Consider the following scenario:
- I use Linux device mapper to create a snapshot of an ext4 file system.
- The snapshot is mounted as read-only; the source volume is mounted as read-write.
- I read the snapshot, and simultaneously write (too much) to the source volume. Eventually, the copy-on-write table fills up.
Now exactly what happens in practice from the user's perspective? What kind of messages should I expect to see in dmesg? How do the applications that read the snapshot behave? Has someone actually tried this to see what would happen?