DM Flakey

Created: 2023-12-08 13:30

The Linux dm-flakey target is the same as the linear target except that it exhibits unreliable behaviour periodically. It’s been found useful in simulating failing devices for testing purposes.

To use it do the following:

sudo dmsetup create <name> --table "<begin-sector> <size> flakey <source-device> <offset> <period-normal> <period-faulty>"

This would create a block device under /dev/mapper/<name> which forwards IO to <source-device> for <period-normal> after which it returns IO errors for <period-fault>.

The module has more options, for example it can drop writes or only error on reads. For the full options see the kernel documentation.

Full example#

Below is a script which builds a disk image. Writes a filesystem to the image, adds a few random files. And creates a flakey device for this image.

#!/usr/bin/env bash

set -o errexit
set -o nounset
set -o pipefail


DISK_IMAGE=${DISK_IMAGE:-"disk_image"}
DISK_SIZE=${DISK_SIZE:-1024}
DISK_MOUNT=${DISK_MOUNT:-"disk_mount"}
NUMBER_OF_FILES=${NUMBER_OF_FILES:-5}
FILE_SIZE=${FILE_SIZE:-100}
FLAKEY_DEVICE=${FLAKEY_DEVICE:-"flakey-test"}
FLAKEY_MOUNT=${FLAKEY_MOUNT:-"flakey_mount"}
FLAKEY_PERIOD=${FLAKEY_PERIOD:-5}


dd if=/dev/zero of="${DISK_IMAGE}" bs=1M count="${DISK_SIZE}" status=progress


sudo losetup -f "${DISK_IMAGE}"
LOOP_DEVICE=$(sudo losetup|grep "${DISK_IMAGE}"|awk '{print $1}')


sudo mkfs.ext4 "${LOOP_DEVICE}"


mkdir "${DISK_MOUNT}" || echo "Mount directory already created"
sudo mount "${LOOP_DEVICE}" "${DISK_MOUNT}"
sudo chown "${USER}":"${USER}" -R "${DISK_MOUNT}"


pushd "${DISK_MOUNT}"
for index in $(seq 0 "${NUMBER_OF_FILES}"); do
    dd if=/dev/random of="file${index}" bs=1M count="${FILE_SIZE}" status=progress
done
popd


sudo umount "${DISK_MOUNT}"


sudo dmsetup create "${FLAKEY_DEVICE}" --table "0 2097152 flakey ${LOOP_DEVICE} 0 ${FLAKEY_PERIOD} ${FLAKEY_PERIOD}"


mkdir "${FLAKEY_MOUNT}" || echo "Mount directory already created"
sudo mount "/dev/mapper/${FLAKEY_DEVICE}" "${FLAKEY_MOUNT}"

References#

dm-flakey