Synology DSM 6 – rebalancing BTRFS

I recently upgraded my Synology NAS from 4x 3TiB to 4x 6TiB disks (WD Red).

I could have simply installed the new disks and created a BTRFS volume (which would have become /volume2) but I decided to take a different route:

1. Install 2x 6TiB disks and format as RAID-0 (mounted on /volume2).
2. Copy all relevant data from the old disks to the new disks, and verify.
   If anything were to go wrong, I still had my data on the old disks.
3. Remove the old /volume1 4x 3TiB disks for safekeeping, insert 2x 6TiB disks and create a new /volume1 using BTRFS.
4. Copy all data from /volume2 (RAID-0) to /volume1 (BTRFS), and verify.
5. Destroy /volume2 and add the remaining disks to /volume1.

Now, all my data is effectively stored on the first 2 disks in the new volume. BTRFS can rebalance the data chunks across all spindles. Open an SSH connection to the Synology NAS, and issue the following command:

    btrfs balance start /volume1/

This operation took several hours to complete – it had to rebalance about 6 terabytes of data… I opened a second SSH session to monitor progress:

    btrfs balance status -v /volume1