GNOP(8) MidnightBSD System Manager’s Manual GNOP(8)


gnop — control utility for NOP GEOM class


gnop create [−v] [−e error] [−o offset] [−r rfailprob] [−s size] [−S secsize] [−w wfailprob] dev ...
gnop configure
[−v] [−e error] [−r rfailprob] [−w wfailprob] prov ...
gnop destroy
[−fv] prov ...
gnop reset
[−v] prov ...
gnop list
gnop status
gnop load
gnop unload


The gnop utility is used for setting up transparent providers on existing ones. Its main purpose is testing other GEOM classes, as it allows forced provider removal and I/O error simulation with a given probability. It also gathers the following statistics: number of read requests, number of write requests, number of bytes read and number of bytes written. In addition, it can be used as a good starting point for implementing new GEOM classes.

The first argument to gnop indicates an action to be performed:


Set up a transparent provider on the given devices. If the operation succeeds, the new provider should appear with name /dev/

dev.nop. The kernel module geom_nop.ko will be loaded if it is not loaded already.


Configure existing transparent provider. At the moment it is only used for changing failure probability.


Turn off the given transparent providers.


Reset statistics for the given transparent providers.


See geom(8).


See geom(8).


See geom(8).


See geom(8).

Additional options:

−e error

Specifies the error number to return on failure.


Force the removal of the specified provider.

−o offset

Where to begin on the original provider.

−r rfailprob

Specifies read failure probability in percent.

−s size

Size of the transparent provider.

−S secsize

Sector size of the transparent provider.

−w wfailprob

Specifies write failure probability in percent.


Be more verbose.


The following sysctl(8) variables can be used to control the behavior of the NOP GEOM class. The default value is shown next to each variable.

kern.geom.nop.debug: 0

Debug level of the NOP GEOM class. This can be set to a number between 0 and 2 inclusive. If set to 0 minimal debug information is printed, and if set to 2 the maximum amount of debug information is printed.


Exit status is 0 on success, and 1 if the command fails.


The following example shows how to create a transparent provider for disk /dev/da0 with 50% failure probability, and how to destroy it.

gnop create -v -f 50 da0
gnop destroy -v da0.nop

The traffic statistics for the given transparent providers can be obtained with the list command. The example below shows the number of bytes written with newfs(8):

gnop create da0
newfs /dev/da0.nop
gnop list


geom(4), geom(8)


The gnop utility appeared in FreeBSD 5.3.


Pawel Jakub Dawidek 〈〉

MidnightBSD 0.3 September 8, 2006 MidnightBSD 0.3