disk plugin: ignore loop devices

[panlinux]

It doesn't make sense to apply tuning parameters to loop devices, since they point at files that live in other block devices, also subject to tuning.

I checked the device structure looking for a telltale of a loop device. I thought that the loop/backing_file attribute would be a good choice, but I had some that had no such attribute. I could also inspect the name of the device, and assume "loop*" is a loop device. I ended up picking the major number, 7, and was a bit annoyed that there was no straight attribute with that number, so there is some ugly parsing of the dev attribute.

Let's use this PR to plant the idea of ignoring loop devices. If that sounds ok and I didn't miss a use case where we want loop devices, we can iterate over the best way to do it.

Thanks!

[panlinux]

Thanks to Athos Ribeiro, here is a much simpler version that I force-pushed.

[yarda]

Thanks. I think the change makes sense, but I am afraid it will break several Beaker functionality tests that use loop devices to safely test disk plugin features. That's why I would prefer it to be configurable, e.g. in /etc/tuned/tuned-main.conf and by default the loopdev tuning can be disabled. In such case if there is anybody who was using it they could easily re-enable it.

I think the loopdev can be detected if the device has the udev disk/by-loop* attribute(s), because IMHO if there is a loopdev there should be associated inode on the filesystem, so the disk/by-loop-inode should be there. But hardcoding the devid 7 is probably also OK.

[yarda]

Thanks. I think the change makes sense, but I am afraid it will break several Beaker functionality tests that use loop devices to safely test disk plugin features. That's why I would prefer it to be configurable, e.g. in /etc/tuned/tuned-main.conf and by default the loopdev tuning can be disabled. In such case if there is anybody who was using it they could easily re-enable it.

Or per plugin configuration when the #626 is merged.