First install 7.1a (if you can find it anymore) was the latest totally working version of truecrypt. You might try to install that and see if it fixes your problem.
However, this product is now basically obsolete as the developers have apparently stopped development of truecrypt for reasons not yet really fully understood. The current release 7.2 contains only a DECRYPT and not any of the encrypt functionality.
I'd be thinking seriously of shifting away from this project to something else. Although, I don't have a good feel on what that will be.
ioctl tends to go hand-in-hand with a /dev entry; your typical code would do
fd=open("/dev/mydevice",O_RDRW);
ioctl(fd,.....);
This is perfectly standard Unix behaviour. Inside the kernel driver you can put access controls (eg only root can do some things, or require a specific capability for more fine grained access) which makes it pretty flexible and powerful.
Of course this means that devices can expose a lot more than use block/character read-write activity; many things can be done via ioctl calls. Not so easy to use from shell scripts, but pretty easy from C or perl or python or similar.
sysfs entries are another way of interacting with drivers. Typically each type of command would have a different entry, so it can be complicated to write the driver but it makes it very easy to access via userspace; simple shell scripts can manipulate lots of stuff, but may not be very efficient
netlink is primarily focused (I think!) on network data transfers, but it could be used for other stuff. It's really good for larger volumes of data transfer and is meant to be a successor to ioctl in some cases.
All the options are good; your use case may better determine which type of interface to expose from your driver.
Best Answer
According to the launchpad thread you linked to, it is a cosmetic error caused by
os-probernot properly ignoring ZFS-managed drives, and if you're not dual-booting you can safely make the message go away withapt purge os-prober. See also here.