Since I have migrated from my DS713+ to a DS1815+, I can't update seamlessly my DSM. Most of the time, my NAS pretends that I have "Not enough storage space". This problem is due to large log files filling my system partition.
After the migration top my new NAS, I have reinstalled JDownloader. This one is using a script to run a java application via "nohup". Nohup's log is growing fast and takes a lot of storage, preventing my NAS to download the DSM updates.
I found it was the culprit via an 'iterative' procedure suggested by maxxfi on the forum of Synology:
Use your admin password. Once authenticated as root, type:
That command will give you for each subdirectory whose name begins with a letter from a to u (so to avoid the volumeX) the total amount of disk space in use. Wait that it finishes and search for which directory shows the biggest numbers, and type:
Repeat the analysis going deeper into the directory tree until you are able to pinpoint the culprit. Finally, use the appropriate "rm <filenames>" command to delete the largest folder/files.
In my case, the largest file was /root/nohup.out
But in the past , I also had to deleted /var/log or /var/log/messages and reboot prior updating the DSM.
In a Putty session, to know the free space in the system partition, type: df -k