pause device removal

[Stephan Mueller <stepmueller@phys.ethz.ch>]

Hello

we have 140T btrfs made from 10 LUNs. This file system is quite full 
with backups (and snapshots). Now we need to free one of the LUNs. A 
`btrfs device remove` is quite slow and also slows down nightly backups 
a lot.

So we run the device remove during the day and the backups during the 
night. On another btrfs this worked quite well. But here, the backups 
fill the device we want to remove and currently we are not moving 
anywhere regarding the usage the LUN.

Do you have an idea how can resolve this?
Can we somehow mark the device as unusable for new writes, or set it 
size, so that it is not written to?
Do you think it's safe to cgroup freeze the removal during the night?

   Have a nice day
   Stephan

Total devices 10 FS bytes used 144.08TiB
devid    1 size 0.00B used 6.81TiB path /dev/sdgb     <--- currently in 
removal
devid    3 size 1.88TiB used 546.00GiB path /dev/sdv
devid    5 size 20.00TiB used 17.16TiB path /dev/sdw
devid    6 size 20.00TiB used 17.16TiB path /dev/sdo
devid    7 size 20.00TiB used 17.16TiB path /dev/sdu
devid    8 size 20.00TiB used 17.16TiB path /dev/sdn
devid    9 size 20.00TiB used 17.16TiB path /dev/sdj
devid   10 size 20.00TiB used 17.16TiB path /dev/sdk
devid   11 size 20.00TiB used 17.16TiB path /dev/sdm
devid   12 size 20.00TiB used 17.16TiB path /dev/sdbc

-- 
Stephan Müller <stephan.mueller@phys.ethz.ch>
IT Services Group, HPT H 8
Physics Department, ETH Zurich
CH-8093 Zurich, Switzerland