On Sun, Aug 19, 2012 at 02:08:17PM +0000, Daniel Pocock wrote:

I created a 1TB RAID1.  So far it is just for testing, no important data
on there.


After a reboot, I tried to mount it again

# mount /dev/mapper/vg00-btrfsvol0_0 /mnt/btrfs0
mount: wrong fs type, bad option, bad superblock on
/dev/mapper/vg00-btrfsvol0_0,
       missing codepage or helper program, or other error
       In some cases useful info is found in syslog - try
       dmesg | tail  or so

   With multi-volume btrfs filesystems, you have to run "btrfs dev
scan" before trying to mount it. Usually, the distribution will do
this in the initrd (if you've installed its btrfs-progs package).

Then I did btrfsck - it reported no errors, but mounted OK:

# btrfsck /dev/mapper/vg00-btrfsvol0_0

[...]

   The first thing that btrfsck does is to do a device scan.

[...]

Can anyone comment on this?

   See above.

Also, df is reporting double the actual RAID1 volume size, and double
the amount of data stored in this filesystem:

# df -lh .
Filesystem                    Size  Used Avail Use% Mounted on
/dev/mapper/vg00-btrfsvol0_0  1.9T   51G  1.8T   3% /mnt/btrfs0

I would expect to see Size=1T, Used=25G

# strace -v -e trace=statfs df -lh /mnt/btrfs0
statfs("/mnt/btrfs0", {f_type=0x9123683e, f_bsize=4096,
f_blocks=488374272, f_bfree=475264720, f_bavail=474749786, f_files=0,
f_ffree=0, f_fsid={2083217090, -1714407264}, f_namelen=255,
f_frsize=4096}) = 0
Filesystem                    Size  Used Avail Use% Mounted on
/dev/mapper/vg00-btrfsvol0_0  1.9T   51G  1.8T   3% /mnt/btrfs0

   This is an FAQ:

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

   tl;dr: It's reporting the total number of raw storage bytes,
because it's impossible to compute actual usable space in the general
case.

   Hugo.

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