Btrfs is *not* using multiple devices.

Izzette · 2017-02-14 04:27:10

I've added a device to my existing Btrfs file system recently, but it doesn't report the new device as having even allocated any space for the data. Take a look:

Before I started:

# btrfs fi show /
Label: 'LINUX'  uuid: f0b4d012-91d8-43bd-90b5-e398a012ccff
	Total devices 1 FS bytes used 126.72GiB
	devid    1 size 272.40GiB used 128.03GiB path /dev/sda3
# lsblk
NAME   MAJ:MIN RM   SIZE RO TYPE MOUNTPOINT
sda      8:0    0 273.4G  0 disk 
├─sda1   8:1    0     1M  0 part 
├─sda2   8:2    0     1G  0 part /boot
└─sda3   8:3    0 272.4G  0 part /pool
sdb      8:16   0  68.3G  0 disk

So now I want to add /dev/sdb:

# btrfs device add /dev/sdb /
# btrfs balance start --full-balance /

< an hour or so later ... >

# btrfs fi usage /
Overall:
    Device size:		 340.73GiB
    Device allocated:		 128.03GiB
    Device unallocated:		 212.70GiB
    Device missing:		     0.00B
    Used:			 126.68GiB
    Free (estimated):		 213.10GiB	(min: 213.10GiB)
    Data ratio:			      1.00
    Metadata ratio:		      1.00
    Global reserve:		 338.14MiB	(used: 0.00B)

Data,single: Size:126.00GiB, Used:125.60GiB
   /dev/sda3	 126.00GiB

Metadata,single: Size:2.00GiB, Used:1.08GiB
   /dev/sda3	   2.00GiB

System,single: Size:32.00MiB, Used:16.00KiB
   /dev/sda3	  32.00MiB

Unallocated:
   /dev/sda3	 144.37GiB
   /dev/sdb	  68.33GiB

Huh, I thought that was supposed to do it, lets try this:

# btrfs balance start -dconvert=single -mconvert=raid1 /

< an hour or so later ... >

# btrfs fi show /
Label: 'LINUX'  uuid: f0b4d012-91d8-43bd-90b5-e398a012ccff
	Total devices 2 FS bytes used 126.72GiB
	devid    1 size 272.40GiB used 128.03GiB path /dev/sda3
	devid    2 size 68.33GiB used 2.03GiB path /dev/sdb
# btrfs fi usage /
Overall:
    Device size:		 340.73GiB
    Device allocated:		 130.06GiB
    Device unallocated:		 210.67GiB
    Device missing:		     0.00B
    Used:			 127.80GiB
    Free (estimated):		 211.03GiB	(min: 105.69GiB)
    Data ratio:			      1.00
    Metadata ratio:		      2.00
    Global reserve:		 338.47MiB	(used: 0.00B)

Data,single: Size:126.00GiB, Used:125.64GiB
   /dev/sda3	 126.00GiB

Metadata,RAID1: Size:2.00GiB, Used:1.08GiB
   /dev/sda3	   2.00GiB
   /dev/sdb	   2.00GiB

System,RAID1: Size:32.00MiB, Used:16.00KiB
   /dev/sda3	  32.00MiB
   /dev/sdb	  32.00MiB

Unallocated:
   /dev/sda3	 144.37GiB
   /dev/sdb	  66.30GiB

What the heck?

Kernel said anything during balance? Just a whole lot of this:

Feb 13 21:55:06 localhost kernel: BTRFS info (device sda3): found 7541 extents
Feb 13 21:55:06 localhost kernel: BTRFS info (device sda3): relocating block group 621927202816 f
Feb 13 21:55:18 localhost kernel: BTRFS info (device sda3): found 7020 extents
Feb 13 21:55:28 localhost kernel: BTRFS info (device sda3): found 7020 extents
Feb 13 21:55:28 localhost kernel: BTRFS info (device sda3): relocating block group 620853460992 f
Feb 13 21:55:39 localhost kernel: BTRFS info (device sda3): found 2540 extents
Feb 13 21:55:48 localhost kernel: BTRFS info (device sda3): found 2540 extents
Feb 13 21:55:48 localhost kernel: BTRFS info (device sda3): relocating block group 619779719168 f
Feb 13 21:55:58 localhost kernel: BTRFS info (device sda3): found 3971 extents

Slithery · 2017-02-14 09:13:26

All your output is as expected, what's the problem?

Last edited by Slithery (2017-02-14 12:03:48)

Izzette · 2017-02-14 16:19:56

All the data is still one one device:

Data,single: Size:126.00GiB, Used:125.64GiB
   /dev/sda3	 126.00GiB

Slithery · 2017-02-14 16:26:03

And? You haven't told it to do otherwise.

The way you have it set up at the moment it's just pooling all of the available space into one volume (as can be seen by the Device Size output).

What are you wanting to happen?

Izzette · 2017-02-14 16:32:15

I was under the impression that it would distribute data between the two devices proportional to their size. Will it not allocate for data on sdb until the unallocated space on sda3 dips below 66.30GiB (the unallocated space on sdb)? I'm kind of in favor of the former behavior, as it would balance the disk load between the two devices. I'm not entirely sure what the consequences of using raid0 for data between these two different devices, but I'm not terribly fond of the potential problems that come to mind.

Arch Linux

#1 2017-02-14 04:27:10