I have a btrfs array of 7 drives that I just finished building. It's currently in "single" mode and I'd like to convert it to raid5 (I understand the risks and the write hole issue). Per the documentation,
The way balance operates, it usually needs to temporarily create a new block group and move the old data there. For that it needs work space, otherwise it fails for ENOSPC reasons. This is not the same ENOSPC as if the free space is exhausted. This refers to the space on the level of block groups.
My current space allocation based on btrfs df is
Data, single: total=20.46TiB, used=19.93TiB
Data, RAID5: total=3.25TiB, used=3.17TiB
System, RAID5: total=96.00MiB, used=2.38MiB
Metadata, RAID5: total=29.91GiB, used=26.54GiB
GlobalReserve, single: total=512.00MiB, used=0.00B
And my filesystem per-disk usage us:
Total devices 7 FS bytes used 23.13TiB
devid 1 size 7.28TiB used 6.06TiB path /dev/sdc
devid 2 size 7.28TiB used 5.95TiB path /dev/sdd
devid 3 size 7.28TiB used 5.99TiB path /dev/sde
devid 4 size 3.64TiB used 2.42TiB path /dev/sdj
devid 5 size 3.64TiB used 2.43TiB path /dev/sdk
devid 6 size 4.55TiB used 909.00GiB path /dev/sdf
devid 7 size 4.55TiB used 559.00GiB path /dev/sdg
Note the 3tb of raid5 storage is from running a convert for about 20 hours before I realized the potential for this to be a problem. Is there any way for me to calculate how much free space I'd potentially need, or is 3tb of my data already converted to raid5 and the small amount of free space on each drive sufficient?
