That's most likely the syslog. Check the settings, you can choose the volume to use for it.

Dirty secrets about you.

Have you run lsof? See also: https://www.brendangregg.com/linuxperf.html

Every 4-5 seconds? Yeah, logging.

You can either move the system dataset to your boot drive/pool or syslog to /var/log:

https://www.truenas.com/docs/core/coretutorials/systemconfiguration/settingthesystemdataset/

I've seem many users recommend a reboot after changing those settings.

If it’s logs, there’s a package called log2ram - it’s designed for small form factor systems to reduce writes to SD cards but does apply anywhere you want to log but not hit disk immediately. It syncs logs to disk on a regular basis so you don’t lose much if the system crashes.

Probably these directories...

/tmp /var/tmp /var/log

Two are easy to migrate to tmpfs if you are trying to reduce disk writes. Logs can be a little tricky because of the permissions. It is worth getting it right if you are concerned about all those little writes on an SSD. Especially if you have plenty of memory.

This is filesystem agnostic btw so the procedure can apply to other filesystems on Linux operating systems.

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