[dm-crypt] Alignment issue with 4K disk

Sven Eschenberg sven at whgl.uni-frankfurt.de
Sat Jan 9 15:47:17 CET 2016

Hi Milan,

Am 09.01.2016 um 13:30 schrieb Milan Broz:
> On 01/07/2016 07:00 PM, .. ink .. wrote:
>> There is another (i think) related problem that was reported here[1]
>> Are these warnings serious or can they be ignored?
>> [1] https://github.com/mhogomchungu/zuluCrypt/issues/25
> If I read this correctly, it s caused by mapping of an unaligned plaintext device-mapper
> device (unlocked VeraCrypt) to underlying partition.

Looks like it...

> That's a bug (or lack of proper alignment) inside VeraCrypt header or in GPT partitioning
> - it should respect underlying device alignment while creating device/partition.
> Anyway, I think the only consequence should be performance drop, it should still work.
> Note that output from GPT says 512B while dm kernel message says 4k block... GPT should
> use 4k block as well.
> Reporter should paste output of "lsblk -t" with device activated to check what underlying
> device report alignment limits are violated in storage stack).

GPT fdisk claims 512 byte logical blocksize, which seems legit for a 
512e drive. It even says that partitions should be aligned to 2048 
sectors (which is MByte boundaries and 4k aligned). This seems all 
legit, as the GPT demands to be LBA aligned and 512e drives do indeed 
use a 512 byte sector addressing.

Even start=131072 is a multiple of 2048 and thus MByte aligned - It does 
indeed look a little odd.
> We cannot do anything with it I guess, cryptsetup just activates what is written inside
> Veracrypt header using alignment limits reported by kernel.

That is indeed true and further info including 'dmsetup table' will be 
necessary to tell what is really going on.

> Milan
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