Comment 7 for bug 791992

I think this is caused by poorly balanced disks. I.e:

nmdbio:~ % sudo btrfs fi show /dev/sdb
Label: 'scratch' uuid: 184706ea-89f5-438a-a9f5-b5e91b3ce267
        Total devices 3 FS bytes used 7.57TB
        devid 3 size 2.73TB used 2.45TB path /dev/sdc
        devid 2 size 2.73TB used 2.73TB path /dev/sdb
        devid 1 size 2.73TB used 2.73TB path /dev/sda

Btrfs v0.20-rc1

Note that two disks are entirely filled up, and only one has free space. If I were to venture a guess, btrfs struggles to shuffle data about in order to write multiple copies of metadata to each spindle.