btrfs RAID1 under Ubuntu 12.04.4 LTS

10-10-20

Howdy,

I have two 1TB disks, sda and sdb, upon which I create a RAID1 through btrfs (mkfs.btrfs).

root@backup:/mnt# mkfs.btrfs -m raid1 -d raid1 /dev/sda /dev/sdb

WARNING! - Btrfs Btrfs v0.19 IS EXPERIMENTAL
WARNING! - see http://btrfs.wiki.kernel.org before using

adding device /dev/sdb id 2
fs created label (null) on /dev/sda
        nodesize 4096 leafsize 4096 sectorsize 4096 size 1.82TB
Btrfs Btrfs v0.19

Ok, so I then check the status.

root@backup:/mnt# btrfs filesystem show
Label: none  uuid: 6de6b9c1-0837-40f1-82df-6fca58544c8c
        Total devices 2 FS bytes used 28.00KB
        devid    1 size 931.51GB used 2.03GB path /dev/sda
        devid    2 size 931.51GB used 2.01GB path /dev/sdb

Label: none  uuid: 38673844-bcc4-48a0-884d-5613ae0ec043
        Total devices 1 FS bytes used 28.00KB
        devid    1 size 1.82TB used 2.04GB path /dev/sdc

Btrfs Btrfs v0.19

Odd, I see 1.82TB as the size...

I now have no additional device created (like as you would with mdadm - /dev/md127...).

lsblk
NAME   MAJ:MIN RM   SIZE RO TYPE MOUNTPOINT
sda      8:0    0 931.5G  0 disk 
sdb      8:16   0 931.5G  0 disk 

So I check out the UUID's...

root@backup:/mnt/raid1# blkid 
/dev/sda: UUID="6de6b9c1-0837-40f1-82df-6fca58544c8c" UUID_SUB="ddb354a2-5562-4b1e-a61b-d0254a438fc0" TYPE="btrfs" 
/dev/sdb: UUID="6de6b9c1-0837-40f1-82df-6fca58544c8c" UUID_SUB="96b611fb-d03b-4795-aa4a-1fbe0cf96d1e" TYPE="btrfs" 

Ok, so all looks ok to a point and I mount it on /mnt/raid1... But when I do a df -h, I see the following:

root@backup:/mnt/raid1# df -h
Filesystem      Size  Used Avail Use% Mounted on
/dev/sda        1.9T   56K  1.9T   1% /mnt/raid1

Can anybody verify that this is a bug (or is it me)? I should be seeing 931GB instead of 1.9TB (as in, RAID1 versus RAID0).

Kinet1c

I would advise against software raid, it won't end well.

10-10-20

Last night I filled the drive by doing a dd from /dev/zero.
Guess where dd stopped?

root@backup:/mnt/raid1# ls -lah
total 930G
-rw-r--r-- 1 root root 930G Jul  9 06:43 test.file

At 930GB.
zlib compression is enabled, btw.

mount
/dev/sda on /mnt/raid1 type btrfs (rw,noatime,compress=zlib)

df -h shows a full drive, at 1.9TB full.

root@backup:/mnt/raid1# df -h
Filesystem      Size  Used Avail Use% Mounted on
/dev/sda        1.9T  1.9T  288K 100% /mnt/raid1

btrfs shows 930GB used.

root@backup:/mnt/raid1# btrfs filesystem show
Label: none  uuid: 6de6b9c1-0837-40f1-82df-6fca58544c8c
	Total devices 2 FS bytes used 930.77GB
	devid    1 size 931.51GB used 931.51GB path /dev/sda
	devid    2 size 931.51GB used 931.49GB path /dev/sdb

Label: none  uuid: 38673844-bcc4-48a0-884d-5613ae0ec043
	Total devices 1 FS bytes used 28.00KB
	devid    1 size 1.82TB used 2.04GB path /dev/sdc

Btrfs Btrfs v0.19

So, it seems to be a disk reporting error on the side of the kernel, in that it views the disks as independent objects and not as part of a RAID group? How odd.

Khannie

Kinet1c wrote: »

I would advise against software raid, it won't end well.

I was about to call you on this, but I see that OP is not using the kernel raid. The kernel raid is brilliant.

OP: I wouldn't be using btrfs build in raid on 12.04. Maybe bump to 14.04 and use the kernel raid with a btrfs filesystem on top?

azzeretti

If I remember correctly there us a bug with df reading btrfs correctly. I think you need to trust 'btrfs fi df'
IIRC this is a known problem and should be on the btrfs wiki.

EDIT: https://btrfs.wiki.kernel.org/index.php/FAQ#Why_does_df_show_incorrect_free_space_for_my_RAID_volume.3F