Simulate a faulty block device with read errors?

Question

I\'m looking for an easier way to test my application against faulty block devices that generate i/o read errors when certain blocks are read. Trying to use a physical hard

Answer 1

I would like to elaborate on Peter Cordes answer.

In bash, setup an image on a loopback device with ext4, then write a file to it named binary.bin.

imageName=faulty.img
mountDir=$(pwd)/mount

sudo umount $mountDir ## make sure nothing is mounted here

dd if=/dev/zero of=$imageName bs=1M count=10
mkfs.ext4 $imageName
loopdev=$(sudo losetup -P -f --show $imageName); echo $loopdev
mkdir $mountDir
sudo mount $loopdev $mountDir
sudo chown -R $USER:$USER mount

echo "2ed99f0039724cd194858869e9debac4" | xxd -r -p > $mountDir/binary.bin

sudo umount $mountDir

in python3 (since bash struggles to deal with binary data) search for the magic binary data in binary.bin

import binascii

with open("faulty.img", "rb") as fd:
    s = fd.read()
    
search = binascii.unhexlify("2ed99f0039724cd194858869e9debac4")

beg=0
find = s.find(search, beg); beg = find+1; print(find)

start_sector = find//512; print(start_sector)

then back in bash mount the faulty block device

start_sector=## copy value from variable start_sector in python
next_sector=$(($start_sector+1))
size=$(($(wc -c $imageName|cut -d ' ' -f1)/512))
len=$(($size-$next_sector))

echo -e "0\t$start_sector\tlinear\t$loopdev\t0" > fault_config
echo -e "$start_sector\t1\terror" >> fault_config
echo -e "$next_sector\t$len\tlinear\t$loopdev\t$next_sector" >> fault_config

cat fault_config | sudo dmsetup create bad_drive
sudo mount /dev/mapper/bad_drive $mountDir

finally we can test the faulty block device by reading a file

cat $mountDir/binary.bin

which produces the error:

cat: /path/to/your/mount/binary.bin: Input/output error

clean up when you're done with testing

sudo umount $mountDir
sudo dmsetup remove bad_drive
sudo losetup -d $loopdev
rm fault_config $imageName