On Wed, 10 Dec 2014, Toby Corkindale <toby@dryft.net> wrote:
Let's say I want to have 2TB of storage. In that case, I'd purchase two 2TB drives, mirror them in btrfs, done.[1] mkfs.btrfs -m raid1 -d raid1 /dev/disk/by-id/foo1 /dev/disk/by-id/foo2
Actually buy at least 3TB disks. The MSY prices are $92 and $123 respectively. It's a good idea to pay an extra $30 per disk to delay your need to buy bigger disks (which is expensive and takes some time and effort) and to give better performance (as a rule of thumb bigger drives are faster, especially if you partition it to put your data on the outer tracks). Even buying 4TB disks if you need to store 2TB of data may be a good idea, it's only $178 for 4TB.
Time goes by, the amount of data I'm collecting ramps up hugely, I need more space. So, I'd buy a couple of 4TB drives, and add them to the pool and then perhaps rebalance: btrfs device add /dev/disk/by-id/foo3 /mnt btrfs device add /dev/disk/by-id/foo4 /mnt btrfs balance start /mnt
Later, I'd remove the original drives as they were getting old, and probably look at replacing them with bigger drives. btrfs device delete /dev/disk/by-id/foo1 /mnt btrfs device delete /dev/disk/by-id/foo2 /mnt
Note that "btrfs replace" is MUCH faster than a balance or delete operation. Also last time I measured it an idle but spinning SATA disk added about 7W to the total power use of a PC (IE the power taken from the power point). Among other things that increases cooling problems in summer, noise, and as you noted more disks means a greater probability of failure. I've currently got 2*4TB disks in a BTRFS RAID-1 array for my home server. When that gets almost full I'll consider adding a pair of 1TB or 2TB disks to the array if 6TB is still the biggest capacity on sale at that time. But if they have 10TB disks on sale and 6TB disks going cheap (as is likely to be the case) then I'll probably just buy new 6TB disks. I could use a couple of 4TB disks for backups.
Footnotes: 1: I'd actually buy three drives, with the third an externally housed one used for off-site backup.
Buy 4 drives and use 1 for local backup and 1 for off-site. Even when your first line backup is BTRFS snapshots you still want to have more than one removable backup device. Another possibility is to use an old PC with a few disks in a RAID-5 or RAID-6 array for local backup. I've been considering getting a large tower PC filled with old disks (~1TB capacity) in a BTRFS RAID-5 configuration for local backup. The noise and heat of all the disks wouldn't matter as I'd only turn it on when doing a backup. -- My Main Blog http://etbe.coker.com.au/ My Documents Blog http://doc.coker.com.au/