I had a filesystem that was over 60%. I found that the file /var/log/messages had more than 4 million lines so I decided to delete 2 million lines in order to clear some space. I executed the command
sed -i '1,2000000d' /var/log/messages
the lines were deleted and the log size decreased about 300MB, but the filesystem usage increased from 60% to 90%.
Then I restarted the service rsyslog but nothing happened, the usage is still 90%.
Note that if I execute the command du -sh * and then I sum the usage of each file the total usage does not correspond with the 'Used' value of the filesystem. Seems like there should be a hidden file occupying the rest of the space but I do not know how to find it or delete it.