Archive > 21.7 Legacy Series

ZFS install - tuning determined at install or at each reboot?

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dinguz:
When 21.7 came out, I reinstalled my system using the ZFS filesystem. The system back then had only 4 GB of RAM, and I vaguely remember the installer stating that certain settings related to ZFS had been applied, this being a 'low memory' system.
I have since upgraded the RAM to 8 GB. Do I need to change any of these settings manually to adjust to the new situation, or are these all dynamically determined at each reboot of the machine?

franco:
It would help to know the message exactly as it was written to trace this in the source code. Not too familiar with that area of bsdinstall yet.


Cheers,
Franco

dinguz:
Replying to my own question here, to perhaps benefit others. I have studied this somewhat and believe this be related to ZFS prefetch. This is automatically disabled at boot time for systems with up to 4 GB of RAM, and enabled for systems with more RAM. It's also tunable with this sysctl: vfs.zfs.prefetch_disable
As far as I could tell the installer doesn't set this sysctl anywhere but leaves it to the boot time heuristics to set this  appropriately.

Patrick M. Hausen:

--- Quote from: dinguz on September 19, 2021, 07:56:17 pm ---As far as I could tell the installer doesn't set this sysctl anywhere but leaves it to the boot time heuristics to set this  appropriately.

--- End quote ---
Correct. If this is indeed the only tunable of concern, it's best to not touch it at all. The system will do the right thing at boot.

HTH,
Patrick

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