I made them from a youtube video:
They will hold coals overnight if there's a good amount in the bottom plus some saw dust to slow the burn. This barrel design helps oxygen get in better so it will burn everything better. If you put any saw dust in, put it over hot coals against one side of the barrel and don't put too much that it smothers all the coals. Also, I'm assuming you guys know the dangers of dumping saw dust on a fire? Dry dust will act like pouring gasoline on a fire and climb up to where you're pouring from. Damp dust is a lot easier to work with and keep your face away from the fire. I have singed my my arm hair a few times, and my head hair once.
They will burn green wood, but I don't put any thick pieces in there only because my off bearer takes the thicker slab material for his wood stove, so only the thinner stuff is left to burn. But yes it will burn green slab wood fine. Green Bark will burn easily, and the heat of the fire will quickly dry the slab pieces and burn them. Sugars in the wood also help them burn.
Again, if you're production sawing boards on a hydraulic mill, two barrels probably can't keep up with your saw dust. But doing timbers plus a few boards and stickers from a log like I do, It can keep up. The other good thing about these barrels is that they are closer to my mill than what we used to do(haul the dust to the back pasture for a burn pile once a year). The barrels also burn much cleaner than an open burn pile because as the material gassifies, the added oxygen burns it like an after burner. So once you get them going you don't have much smoke to speak of (until you dump some wet dust and smother the coals). Open fires just create so much smoke, and are less safe. With the burn barrels I have them on the gravel so less chance of catching anything else on fire. Keep away from the grass to you don't get a grass fire! I also elevate the barrels on 3 bricks to make sure they get enough air from underneath.