"
Turns out the tunable name for OPNsense is (the older - according to most info I found) vfs.zfs.arc_max - so an underscore not a dot between "arc" and "max". Working as expected now.
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25.1, 25.4 Series / Re: High ARC RAM usage but vfs.zfs.arc.max doesn't help
September 03, 2025, 06:22:33 PM #2
25.1, 25.4 Series / High ARC RAM usage but vfs.zfs.arc.max doesn't help
September 02, 2025, 11:11:16 AM
We recently had a big problem after a upgrade to the latest business release, the master of our HA pair was fairly well trashed.
We managed to recover from that using the configuration importer, but realised that we should swap from UFS to ZFS to enable snapshots. Which we did.
Now we're getting alerts from our monitoring system because all the RAM in the box (16GB, physical server) is being gobbled up by ARC. I know that's what it is supposed to do, but it isn't necessary to have more than about an 8GB cache on a box with hardly any disk IO.
So, I added a System: Settings: Tunables entry for vfs.zfs.arc.max and set it to 8589934592 (which is 8GiB) and clicked save. No change. Rebooted, no change - definitely more than 8GB RAM is being used by ARC - both as shown in the dashboard and via top.
Is that tunable broken? Do I need to do anything else to make it take effect? Have I misread/misunderstood the documentation on it? I'd rather not fiddle around in the OS if possible, but am happy to try short-term tweaks if that helps figure out why the ARC RAM limit isn't being respected.
Thanks in advance.
We managed to recover from that using the configuration importer, but realised that we should swap from UFS to ZFS to enable snapshots. Which we did.
Now we're getting alerts from our monitoring system because all the RAM in the box (16GB, physical server) is being gobbled up by ARC. I know that's what it is supposed to do, but it isn't necessary to have more than about an 8GB cache on a box with hardly any disk IO.
So, I added a System: Settings: Tunables entry for vfs.zfs.arc.max and set it to 8589934592 (which is 8GiB) and clicked save. No change. Rebooted, no change - definitely more than 8GB RAM is being used by ARC - both as shown in the dashboard and via top.
Is that tunable broken? Do I need to do anything else to make it take effect? Have I misread/misunderstood the documentation on it? I'd rather not fiddle around in the OS if possible, but am happy to try short-term tweaks if that helps figure out why the ARC RAM limit isn't being respected.
Thanks in advance.
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