Eric H. Johnson on 8 Jul 2016 08:04:40 -0700

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Re: [PLUG] RAID striping


Thanks for the detailed answer. The most sensitive data on these drives is
payroll information that is several years old, but nothing that rises to NSA
level secrecy.  :)


Setting aside the good advice to destroy a drive if you can't wipe it...

In traditional raid (including mdadm) the data will be striped across all
your drives in stripes.  So for any particular byte on the drive parts of
the byte are stored on all of the drives in the array.

Generally speaking it should not be possible to reconstruct any data from a
single drive in a 6-drive raid5.

However, there certainly could be exceptions to that.  If you had data with
a very low density (imagine a file where you stored one bit surrounded by
zeros in every few MB range of a file), then it probably would be possible
to read the file from a single drive (most likely the encoded version of the
file would be identical in all blocks except where a 1 was stored, with no
more than one bit per stripe, so you can read it off by the presence/absence
of the one/zero pattern).
Besides the space savings, this is why there is often a security benefit to
compressing files before encrypting them (something built into many
encryption programs).  When you're only storing a few bits of data per block
of encoding things can sometimes break down.  And unlike crypto systems the
raid algorithms are probably not engineered to avoid leaking information (if
anything, you want a raid to be as easy to recover as possible).

I'm sure there are other things that would cause it to break down, like
files with internal relationships (maybe you can narrow down every byte in a
file to a list of certain possibilities, and then relationships within the
file let you narrow that down further).

So, if your threat model is credit card thieves and such, you probably don't
need to worry.  If you're storing highly valuable data or the NSA is
interested in you, I wouldn't take chances.

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