nbdkit-evil-filter - add random data corruption to reads
nbdkit --filter=evil PLUGIN [PLUGIN-ARGS...]
evil=[cosmic-rays|stuck-bits|stuck-wires]
[evil-probability=PROB] [evil-stuck-probability=PROB]
[evil-seed=SEED]
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).
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'
Plugins can be sparse. This filter only corrupts bits in non-sparse parts of the backing disk and it leaves sparse regions unchanged (which is realistic behaviour). If you wish to use this filter to corrupt sparse regions, then combine this filter with nbdkit-noextents-filter(1). For example:
nbdkit --filter=evil --filter=noextents memory 1G
The current implementation limits probabilities to the range [ 1e-12 .. 1/8 ]. Values below this range are treated the same as 0%. Values above this range are treated the same as 100%.
Select the mode of evil. See the "DESCRIPTION" above. The default is stuck-bits
.
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.
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.
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.
The filter.
Use nbdkit --dump-config
to find the location of $filterdir
.
nbdkit-evil-filter
first appeared in nbdkit 1.36.
nbdkit(1), nbdkit-filter(3), nbdkit-delay-filter(1), nbdkit-noextents-filter(1), nbdkit-error-filter(1).
Richard W.M. Jones
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