[User Guide] Device Mapper Proxy

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Kernel module compile, install & testing

Step 0: Prerequisites

This module is developed and tested in Fedora 33.
Kernel used in tests is Linux 5.9.8-200.fc33.x86_64

For successful module compilation you need gcc and make tools installed. Use dnf install gcc make to install it.

Also corresponding version of linux kernel headers is needed. In Fedora they are provided by kernel-devel package. It can be installed with the command dnf install kernel-devel.

Step 1: Build

Clone this repository and navigate into its directory. Use make command to compile module:

    $ make

Step 2: Install

Use insmod command to insert module into a running kernel. In this and next steps superuser permissions is required, use sudo or switch to root using su.

    $ su
    $ insmod device_mapper_proxy.ko

If something went wrong you can check log using journalctl or dmesg for kernel or module error messages.

Check module is installed by running lsmod:

    $ lsmod | grep "device_mapper_proxy"

Step 3: Module testing

You can export device size in sectors for convenience reasons:

	$ size=20000

Create a new test block device:

    $ dmsetup create zero1 --table "0 $size zero"

Create a new device mapper proxy device:

    $ dmsetup create dmp1 --table "0 $size dmp /dev/mapper/zero1"

Check devices listed in /dev/mapper:

    $ ls -al /dev/mapper

Make some reading and writing using dd utility:

    $ dd if=/dev/random of=/dev/mapper/dmp1 bs=4k count=1
    $ dd of=/dev/null if=/dev/mapper/dmp1 bs=4k count=1

Statistics available through sysfs:

	$ cat /sys/module/device_mapper_proxy/stat/volumes

Example output looks like this:

read:
 reqs: 549
 avg size: 916
write:
 reqs: 8
 avg size: 512
total:
 reqs: 557
 avg size: 916

For comparison, I/O request stats for /dev/mapper/dmp1 (write and read request number should be equal):

    $ cat /sys/block/dm-1/stat