Deploy nginx container results in a sustained write for approx 15 minutes

Each time I upgrade my nginx app, the deploy step takes about 15 minutes to complete. During that time, there is a sustained write to the data pool. What is happening in the deploy step that requires it to write that much data?

All my other containers - file browser, homepage, jellyfin, syncthing etc… deploy in a few seconds.

This is reproducible. Below is the disk report from the last upgrade of the nginx app.

Below is the screen cap of the current disk report with the nginx deploy still in progress just now.

Once it does what it does, the total capacity used on the pool does not increase, so I don’t think it is writing new data.

Anyone understand what is going on here? TIA