qcow2 overlay size on cuttlefish crosvm

[Mikko Koivisto]

I'm using crosvm as VMM for Cuttlefish device.

The setup creates overlay image with

crosvm create_qcow2 --backing-file os_composite.img overlay.img

...and the content of backing file is described in readme in 

https://android.googlesource.com/device/google/cuttlefish/+/refs/heads/master/host/commands/assemble_cvd/

the size of the overlay file becomes same as size of the cuttlefish device userdata (6GB by default) after Cuttlefish has started. Is this expected?

I would like to have a setup in which overlay only grows when there is some difference between overlay and underlying images (i.e. when something is written to the disk)

I'm using aosp master branch in which

platform/external/crosvm is at 7716a71

device/google/cuttlefish is at 6afb824