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The issue with an ext3/4 filesystem from a shred perspective is that the journal has a copy of the old data (for awhile). To use shred effectively, alter the /etc/fstab entry for your system from (showing home for an example):

/dev/mapper/fedora-home /home                   ext4    defaults                     1 2

to

/dev/mapper/fedora-home /home                   ext4    defaults,data=ordered        1 2

where the only change made is to the fourth item.

Reboot and shred as required. Then, if there is no cause to shred for awhile, change the filesystem table back and reboot.

The source for my suggestion is from man shred near the bottom:

In the case of ext3 file systems, the above disclaimer applies (and shred is thus of limited effectiveness) only in data=journal mode, which journals file data in addition to just metadata. In both the data=ordered (default) and data=writeback modes, shred works as usual. Ext3 journaling modes can be changed by adding the data=something option to the mount options for a particular file system in the /etc/fstab file, as documented in the mount man page (man mount).