I was wondering, after a delete some log files, the disk is furthermore full.

Show which process is using a deleted files

lsof +L1

show something like that

filebeat 2416 root 608r REG 202,1 36527 0 135333 /var/log/syslog-20170509 (deleted)
filebeat 2416 root 615r REG 202,1 28672 0 135049 /var/log/syslog-20170511 (deleted)
filebeat 2416 root 618r REG 202,1 12288 0 135054 /var/log/syslog-20170512 (deleted)
filebeat 2416 root 621r REG 202,1 262144 0 140842 /var/log/syslog-20170513 (deleted)
filebeat 2416 root 623r REG 202,1 28672 0 134561 /var/log/syslog-20170521 (deleted)
filebeat 2416 root 628r REG 202,1 32768 0 134341 /var/log/syslog-20170515 (deleted)
filebeat 2416 root 634r REG 202,1 8192 0 133773 /var/log/syslog-20170517 (deleted)
filebeat 2416 root 638r REG 202,1 8192 0 136486 /var/log/syslog-20170518 (deleted)
filebeat 2416 root 640r REG 202,1 16384 0 135055 /var/log/syslog-20170519 (deleted)

Restart your process and all is fine

Found here

https://serverfault.com/questions/232525/df-in-linux-not-showing-correct-free-space-after-file-removal