Re: [dm-crypt] What happen if hard drive has a read error?

[Arno Wagner <arno@wagner.name>]

On Thu, Oct 07, 2010 at 11:29:02PM +0200, Milan Broz wrote:
> On 10/07/2010 10:20 PM, octane indice wrote:
> > It's just a theorical question. I encrypted some data with dm-crypt on an
> > hard drive.
> > But what happens if the hard drive has an error? Not a kind of a big error
> > which cause  the hard drive hangs, but just a small error that make the
> > system doesn't read the bytes all right?
> > 
> > Obviously, dm-crypt won't decrypt the data. But what happens next?
> 
> Basically the same like if you have IO error on plain disk - the 
> IO operation returns IO error. Upper layer decides what to do next.

Just to clarify this a bit further: disk error correction
capability is limited, an unrecoverable read error is a real 
possibility. The datasheets say once every 10^15 bits read,
which whould translate to an unreadable sector once every 
125 TB read. I think this is realistic, but still a bit of a 
worst case scenario. Also keep in mind that this is for a 
healthy disk only. 

The error detection capabilities of the coding uses is far, far 
larger.  An undetected unrecovrable error (i.e. you get wrong 
data but no error even on retry) is something unlikely enough 
that you can safely ignore the possibility. There are special
safeguards against this, as error correction by itself can 
correct to the wrong value.

Nonetheless, you can find wrong data on disks without any 
explanation or detected errors. The reason is typically 
corruption in RAM or in busses before the data is written.
For a complex data aquisition system (compression -> 
network -> disk-buffer -> tape-library), I have observed 
something like one  unexplained bit error every 5-10TB 
written. This number may vary wildly in practice. Especially
overclocking and cheap non-ECC RAM may drive it up.

Arno
-- 
Arno Wagner, Dr. sc. techn., Dipl. Inform., CISSP -- Email: arno@wagner.name 
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