrink its ARC when any actual applications require it.