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

From: Mikulas Patocka
Date: Tue Jan 12 2016 - 21:18:18 EST

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.