Re: [dm-devel] [PATCH v2 0/2] Introduce the bulk IV mode for improving the crypto engine efficiency

[Mikulas Patocka]

On Wed, 13 Jan 2016, Arnd Bergmann wrote:

> On Tuesday 12 January 2016 18:31:19 Mikulas Patocka wrote:
> > 
> > Another possibility is to use dm-crypt block size 4k and use a filesystem 
> > with 4k blocksize on it (it will never send requests not aligned on 4k 
> > boundary, so we could reject such requests with an error).
> 
> Is there ever a reason to use something other than 4K block size on
> dm-crypt?
> 
> 	Arnd

You can't use 4k block on CBC (and most other encryption modes). If only a 
part of 4k block is written (and then system crash happens), CBC would 
corrupt the block completely.

For example, suppose that EXT2 directory block is updated, the first 
512-byte sector is written and the rest of the sectors is not written 
because of a crash. CBC would corrupt all sectors except the first one in 
this case.

You could use 4k block on XTS and ECB.

Mikulas