I test the mint ISO file with VM myself with persistence and later extend the persistence file, everything goes well.
When you boot with persistence file, you see /cow is the rootfs, but we can not see where is /cow mounted.
We can add a break=bottom boot option to the boot menu and make it break to a shell during boot
then we can see that /cow is actually mounted from /dev/dm-X (vtoy_persistent).
This means that the persistence mechanism actually take affect (your data file were successfully saved).
I take some screenshots, hope that can help.
(the forums limit up to 5 attachments, so the screenshots are in 3 posts)
When you boot with persistence file, you see /cow is the rootfs, but we can not see where is /cow mounted.
We can add a break=bottom boot option to the boot menu and make it break to a shell during boot
then we can see that /cow is actually mounted from /dev/dm-X (vtoy_persistent).
This means that the persistence mechanism actually take affect (your data file were successfully saved).
I take some screenshots, hope that can help.
(the forums limit up to 5 attachments, so the screenshots are in 3 posts)