Cant understand the File system working nature

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http://www.unix.com – Hi, We have a filesystem whose usage has gone above 81%, i tried to zip some files in the sub directories but the space is not released in the main file system. > df -h Filesystem size used avail capacity Mounted on /u01/app 5.8G 4.6G 1.2G 81% /u01/app /u01/app/oracle/exp 16G 582M 15G 4% /u01/app/oracle/exp Here we have FS /u01/app with size 5.8 GB and /u01/app/oracle/exp with size 16 GB. So if i delete or zip files in /u01/app/oracle/exp FS usage should reduce in /u01/app right. Please advice. (HowTos)