Pretty new diver here, about 40 dives, and looking for advice.
Just finished up a week of dives in Grenada, and made a point of paying attention to air consumption. Based on Internet advice, I focused on breathing deeply and exhaling completely, counting 4 seconds in, 6 seconds out. Doing this, my computer reported average SAC has dropped from about 0.8 to 0.5, and I'm not the one calling dives for gas anymore. This seems like a great improvement.
However, my buoyancy goes to shit when I'm doing this. Breathing more "normally", I can maintain a neutral depth with good trim. But with this more efficient breath control, I go up and down several feet with every breath. This actually makes it pretty easy to control when I ascend and descend, but obviously isn't great for most of the dive.
If I try to breathe normally-but-slow, I feel like I'm hyperventilating.
So what's the trick here? How do you both breathe efficiently and control your buoyancy?
I think I'm pretty well weighted, since I have no problem maintaining my safety stop with the shallower breaths.
Check your sector boundaries. You can't resize a physical partition if it overlaps another one. That lvm shrink likely lowered the top sector, rather than raising the bottom.
Also, 488mb should be plenty in /boot. You can purge old kernels with 'sudo apt --purge autoremove' (that'll take any other unused packages with it, so use caution.)