Boot error due to No space left on device

[loveho]

Hi guys
Just used centos7 in a virtual box for months, but yesterday it suddenly cannot show the login screen. From the log can see the error is because of no space left on device.
Can you teach me 1) how to login in the system first. Is there anything like safe mode to login in like windows? and 2)how can i resolve this issue by deleting some files?
Thanks a lot.