du vs df giving wrong disk usage results

du vs df giving wrong disk usage results

In OS, oracle is reported as using 168 Gb

While in oracle, the usage is only 78 Gb

How can that be?

Space usage in Linux:

find . -type f -printf '%s %p\n'| sort -nr | head -40

Filesystem                     Size  Used Avail Use% Mounted on

/dev/mapper/oravg2-ora_db      196G  168G   19G  91% /oracle_db/db3

At this point , many files were dropped using the Oracle syntax

Example:

Drop some big tablespaces, each one is several Gb in size.

DROP TABLESPACE OLD_TRANSACTION_202401 INCLUDING CONTENTS AND DATAFILES;

DROP TABLESPACE OLD_TRANSACTION_202402 INCLUDING CONTENTS AND DATAFILES;

SELECT tablespace_name FROM dba_tablespaces WHERE tablespace_name = 'OLD_TRANSACTION_202401';

No Data Found

See now space usage in Linux:

df -hP | grep db3

Filesystem                     Size  Used Avail Use% Mounted on

/dev/mapper/oravg2-ora_db      196G  168G   19G  91% /oracle_db/db3

No change in space usage, how can that be?

Check space usage with du

>cd /oracle_db

>du -sh *

78G     db3

du report usage of 78Gb, which is in sync with oracle stats, but df report a wrong result.

A common cause is that the file was deleted by a process, but is still in a deleted status, and was not deleted by the OS.
indeed this was the case here:

>lsof | grep deleted

/oracle_db/db3/db_igt/OLD_TRANSACTION_202401_1.dbf (deleted)

oracle     37484    oracle  424u      REG             253,17  4404027392    2621478 /oracle_db/db3/db_igt/OLD_TRANSACTION_202402_1.dbf (deleted)

oracle     37484    oracle  428u      REG             253,17  3670024192    2621524 

Theory:

When Linux mount point usage as reported by df does not align with the sum of file sizes reported by du, several common scenarios can explain the discrepancy

1. Open but Deleted Files

A file may be deleted using rm, but if a process still holds an open file descriptor to it, the disk space occupied by that file is not immediately released. 

It remains allocated until the process either closes the file or terminates. 

This is a very common reason for df showing higher usage than du.

For Oracle, a simple solution is to bounce the instance.

There are other options, not related to Oracle scenario

After oracle restart

>df -hP | grep ora

Filesystem                     Size  Used Avail Use% Mounted on

/dev/mapper/oravg2-ora_db      196G   78G  109G  42% /oracle_db/db3

Issue fixed!