[dm-devel] Help request: setting up /dev/zero using dm-crypt ...

[Milan Broz]

On 02/07/2018 09:15 AM, Ravi Prakash Putchala wrote:
> We would like to experiment and see the performance results with different options of dm-crypt. To do this we would like to setup /dev/zero with dm-crypt as mentioned in https://www.redhat.com/archives/dm-devel/2015-February/msg00106.html
> However, when I did the following no new mapping gets created in /dev/mapper:
> 
> # cryptsetup -y luksFormat /dev/zero

This command cannot work, /dev/zero is a character device, moreover all writes there are discarded.

Seems cryptsetup does not print error message here (that is a bug) but return value of this command
indicates the problem: 4 - that means "wrong device specified" (see man page).

If you need experiment over a memory device, use either ramdisk, scsi_debug module to it.
Another alternative is to create dm-zero device and use plain dm-crypt (not LUKS).

Milan