2 enospc errors during balance, with some 20+ TB free space

2 enospc errors during balance, with some 20+ TB free space

[Tomasz Chmielewski]

Tried to run metadata-only balance a btrfs filesystem with RAID-10 
metadata to RAID-1 metadata, it failed with "No space left on device".
Seriously, 20 TB of free disk space is not enough to convert some 25 GB 
of metadata to a different format? O_o

Kernel is 4.18.20.


# time btrfs balance start -mconvert=raid1 /data
ERROR: error during balancing '/data': No space left on device
There may be more info in syslog - try dmesg | tail

real    663m37.901s
user    0m0.000s
sys     6m36.804s


In dmesg:

[4020028.680326] BTRFS info (device sda2): found 62605 extents
[4020093.430147] BTRFS info (device sda2): relocating block group 
4778521264128 flags system|raid10
[4020286.363882] BTRFS info (device sda2): found 35 extents
[4020292.904640] BTRFS info (device sda2): 2 enospc errors during 
balance


# btrfs fi usage /data
WARNING: RAID56 detected, not implemented
Overall:
     Device size:                  35.80TiB
     Device allocated:             48.06GiB
     Device unallocated:           35.75TiB
     Device missing:                  0.00B
     Used:                         33.61GiB
     Free (estimated):                0.00B      (min: 8.00EiB)
     Data ratio:                       0.00
     Metadata ratio:                   2.00
     Global reserve:              512.00MiB      (used: 0.00B)

Data,RAID5: Size:6.71TiB, Used:6.33TiB
    /dev/sda2       2.24TiB
    /dev/sdb2       2.24TiB
    /dev/sdc2       2.24TiB
    /dev/sdd2       2.24TiB

Metadata,RAID1: Size:22.00GiB, Used:14.92GiB
    /dev/sda2      11.00GiB
    /dev/sdb2      11.00GiB
    /dev/sdc2      11.00GiB
    /dev/sdd2      11.00GiB

Metadata,RAID10: Size:2.00GiB, Used:1.88GiB
    /dev/sda2       1.00GiB
    /dev/sdb2       1.00GiB
    /dev/sdc2       1.00GiB
    /dev/sdd2       1.00GiB

System,RAID1: Size:32.00MiB, Used:592.00KiB
    /dev/sdb2      32.00MiB
    /dev/sdc2      32.00MiB

Unallocated:
    /dev/sda2       6.70TiB
    /dev/sdb2       6.70TiB
    /dev/sdc2       6.70TiB
    /dev/sdd2       6.70TiB


# df -h /data
Filesystem      Size  Used Avail Use% Mounted on
/dev/sda2        36T  6.4T   28T  19% /data