If there's redundancy, you might shred one disk but not the other(s), or you might find that different passes have affected different disks such that each disk is partly shredded. RAID filesystems (depending on the RAID mode) might not overwrite all of the copies of the original blocks. If data is journaled, then each pass of shred might be written to a new location on disk, in which case nothing is shredded. So, journaling filesystems won't overwrite the original blocks in place, because that would stop them recovering cleanly from errors where the change is half-written. This is because without knowing non-standard things about the underlying filesystem, it can't. It does absolutely nothing to find out whether overwriting a file actually results in the blocks which contained the original data being overwritten. Particular file system in the /etc/fstab file, as documented in theĪll shred does is overwrite, flush, check success, and repeat. The data=something option to the mount options for a Ext3 journaling modes can be changed by adding Inīoth the data=ordered (default) and data=writeback modes, shred Mode, which journals file data in addition to just metadata. (and shred is thus of limited effectiveness) only in data=journal In the case of ext3 file systems, the above disclaimer applies Log-structured or journaled file systems, such as those supplied with AIX and Solaris (and JFS, ReiserFS, XFS, Ext3, etc.)įile systems that write redundant data and carry on even if some writes fail, such as RAID-based file systemsįile systems that make snapshots, such as Network Appliance’s NFS serverįile systems that cache in temporary locations, such as NFS version 3 clients Systems on which shred is not effective, or is not guaranteed to be

Traditional way to do things, but many modern file system designsĭo not satisfy this assumption.

That the file system overwrites data in place. So if I shred a document on my Ext3 filesystem or on a Raid, what happens? Do I shred part of the file? Does it sometimes shred the whole thing and sometimes not? Can it shred other stuff? Does it only shred the file header?ĬAUTION: Note that shred relies on a very important assumption: Shred documentation says shred is 'not guaranteed to be effective' (See bottom).

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