Adding a new device to a btrfs volume, but the available size is hardly growing

Question

I have a btrfs volume which already consists of two different devices and which is mounted on /samples. Its total size is 194GB as reported by df.

$ df -h
Filesystem      Size  Used Avail Use% Mounted on
...
/dev/sdc1       194G  165G   20G  90% /samples

Now, I would like to add another 500GB to that volume, from another device. I did

$ sudo mkfs.btrfs -m raid0 -d raid0 /dev/sdb
$ sudo btrfs device add /dev/sdb /samples

My filesystem now correctly reports:

$ sudo btrfs filesystem show
Label: none  uuid: 545e95c6-d347-4a8c-8a49-38b9f9cb9add
    Total devices 3 FS bytes used 161.98GB
    devid    3 size 465.76GB used 0.00 path /dev/sdb
    devid    2 size 93.13GB used 84.51GB path /dev/sdc1
    devid    1 size 100.61GB used 84.53GB path /dev/sdc6

But I miss some space when I do:

$ df -h
Filesystem      Size  Used Avail Use% Mounted on
...
/dev/sdc1       660G  165G   43G  80% /samples

I added 500GB! Why haven't I got more available??

To debug, I ran this command:

$ sudo btrfs filesystem df /samples
Data, RAID0: total=162.00GB, used=159.79GB
Data: total=8.00MB, used=7.48MB
System, RAID1: total=8.00MB, used=24.00KB
System: total=4.00MB, used=0.00
Metadata, RAID1: total=3.50GB, used=2.19GB
Metadata: total=8.00MB, used=0.00

My data is in RAID0, that's ok. So where have my 500GB gone, and how can I fix this?

Thanks

Answer 1

Thanks for your answers. I am computing the answer from the various responses I got here and on the btrfs mailing-list.

1. My new 500G aren't showing because they cannot be used initially.
2. I need to balance the data on the three partitions:

btrfs filesystem balance /samples

3. Using two partitions on the same disk (/dev/sdc1 and /dev/sdc6) is not optimal at all because they will spend time seeking between the two partitions. In that case, a better option is to use 'single', where the partitions are filled one after the other (no striping). With new btrfs tools, this is possible using this command:

btrfs balance start -dconvert=single,soft /samples

4. On Debian/Ubuntu hosts, the newest btrfs tools which support balance conversion are usually not available in packaged format and need to be compiled from source.

Thanks again for your answers.