DeviceMapperProxy (dmp)

Description

dmp is a kernel module for linux that creates virtual block devices on top of an existing device mapper-based device and monitors the statistics of operations performed on the device. The statistics are accessible through the sysfs module.

Instalation

To install dmp module to your system use make script to build kernel module and insmod command to insert it:

make
insmod dmp.ko

Check the availability of dmp module by command dmsetup targets

Usage

Virtual device creation

To create virtual device using dmp:

sudo dmsetup create $virtual_dev_name --table "0 $num_sectors dmp /dev/$some_dev"

Where:

• $virtual_dev_name - name of a new virtual device (e.g. dmp1)
• $num_sectors - size of a new virtual device in sectors
• $some_dev - underlying device

For more information see dmsetup manual.

Statistics

Using sysfs, can be obtained the following statistics about created virtual devices:

• read_cnt - number of read requests
• write_cnt - number of write requests
• total_cnt - total number of requests
• read_avr_bs - average block size per read
• write_avr_bs - average block size per write
• total_avr_bs - total average block size

These userspace statistics are accessible via sysfs attributes:

cat /sys/module/dmp/stat/$device_name/$attribute

Where:

• $attribute - some statistical information
• $device_name - virtual device kernel name

Summary statistics are also available in formatted form:

cat /sys/module/dmp/stat/$device_name/summary

Output:

read:
       reqs: <number>
       avg size: <number>
write:
       reqs: <number>
       avg size: <number>
total:
       reqs: <number>
       avg size: <number>

All statistics are available both for each device and for all devices simultaneously. To access common statistics see /sys/module/dmp/stat/all_devs/

Spinlocks are used to maintain consistency of statistical data.

Testing

Tests were carried out on the kernel: Linux 6.2.0-39-generic x86_64

Example test script (with dd):

sudo dmsetup create zero1 --table "0 $size zero" 
sudo dmsetup create dmp1 --table "0 $size dmp /dev/mapper/zero1"
sudo dd if=/dev/random of=/dev/mapper/dmp1 bs=4k count=1
cat /sys/module/dmp/stat/all_devs/summary

This script tests writing to the device. First, dmp1 is created based on the virtual block device zero1. Then a record is made to it through the dd utility. At the very end, statistics are displayed.

Strongly recomended insted of dd using . It is strongly recommended to use fio utility instead of dd.

License

Distributed under the GPLv2 License. See LICENSE for more information.