For Btrfs redundancy to work, each file needs to be stored on two separate drives. Thus, a pool consisting of a 2, 4 and 8 TB disk could at most store 6TB in mirror mode. On the other hand, a pool of two 8 TB disks and one 4 TB disk could hold about 10 TB with no space wasted.Maybe do some trials at: https://carfax.org.uk/btrfs-usage/
I usually bought bigger HDD, typically twice the size, so long time ago 500GB then some 2TB then several 4TB
I currently do not use RAID anymore, instead clone max 8TB filesystems, all Btrfs.
With the btrfs-progs tooling, it is possible to convert the filesystem on-the-fly, no unmounting needed. Add an 8TB HDD for example and make the 2TB+4TB+8TB RAID1
If not Btrfs yet, offline btrfs-convert can convert from Ext4 to Btrfs. You need some free space for that as new Btrfs metadata needs space. This is also true for online conversion (via btrfs balance).
As already indicated, all via USB is far from ideal, it works, but risk that something will fail sometime w.r.t. RPi4 UBS3 or USB-SATA chipset is not unthinkable. Then RAID won't help as the hardware USB chain is also not multi-path.
The idea of doing an in-place conversion of the Ext4 filesystem on the existing 4TB hard disk to Btrfs and then adding the new drives to the resulting drive pool is tempting. In my opinion, that is likely to work but one should have at least one current backup and preferably another before attempting such a conversion.
As already mentioned, after the existing filesystem has been converted and the new drives added to the pool, one would then convert the resulting Btrfs filesystem to mirror mode to enable redundancy.
Those three kittens looking over my shoulder as I typed immediately entered into a discussion on the technical merits of Btrfs versus ZFS versus Bcachefs. Shy mewed that Btrfs is built in to Linux and the possibility of in-place conversion makes it the obvious choice. Purr purred that ZFS is perfect due its long history of enterprise use. Scratchy hissed, it's simpler to just delete the data. JFS also has a long history of enterprise use, yet even Bcachefs is more reliable.
Statistics: Posted by ejolson — Sat Jan 10, 2026 9:13 pm