That sounds unreasonably fragile. Especially if you are unable to ever do a balance.
You should only ever need to do a balance if you have too much space allocated to one of data/metadata and need to free some for the other or when you are doing things like changing RAID levels. In normal use you shouldn't need to do it. The fact that it is sometimes needed in normal use is due to deficiencies in BTRFS that might have been fixed now.
I thought you needed to run it after adding an additional disk, so that you wouldn't get into a situation where the new disk had space, but all the other available disks were full so there was no second disk to write the copy to?
Btw, when you say 5TB RAID1, what exactly do you mean? Is the 5TB referring to the raw disks or the usable redundant space? I'm never quite sure.
5TB disks are quite affordable nowadays. 6TB is still a little expensive. So a RAID-1 array of 5TB disks is a good option.
I guess it depends on your performance vs capacity vs power requirements. For performance, you want as many spindles as possible (and preferably 2.5", all other things being equal), for power greenness, you want as few as possible (and again, preferably 2.5"), and for capacity you want a number of disks where $/GB is the best to give you the capacity you want in the number of drive bays you have. Maybe the spindle count isn't as important if you use bcache though, assuming typical access patterns. 3TB still appears to be the best $/GB right now, but obviously that's not the only factor. But if I didn't have a use for 5TB in the next few years, I'd be going with the cheaper disks and buying more of them to fill all my bays (With BTRFS, I just get whatever disks I can get my hands on when I need them, or when surplus disks come my way, and cram them into my server. BTRFS knows what to do with them :) Has anyone ever put the numbers together for the optimal buying strategy? Eg I need x storage now, and my projected storage growth is yTB/year, should I buy the bigger (more expensive) disks now that will last me longer, or smaller (cheaper) disks now and bigger disks later as I need them and when they are cheaper? James