balance conversion from RAID-1 to RAID-10 leaves some metadata in RAID-1?

Tomasz Chmielewski
2020-03-01T10:41:05Z

Just finished a conversion from RAID-1 to RAID-10, on Linux 5.5.6:

# time btrfs balance start -dconvert=raid10 -mconvert=raid10 /data/lxd
Done, had to relocate 2064 out of 2064 chunks

real 1811m51.427s
user 0m0.000s
sys 1259m34.448s

# echo $?
0

# dmesg
(...)
[211862.205521] BTRFS info (device sda2): found 40805 extents
[211910.773888] BTRFS info (device sda2): found 40805 extents
[211965.321150] BTRFS info (device sda2): relocating block group
3492979671040 flags data|raid1
[211972.743476] BTRFS info (device sda2): found 37221 extents
[212020.193342] BTRFS info (device sda2): found 37221 extents
[212084.580739] BTRFS info (device sda2): balance: ended with status: 0

To my surprise, some metadata is still RAID-1 - is it expected?

# btrfs fi usage /data/lxd
Overall:
Device size: 6.91TiB
Device allocated: 6.70TiB
Device unallocated: 210.40GiB
Device missing: 0.00B
Used: 3.96TiB
Free (estimated): 1.46TiB (min: 1.46TiB)
Data ratio: 2.00
Metadata ratio: 2.00
Global reserve: 512.00MiB (used: 0.00B)

Data,RAID10: Size:3.32TiB, Used:1.97TiB (59.20%)
/dev/sda2 1.66TiB
/dev/sdd2 1.66TiB
/dev/sdc2 1.66TiB
/dev/sdb2 1.66TiB

Metadata,RAID1: Size:2.00GiB, Used:85.02MiB (4.15%)
/dev/sdc2 2.00GiB
/dev/sdb2 2.00GiB

Metadata,RAID10: Size:29.00GiB, Used:12.55GiB (43.27%)
/dev/sda2 14.50GiB
/dev/sdd2 14.50GiB
/dev/sdc2 14.50GiB
/dev/sdb2 14.50GiB

System,RAID10: Size:64.00MiB, Used:400.00KiB (0.61%)
/dev/sda2 32.00MiB
/dev/sdd2 32.00MiB
/dev/sdc2 32.00MiB
/dev/sdb2 32.00MiB

Unallocated:
/dev/sda2 53.60GiB
/dev/sdd2 53.60GiB
/dev/sdc2 51.60GiB
/dev/sdb2 51.60GiB

# btrfs fi df /data/lxd
Data, RAID10: total=3.32TiB, used=1.97TiB
System, RAID10: total=64.00MiB, used=400.00KiB
Metadata, RAID10: total=29.00GiB, used=12.55GiB
Metadata, RAID1: total=2.00GiB, used=85.02MiB
GlobalReserve, single: total=512.00MiB, used=0.00B

# btrfs fi show /data/lxd
Label: 'lxd5' uuid: 2b77b498-a644-430b-9dd9-2ad3d381448a
Total devices 4 FS bytes used 1.98TiB
devid 1 size 1.73TiB used 1.67TiB path /dev/sda2
devid 3 size 1.73TiB used 1.67TiB path /dev/sdd2
devid 4 size 1.73TiB used 1.68TiB path /dev/sdc2
devid 5 size 1.73TiB used 1.68TiB path /dev/sdb2

Tomasz Chmielewski