[dm-crypt] dm-integrity standalone - lack of metadata

[Harald Braumann <harry@unheit.net>]

Hi!

I'm using the dm-integrity target standalone with crc32c for component
devices of a RAID. I don't use journaling, because the perfomance hit
is too big and I don't really need it in that case.

I want to automatically activate these targets using udev
rules (see below). However, the superblock lacks some crucial
information:

- Integrity algorithm
The algorithm is not stored in the superblock and so I have to use
crc32 hard-coded in my udev rule. This is of course no general
solution. I don't quite understand why it is not stored in the
superblock. Is there a technical reason for that?

- Journal mode
I open the integrity devices without journaling. But again, that's
hard-coded and it would a be problem if I had other devices, where I
actually want journaling. It would be nice, if the superblock
contained a use_journal flag so the default could be configured on the
device itself.

- UUID/label support
Not strictly required for my use case, but would be nice for
persistent naming.

While playing around with that, I also noticed that integritysetup
format is extremly slow (this is on an HDD):

integritysetup format /dev/sdb2 -v --integrity crc32c --sector-size
4096
=> 39.6 MiB/s

Format with --no-wipe, open the target and use dd:
dd if=/dev/zero of=/dev/mapper/int bs=1M
=> 63 MiB/s

Same as above, but opening without journal (which really isn't
required for that step, even if the final target should be journaled):
=> 135 MiB/s

I have absolutely no problem with using the dd method, but it kind of
makes the built-in format method redundant. 

Best regards
harry

/etc/udev/rules.d/70-dm-integrity.rules:

ACTION!="add", GOTO="dm_integrity_end"
SUBSYSTEM!="block", GOTO="dm_integrity_end"
ENV{ID_FS_TYPE}!="DM_integrity", GOTO="dm_integrity_end"

RUN+="/sbin/integritysetup open $devnode $kernel-integrity --integrity crc32c --integrity-no-journal"

LABEL="dm_integrity_end"