start Btrfs with one disk and add second for RAID1 setup

The new file server will contain two 4TB disks as RAID1 and a single 4TB disk for of side backup. All three disks are different to spread the risk of failing at the same time. The RAID disks will be a WD red and a Segate NAS. The backup disk a WD green. Unfortunately the Segate was out of stock.

To test how simple it is to start with only one RAID disk I added two disk to a VM and did all needed steps there. From the starting point off the simple creation to starting the RAID.

To verify this, I created some files on the first disk and compared the hash of the files after the test.

1. create the file system with one disk

[root@centos ~]# mkfs.btrfs -L test_merge_HDs /dev/sdb

SMALL VOLUME: forcing mixed metadata/data groups

WARNING! - Btrfs v3.12 IS EXPERIMENTAL

WARNING! - see http://btrfs.wiki.kernel.org before using

Turning ON incompat feature 'mixed-bg': mixed data and metadata block groups

Turning ON incompat feature 'extref': increased hardlink limit per file to 65536

Created a data/metadata chunk of size 8388608

fs created label test_merge_HDs on /dev/sdb

nodesize 4096 leafsize 4096 sectorsize 4096 size 500.00MiB

Btrfs v3.12

[root@centos /]# btrfs fi df /mnt/merge/

System, single: total=4.00MiB, used=4.00KiB

Data+Metadata, single: total=8.00MiB, used=28.00KiB

[root@centos /]# btrfs fi show 

Label: test_merge_HDs  uuid: b5aa052e-b44c-4e80-8e2b-3b7d04eb19e4

Total devices 1 FS bytes used 32.00KiB

devid    1 size 500.00MiB used 12.00MiB path /dev/sdb

Btrfs v3.12

2. add the second disk

[root@centos /]# btrfs device add /dev/sdc /mnt/merge/

SMALL VOLUME: forcing mixed metadata/data groups

[root@centos /]# btrfs fi df /mnt/merge/

System, single: total=4.00MiB, used=4.00KiB

Data+Metadata, single: total=8.00MiB, used=28.00KiB

[root@centos /]# btrfs fi show 

Label: test_merge_HDs  uuid: b5aa052e-b44c-4e80-8e2b-3b7d04eb19e4

Total devices 2 FS bytes used 32.00KiB

devid    1 size 500.00MiB used 12.00MiB path /dev/sdb

devid    2 size 500.00MiB used 0.00 path /dev/sdc

Btrfs v3.12

3. start the RAID

[root@centos /]# btrfs balance start -dconvert=raid1 -mconvert=raid1 /mnt/merge/

Done, had to relocate 2 out of 2 chunks

[root@centos /]# btrfs fi df /mnt/merge/

System, RAID1: total=32.00MiB, used=4.00KiB

Data+Metadata, RAID1: total=112.00MiB, used=100.00KiB

[root@centos /]# btrfs fi show 

Label: test_merge_HDs  uuid: b5aa052e-b44c-4e80-8e2b-3b7d04eb19e4

Total devices 2 FS bytes used 104.00KiB

devid    1 size 500.00MiB used 144.00MiB path /dev/sdb

devid    2 size 500.00MiB used 144.00MiB path /dev/sdc

Btrfs v3.12