I have three drives (8TB, 4TB, 3TB). Originally, I created a btrfs partition on the 8TB drive and copied all my data there. I added the 4TB and 3TB drives by using btrfs device add, and then then ran a balance conversion:btrfs balance start -dconvert=raid1 -mconvert=raid1 /mntNow the balance is done, but it still shows some data in single and DUP mode on the original drive. Here's the output from btrfs fi usage /mnt/btrfs:Overall:    Device size:          13.37TiB    Device allocated:          4.62TiB    Device unallocated:        8.75TiB    Device missing:          0.00B    Used:              4.60TiB    Free (estimated):          4.98TiB  (min: 4.38TiB)    Data ratio:               1.76    Metadata ratio:           2.00    Global reserve:      512.00MiB  (used: 0.00B)Data,single: Size:645.00GiB, Used:645.00GiB   /dev/mapper/8TB   645.00GiBData,RAID1: Size:1.98TiB, Used:1.98TiB   /dev/mapper/3TB   551.00GiB   /dev/mapper/4TB     1.44TiB   /dev/mapper/8TB     1.98TiBMetadata,RAID1: Size:8.00GiB, Used:3.84GiB   /dev/mapper/4TB     8.00GiB   /dev/mapper/8TB     8.00GiBMetadata,DUP: Size:7.00GiB, Used:6.41GiB   /dev/mapper/8TB    14.00GiBSystem,DUP: Size:8.00MiB, Used:400.00KiB   /dev/mapper/8TB    16.00MiBUnallocated:   /dev/mapper/3TB     2.19TiB   /dev/mapper/4TB     2.19TiB   /dev/mapper/Seagate_Archive_8TB-btrfs       4.37TiBQuestions:Is there any data which is not stored on more than one disk? So, in other words, is there any data which would be lost if one disk failed? If so, how can I coerce this lingering single stored data into RAID1?Assuming the single and DUP data stores are unnecessary, now that everything has been converted to raid, is there any way to clear them up?Edit: here is some system info:uname -a Linux 4.8.0-0.bpo.2-amd64 #1 SMP Debian 4.8.11-1~bpo8+1 (2016-12-14) x86_64 GNU/Linuxbtrfs --versionbtrfs-progs v4.9I also should mention that this computer was restarted during the balance, and when it came back up I wasn't able to get the btrfs volume to mount at all (it would just hang). I tried with a number of different mount parameters (skip-param, recovery) and the only one which would work was to mount it read only (using -o ro). After much frustration, I booted wth an Antergos live USB, which had the newest kernel and btrfs progs on it, and it mounted without a problem. I paused the balance operation which had automatically been started and then booted back into Debian, and it mounted without an issue, so I resumed the balance again.
BTRFS balance completed, but still shows data stored in single mode
filesystems;raid;btrfs
With the help of a user on the btrfs irc, I was able to answer question (1). It seems unrelated to the reboot and unsuccessful mount attempt (still not sure what that was about). Instead, it seems the 645GB of data stored as single was data that was added to the btrfs volume after the raid1 conversion had been initiated. Therefore, it seems good practice to check the output of btrfs fi usage before you assume all your data is stored as raid1 after a conversion. Furthermore, the soft filter will allow you to rebalance data which is not being stored according to the target profile, so for example I ran:btrfs balance start --bg -mconvert=raid1,soft /mnt/btrfsbtrfs balance start --bg -dconvert=raid1,soft /mnt/btrfs(following a suggestion by the user on the btrfs irc forum to perform the balance on the metadata first, and then on the data) and this is in the process of converting the remaining data to raid1.Furthermore, to answer question (2), the answer is that it is possible to end up with some single chunks in a raid1 filesystem, but they should have 0 usage. If this happens, you can clean them up by runningbtrfs balance start -dusage=0 -musage=0 /mnt/btrfs(see the btrfs FAQ)