[dm-crypt] The future of disk encryption with LUKS2

Sven Eschenberg sven at whgl.uni-frankfurt.de
Mon Feb 8 18:26:49 CET 2016


Indeed usually a disk should be able to finish the sector write with 
remaining power. Actually most modern disks do have voltage shifters and 
most parts operate on lower voltage. Thus a drop on the changer's input 
does not immediately lead to a drop on the output of the voltage 
shifter. If's theres enough power left for the physical layer scrambler 
and the head to write, then everything should be fine. I was rather 
wondering if there's definite sources on that?

BTW. The burst errors I mentioned did not happen on a power loss, but 
rather during operation. Reading twice, one time with burst errors, one 
time without. I checked the RAM for ages - no failures. That was really 
weird.

Regards

-Sven


Am 08.02.2016 um 17:41 schrieb Arno Wagner:
> On Mon, Feb 08, 2016 at 01:25:31 CET, Sven Eschenberg wrote:
> [...]
>> Concerning disks, I thought with ACS2/ATA-8 real write barriers were
>> introduced. On the other hand I've seen disks returning successfull
>> reads with long zero-burst-errors undetected - no fun. I always
>> wondered how a HDD exactly behaves when power fails, while a sector
>> is in transit. My best hope is, that the CRC at the end of the
>> sector does not match and an error is returned on the next read?
>
> For these you should have intact data on disk, but
> your data never made it there. If data after the zeros
> did get written fine, there is a simple explanation:
> Modern disks may reorder sectors in order to be able
> to begin writing as soon as the heads are stable in
> the track.
>
> Behavior on power failure used to be that the disk will
> notice the power failing early enough that it has enough
> time left with hood power to finish a sector-write in
> progress. I think that still applies. The zeros would
> then be sector-aligned and/or the data that was in
> those sectors before, hence the checksums are fine.
>
> The thing is that in a typical PC, power drops relatively
> slowly and disks work non-seeking for a lower voltage
> that the thresholds. Add to that that a single sector
> write takes less than 1ms (probably much less), and
> you get ample time to finish a write in progress.
>
> Regards,
> Arno
>


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