Is there a way to monitor progress of a btrfs rebalance?

btrfs balance status /mountpoint

man 8 btrfs

 [filesystem] balance status [-v] <path>
        Show status of running or paused balance.

        Options

        -v   be verbose

sudo btrfs fi show

this will output something like so:

Label: none  uuid: 2c97e7cd-06d4-4df0-b1bc-651397edf74c
        Total devices 16 FS bytes used 5.36TiB
        devid    1 size 931.51GiB used 770.48GiB path /dev/sdc
        devid    2 size 931.51GiB used 770.48GiB path /dev/sdg
        devid    3 size 931.51GiB used 770.48GiB path /dev/sdj
        devid    4 size 0.00 used 10.02GiB path
        devid    5 size 931.51GiB used 770.48GiB path /dev/sdh
        devid    6 size 931.51GiB used 770.48GiB path /dev/sdi
        devid    7 size 931.51GiB used 770.48GiB path /dev/sdd
        devid    8 size 931.51GiB used 770.48GiB path /dev/sdo
        devid    9 size 465.76GiB used 384.31GiB path /dev/sdn
        devid    10 size 931.51GiB used 770.48GiB path /dev/sdp
        devid    11 size 931.51GiB used 770.48GiB path /dev/sdr
        devid    12 size 931.51GiB used 770.48GiB path /dev/sdm
        devid    13 size 931.51GiB used 769.48GiB path /dev/sdq
        devid    14 size 931.51GiB used 770.48GiB path /dev/sdl
        devid    15 size 931.51GiB used 770.48GiB path /dev/sde
        devid    16 size 3.64TiB used 587.16GiB path /dev/sdf

Btrfs v3.12

And if you notice that device id #4 looks a little bit different than the rest. when you do "btrfs device delete missing /mntpoint" then it will start to regenerate the raid meta/data necessary to free up that "missing" drive.

if you do something like

"watch -n 10 sudo btrfs fi show"

then you can see the space on the offending "missing" device gradually getting smaller and smaller until the operation completes and it will be removed from the fi.

BTRFS may take some time reading or rearranging data prior to writing data to the drive you expect it to write to.

You can see how much CPU time is being devoted to BTRFS operations including rebalance, add, delete, convert, etc:

ps -ef | grep btrfs

To see how busy each drive is, install sysstat, and run:

iostat

Add some options to make iostat show stats in megabytes and update every 30 seconds:

iostat -m -d 30

Sample output from scrub so no writes during this interval:

Device:            tps    MB_read/s    MB_wrtn/s    MB_read    MB_wrtn
sda             700.30       170.10         0.00       6804          0
sdb               0.78         0.00         0.01          0          0
sdc             520.20       127.98         0.00       5119          0
sdd             405.72        92.02         0.00       3680          0
sde             630.05       153.66         0.00       6146          0
sdf             627.43       153.60         0.00       6144          0

Install and run munin to see historical graphs of drive activity and lots of other info. https://www.digitalocean.com/community/tutorials/how-to-install-the-munin-monitoring-tool-on-ubuntu-14-04