macOS is writing 32GB of data during every boot, on new installs of Big Sur and Monterey

Question

I have an issue where brand new installations, both minutes old and weeks old of Big Sur and Monterey (not tried other major releases) are writing exactly 32GB of data to the SSD boot drive during boot, on every single boot.

I boot the machine (2017 5K iMac 64GB RAM (4x16GB)) and there's a delay during the boot progress bar for around 40 seconds. When booted I check Activity Monitor and it shows what appears to be 32GB+the usual 0.2GB to 0.3GB of typical boot writes and a couple of GB of reads.

I've had it once during a verbose boot (Cmd+V) behaves normally, takes seconds to boot, and shows .2 to .3GB of writes and the usual ~2GB reads to get to the desktop, but I've been unable to replicate this.

I've looked through the console logs trying to find a pause of 40~ seconds between log entries but can't find it.

It seems odd it's writing 32GB of data, it seems to be written to /dev/null because there's no trace of any space being used and no files of 1GB+ on the boot volume.

What could this be and what can I try next to troubleshoot this issue?

This happens with or without FileVault enabled and doesn't affect the performance of the machine in any way once booted.

Storage is the internal PCI 3.0 X4 128GB blade SSD.

The first screenshot shows that powerd is writing the data. The second screenshot shows a safe boot screenshot and the third is a normal boot after a safe boot.

Answer 1

After much investigation, I've found the solution.

I still don't know what exactly powerd process was doing, or why or where it was writing the 32GB of data.

At some point I've obviously tweaked the power management settings, and on new installs restoring my data (which I assumed was independent from settings such as that) has restored my tweak to pmset.

I ran sudo pmset restoredefaults and this has fixed the issue. The system now boots normally, reading around 1.5GB and writing 280~MB.

It still doesn't make sense that this is the hibernate file being written, because the system has 64GB of ram, not 32GB. So I still don't know which pmset option I changed to cause this issue, but if anyone else encounters it, at least now we know the fix.