[ale] Recommendation for off-line storage?

Thu May 11 11:07:12 EDT 2006

James,

I have been to several Enterprise class tape drive presentations where
the theory of data degradation on a non-spinning disk drive was
discussed.  I have not heard of any formal studies on the issue, so it
is theoretical.  OTOH, I have some personal experience, and we as a
company have several hundred disk drives "sitting on the shelf", some
from 3 or more years ago.  (Again, we have tape backups of these.)

The issue as I understand is that their are numerous lubicants inside
of a disk drive.

The lubricants naturally contaminate the air inside the sealed drive.

When the drive is running there is a small filter that cleans the air.
 I have opened numerous disk drives and they all have this filter.
Look for a small white plastic piece in one of the corners of the
drive cavity.

When the disk drive is not spinning, the air quits moving and the air
becomes saturated with lubricants and from the air starts depositing
on the drive platters and the drive heads.  I don't know what exactly
causes the problem. ie. lubricant on the head, the platter, or simply
in the air itself.

Also, the lubricant can get hard and cause things not to work
correctly from a mechanical perspective.  I think that is what the
supposed disk drive warmer is designed to address.  Again, I treat
this as an urban legend.

Anyway, I have powered on drives that had been "on the shelf" for a
couple of years and for the first hour or two the data was bad.  After
that the CRC errors quit happening.

I've also had several drives that simply would not give good MD5 hash
verification on the files after being on the shelf for a long time.
ie. We md5 hash most of our files prior to backing them up to tape and
putting the drives on the shelf.  When we need the data off the drives
again, we verify the hash before we proceed to use the data.

The numerous MD5 hash issues we have had has us currently deciding if
we should simply treat all drives as unreliable at the 12 month period
and start wiping and reusing them.  Obviously the tape backups then
become our primary repository for the data.

Greg

On 5/11/06, James P. Kinney III <jkinney at localnetsolutions.com> wrote:
> > At 03:48 PM 5/10/2006, Greg Freemyer wrote:
> >
> >>If so, there is NO data or gaurentee on disk drives maintaining data
> >>with power off.
>
> I have to disagree with this. The greatest data loss issue with any
> magnetic medium is the presence of an unplanned magnetic field that
> changes the bits on the medium. It doesn't matter if it's tape or hard
> drive, the effect is the same.
>
> The hard drive platter is made from a very stable material that hold it's
> magnetic domains intact for very long times. While all magnetic domain do
> tend to spread over time, there is nothing about powered on status that
> prevents this occurance.
>
> Hard drives fail in use due to mechanical issues (platter bearings,
> armature mechanicals, motor, etc), electrical (control electronics board
> fails) or platter meduim failure. The mechanical failure rate can be
> attenuated by infrequent use which reduces the strain on components.
> Electrical component failure seems to be of two types: sudden failure
> related to spike damage and manufacturing errors that cause poor joints,
> poor components, and improper signalling.
>
> The latter failure, platter medium failure, is more complicated. It
> involves the manufacturing aspects of the physical dimensions of the disk
> (basically it's flatness, it's roundness and it's uniform density) and the
> uniformity of the magnetic medium coating. The physical characteristics
> will have a great effect on the mechanical longevity of the drive. Out of
> balance spinning objects put excessive stress on mechanical systems.
> Flatness errors can lead to head impacts with the disk (which are nearly
> always catastophic). The magnetic medium is quite stable in use and in
> storage. Manufacturing errors can have coatings that are "thin" and this
> often results in bad sectors. Writing to the platter has more of an effect
> on the magnetic medium as the heads magnetic field exerts a mechanical
> force on the particles in the medium. Reading is passive and exerts no
> force on the medium. Since hard drive are not handled like RAM (they don't
> get refreshed periodically) the long term storage has no detrimental
> effects on the medium itself. The magnetic dispersal rate is comparable to
> that of high-speed tapes.
>
> What does damage hard drive is mechanical shock of moving between storage
> and active use. During mechanical shock, a phonon wave is propogated
> throughout the entire hard drive. Just like waves in a water tank, phonon
> waves can be reflected and refracted and they can be constructively and
> destructively combined. When the wave moves through a magnetic material,
> the magnetic fields are reoriented at the atomic level for a brief time.
> When the wave has passed, the material "relaxes" back to it's undisturbed
> state. Almost. As the wave passes through, the alignment of the magnetic
> domains is changed. This causes internal stresses that affect the final
> relaxed state. So the magnetic m edium gets altered with each passing
> phonon wave. The stronger the phonon wave, the greater the overall
> disturbance. Given enough of a shock, the entire magnetic domain for a
> cluster can be distrupted enough to render it unreadable. Magnitize a
> sewing needle by rubbing it on a magnet. Then strike it with a hammer and
> notice it looses it magnetic field.
>
>
> >>
> >>We do it a lot around here, but we also make a tape backup before we
> >>put the drives on the shelf.
> >
> > Well that certainly changes the equation.  I was indeed planning to leave
> > these drives on the shelf.
> >
> > I was planning to make two copies before deleting the on-line data -- so
> > maybe I need to send one copy to tape (for long-term retention), and one
> > to disk (for less-reliable retention, but easy searching and restoration).
> >
> > So now I'm looking for a tape-based solution too?  Same parameters apply,
> > I suppose:  Linux-friendly, non-proprietary, rack-mountable, minimal
> > hassle.
> >
> > Although the last time I dealt with tape drives, the tapes were only 20-40
> > GB.  I have 400 GB and the data center is a remote facility.  Now I'm
> > talking a tape carousel?  Am I going about this all wrong?  Should just
> > make two copies on IDE drives, take them off-line, and hope for the best?
> >
> That would work just fine.
>
> > A----
> >
> >
> >
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> >
>
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-- 
Greg Freemyer
The Norcross Group
Forensics for the 21st Century