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I recently noticed a strange behaviour. I could not restart a productive webserver running since more than a year because the configuration was invalid:

# /etc/init.d/apache2 restart Syntax error on line 161 of /etc/apache2/apache2.conf:
Invalid command 'Order', perhaps misspelled or defined by a module not included in the server configuration Action 'configtest' failed.
The Apache error log may have more information.
   ...fail!

After further investigation I found a couple of files unter /etc/apache2/mods-availabe to have a file size of zero bytes

...
-rw-r--r-- 1 root root     0 Jun 15 15:14 authz_host.load
....

Is this an indication of file system corruption? I'm afraid to umount the filesystem in order to run fsck. Of cause the system is backuped but how can we spot something like this in the future?

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No, a zero bytes file is not an indication of file system corruption.

Likely symptoms of file system corruption include:

  • Error messages from the file system showing up on the kernel log
  • I/O errors when trying to read/write on the file system
  • Bogus file attributes, owner, sizes, names showing up in directory listings.

The most reliable way to identify file system corruptions is by running a file system check.

At some point ext4 developers decided to change semantics of certain system calls in a way which deviated from what earlier file systems did and a large number of applications expected. The result was that some combinations of ext4 code and applications could result in data loss when a file was updated. This would often result in an empty file - but not in any corruption of the file system.

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