nbdkit-evil-filter1

Usage

Overview

nbdkit-evil-filter is a Byzantine filter for nbdkit(1) that randomly corrupts data when reading from the underlying plugin. This can be used for testing filesystem checksums. Note that it does not change write operations, so the underlying plugin contains the correct data.

nbdkit-error-filter(1) is a related filter that injects hard errors into the NBD protocol.

This filter has several modes, controlled using the "evil=..." parameter. These are:

"evil=cosmic-rays"

Bits are flipped at random when reading data. The probability that a bit is flipped is controlled using the "evil-probability" parameter, defaulting to 1e-8 (on average 1 in every 100 million bits read is flipped).

"evil=stuck-bits"

This is the default mode.

Fixed bits in the backing file are stuck randomly high or low. The "evil-probability" parameter controls the expected probability that a particular bit is stuck, defaulting in this mode to 1e-8 (1 in 100 million). "evil-stuck-probability" controls the probability that a stuck bit is read as its stuck value or its correct value, defaulting to 100% (always read as a stuck bit).

"evil=stuck-wires"

This is similar to "stuck-bits" but instead of simulating bad backing data, it simulates stuck wires along the data path (eg. in a register). The difference is that when reading, the stuck bit always happens at the same position in the packet of data being read, regardless of where on the underlying disk it is being read from. "evil-probability" controls the probability of a stuck wire, defaulting in this mode to 1e-6 (1 in 1 million). "evil-stuck-probability" controls the probability that a stuck bit is read as its stuck value or its correct value, defaulting to 100% (always read as a stuck bit).

Examples

Add some stuck bits to the backing file at random:

 nbdkit --filter=evil file disk.img

Cosmic rays will flip (on average) one in every 100 million bits copied from the backing file over NBD:

 nbdkit --filter=evil file disk.img evil=cosmic-rays \
        --run 'nbdcopy $uri output.img'

Notes

 nbdkit --filter=evil --filter=noextents memory 1G

Parameters

evil=cosmic-rays

evil=stuck-bits

evil=stuck-wires

Select the mode of evil. See the "DESCRIPTION" above. The default is "stuck-bits".

evil-probability=N

evil-probability=N:M

evil-probability=N%

Set the probability for the mode. You can either use a floating point number between 0 and 1, eg. "evil-probability=0.001" or "evil-probability=1e-6". Or you can write it as N in M, eg. "evil-probability=1:1000000" or "evil-probability=3.33:100000". Or you can write this as a percentage, eg. "evil-probability=1%".

The default probability depends on the mode.

evil-seed=SEED

To make runs repeatable, use this to set a seed for the random number generator. Note that runs are repeatable only if you use the same version of nbdkit, since in future we may change the algorithm.

The default is to choose a seed at random.

evil-stuck-probability=N

evil-stuck-probability=N:M

evil-stuck-probability=N%

For the "stuck-*" modes, the probability that when reading a stuck bit you will read the stuck bit or the correct value. This defaults to 1 (ie. 100%) which means the bit is always stuck. Setting it to 0.5 for example will mean that half the time the bit appears stuck and half the time you see the correct value.

Files

$filterdir/nbdkit-evil-filter.so

The filter.

Use "nbdkit --dump-config" to find the location of $filterdir.

Version

"nbdkit-evil-filter" first appeared in nbdkit 1.36.