Wow, thank to everyone for all that information, I'm going to have to take some time to digest everything. :) I just wanted to quickly say one thing: As Duncan surmised, I'm not treating this as my primary backup, but more of an experimental add-on feature. The primary backup goes to an ext4 partition that is then rsynced to a 'current' btrfs subvolume, the latter is what I'm taking snapshots of. These both share a RAID 1+0 enclosure. The partitions are oversized, 3TB each, so based on this thread that size is something to watch out for if I use up a lot more of the space. For other backups I've also got a Amazon Glacier based backup (large "offsite" but painful to recover with) via Arq, and an Apple TimeCapsule backup (easy point-and-click restore for a single item, but damn flakey and painfully slow for anything complicated). So I'm not trusting any one device or scheme. As an aside, one of the things I loved about reading the Plan 9 papers was their description of their three-tier storage. Where they had "infinite" long term storage via a WORM array, SCSI disks on big servers for the "cache," and then local disk / memory for the files they were working on right at that moment (http://doc.cat-v.org/plan_9/misc/cw/cw.pdf). I do like the idea of periodic "copy to a clean disk" and "mkfs of the old disk" scheme instead of a complicated in-place rebuilding and/or defragmentation, I think I have enough capacity that I will probably try that if/when it becomes necessary. I've got to read up a bit more on subvolumes, I am missing some context from the warnings given by Chris regarding per-subvolume options.
