Just saw this post at adafruit:
learn.adafruit.com/read-only-raspberry-pi/
I think it would be a good idea to add this option to Volumio.org if possible… This way the less tech minded people can power down a pi in the same way the power down an Amp.
Volumio does not need this: the system runs on RAM, and SD Card is written only when saving settings
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amazing.
Thnx, I use a powerbank that cuts the power to all appliances (cd player, gramophone, little lamp) and volumio pi when I power off the Amp.
so far I always needed to shutdown the pi before switch off the Amp.
But no longer!
Thnx again for this info.
Just tested your statement. That is not true.
Volumio version: volumio-2.389-2018-03-26-pi
RPi3
IQaudIO Digi+
Turned off the power for RPi3(Volumio 2.389) without shutting down Volumio from Web interface.
Volumio is not accessible anymore. I guess I have to write SD card again. This issue is not solved 
anymore clarity on this?
At Moode they say that:
There is a recipe for running moOde under the overlayfs read-only file system which IIRC works but has not been reproduced and tested by others.
The main drawback to RAM / read-only mode’s for OS’s is that you have to go through hoops and revert to read-write mode every time a setting or option is changed, then revert back to read-only mode. IMO it’s a convenience killer plus added complexity without a matching compensating benefit.
Then Volumio obviously implemented it with a bit more sense😀.
I think you might be mixing squashfs (read-only) and overlayfs.
The overlay filesystem allows you to dynamically create a filesystem (overlay) with a read-only squash filesystem and a writable data part without “going through loops”.
The overlay is re-created at every boot start before Volumio is started up, for Volumio it is a normal read-write system.