btrfs send from a 3tb drive ti btrfs-receive on a 1tb drive

Question

I have question about the nature of a subvolume Snapshot sent and received.

I have 2 3tb raid1 drives with only about 300gbs on them. I have a spare 1tb drive I placed in an enclosure for backups.

Succinctly, does the receive mountpoint have to be at least as big as the (undefraged) partition size of the sent snapshot?

I had planned to do rsync backups, but like what btrfs send -p parentvol subvol | btrfs receive /mnt/backup could do esp throwing incremental sends in mix. However, I've been unable to determine the exact nature of the sent subvolumes received to the mountpoint. Do they mirror the snapsot of the original such that if any data is at the end of the partition outside the size of the backup partition, it can't be written?

Should I just stick with rsync until I spring f a backup drive the same size?

Kindly,

Narnie

Answer 1

The receiving drive does not have to be the same size as the drive the subvolume is being sent from. It only needs to have enough space to store the subvolume itself.

For incremental sends, btrfs-send will only send the difference between the subvolume and the parent subvoume, and you only need to have enough space to store that difference.

In short, if you have 300G of data on the raid1 array, you should only need approximately 300G of space on the receiving array, provided that any incremental sends are sent specifying an appropriate parent snapshot.