Re: Creating detached dm-integrity hashes for existing disk

Hello all!  (Please keep me in CC, as I'm not subscribed to the mailing list.)

I've sent the following email two months ago, but I assume that either
it slipped through the cracks or nobody has any experience with the
use case I'm mentioning.  However, I'll try to submit this question
one more time, perhaps with more luck this time.  Thanks for the time.


~~~~


I already have a large disk containing important data (backup data)
and I would like to use `dm-integrity` (with meta-data on a separate
device) to make sure that when reading from that disk I actually get
the original, non-corrupted, data.  (It seems that just the SATA
provided CRC isn't enough sometime with a problematic USB-to-SATA
adapter, and I get successful reads with corrupted data.)

Thus, I've tried using `integritysetup format` with `--no-wipe`,
`--data-device` and `--integrity-bitmap-mode`, and then
`integritysetup open` with `--integrity-recalculate`.  However upon
subsequent `integritysetup open` without the recalculate flag, the
`dump` command still shows the recalculating flag.  Searching on the
internet about this I've found only a few issues about this, but no
actual solution or more insight.

As such my question:  is there a way to actually force `dm-integrity`
to compute the hashes without wiping my original data?




Here are the snippets I've tried (on blank files via loop devices):


# create empty files
truncate --size 4GiB -- /tmp/dm-integrity-tests-data
truncate --size 512MiB -- /tmp/dm-integrity-tests-meta

# fill the data with something to make sure dm-integrity doesn't touch
the original data
dd if=/dev/urandom of=/tmp/dm-integrity-tests-dat
a bs=1M count=128
iflag=fullblock status=progress

# compute a hash of the original data
md5sum -b -- /tmp/dm-integrity-tests-data /tmp/dm-integrity-tests-meta


# the data is loop0, and the meta-data is loop1
losetup --show --sector-size 4096 --find /tmp/dm-integrity-tests-data
losetup --show --sector-size 4096 --find /tmp/dm-integrity-tests-meta


# format, but DO NOT TOUCH the original data
# both the bitmap and hash should use 4096 bytes
integritysetup format \
        --batch-mode \
        --verbose \
        --no-wipe \
        --sector-size 4096 \
        --integrity sha256 \
        --integrity-bitmap-mode \
        --bitmap-sectors-per-bit 8 \
        --data-device /dev/loop0 \
        -- \
        /dev/loop1 \
#


# open, requesting a recalculation of the hashes
integritysetup open \
        --verbose \
        --integrity sha256 \
        --data-device /dev/loop0 \
        --integrity-recalculate \
        -- \
        /dev/loop1 \
        dm-integrity-tests \
#

# read the target, checking if we get our original data
dd if=/dev/mapper/dm-integrity-tests bs=1M iflag=fullblock status=progress \
| md5sum -b

integritysetup close dm-integrity-tests


# open, without the recalculation flag
integritysetup open \
        --verbose \
        --integrity sha256 \
        --data-device /dev/loop0 \
        -- \
        /dev/loop1 \
        dm-integrity-tests \
#

integritysetup close dm-integrity-tests


# recalculation still shows up
integritysetup dump -- /tmp/dm-integrity-tests-meta


Am I missing something?  Shouldn't the recalculation flag disappear?

Thanks,
Ciprian.