What I did
When I want to change my nas,
I use Hyper backup software to backup my nas,
Copy from my nas to USB device.
But when I run the copy task,
it tells me "Run command fail"
So I reboot the nas,
and then I see "The storage is full, you can't login." message.
Login with SSH,
I see the partation /dev/md0 100% used.
I cost a lot time to find the reason,
finally I figure it out.
Steps
Run df -h command you will see something like this,
the /dev/md0 is 100% used.
root@SynologyDS416j:/# df -h
Filesystem Size Used Avail Use% Mounted on
/dev/md0 2.3G 1.1G 1.1G 100% /
none 1.9G 0 1.9G 0% /dev
/tmp 1.9G 2.3M 1.9G 1% /tmp
/run 1.9G 4.1M 1.9G 1% /run
/dev/shm 1.9G 548K 1.9G 1% /dev/shm
none 4.0K 0 4.0K 0% /sys/fs/cgroup
cgmfs 100K 0 100K 0% /run/cgmanager/fs
/dev/vg1000/lv 5.5T 4.3T 1.2T 80% /volume1
/dev/sdq2 7.3T 4.4T 2.9T 61% /volumeUSB1/usbshare1-2
Now the purpose is to solve this problem.
First, You can see the /dev/md0 is mounted on /,
the root folder.
It means every folder in root may used the space,
You must find it out yourself.
In my case,
the Hyper backup software copy file into /volumeUSB1 folder,
This is the problem:
the /volumeUSB1/ folder is under the /, not USB device!
The right USB device folder is /volumeUSB1/usbshare1-2/
So I delete the files under /volumeUSB1/,
and reboot nas,
I can login again.
Summary
1, Find where the /dev/md0 is mounted.
2, Find which folder used the space.
3, Delete the files.
4, Reboot.