btrfs-raid10 <-> btrfs-raid1 confusion

Greetings,

until yesterday I was running a btrfs filesystem across two 2.0 TiB disks in RAID1 mode for both metadata and data without any problems.

As space was getting short I wanted to extend the filesystem by two additional drives lying around, which both are 1.0 TiB in size.

Knowing little about the btrfs RAID implementation I thought I had to switch to RAID10 mode, which I was told is currently not possible (and later found out that it is indeed).
Then I read this [1] mailing list post basically saying that, in the special case of four disks, btrfs-raid1 behaves exactly like RAID10.

So I added the two new disks to my existing filesystem

    $ btrfs device add /dev/sde1 /dev/sdf1 /mnt/archive

and as the capacity reported by 'btrfs filesystem df' did not increase,

I started a balancing run:

    $ btrfs filesystem balance start /mnt/archive

Waiting for the balancing run to finish (which will take much longer than I thought; still running) I found out that as of kernel 3.3 changing the RAID level (aka restriping) is now possible: [2].

I got two questions now:

1.) Is there really no difference between btrfs-raid1 and btrfs-raid10
in my case (2 x 2TiB, 2 x 1TiB disks)? Same degree of fault tolerance?

2.) Summing up the capacities reported by 'btrfs filesystem df' I only
get ~2.25 TiB for my filesystem, is that a realistic net size for 3 TiB gross?

    $ btrfs filesystem df /mnt/archive
    Data, RAID1: total=2.10TB, used=1.68TB
    Data: total=8.00MB, used=0.00
    System, RAID1: total=40.00MB, used=324.00KB
    System: total=4.00MB, used=0.00
    Metadata, RAID1: total=112.50GB, used=3.21GB
    Metadata: total=8.00MB, used=0.00

It won't -- "btrfs fi df" reports what's been allocated out of the
raw pool. To check that the disks have been added, you need "btrfs fi show" (no parameters).

You're not comparing the right numbers here. "btrfs fi show" shows the raw available unallocated space that the filesystem has to play with. "btrfs fi df" shows only what it's allocated so far, and how much of the atllocation it has used -- in this case, because you've added new disks, there's quite a bit of free space unallocated still, so the numbers below won't add up to anything like 3TB.

Thanks for clarifying things, Hugo :)

Quoted 3 lines

It won't -- "btrfs fi df" reports what's been allocated out of the
raw pool. To check that the disks have been added, you need "btrfs fi show" (no parameters).

Okay, that gives me

    Label: 'archive'  uuid: 3818eedb-5379-4c40-9d3d-bd91f60d9094
        Total devices 4 FS bytes used 1.68TB
        devid    4 size 931.51GB used 664.03GB path /dev/dm-10
        devid    3 size 931.51GB used 664.03GB path /dev/dm-9
        devid    2 size 1.82TB used 1.56TB path /dev/dm-8
        devid    1 size 1.82TB used 1.56TB path /dev/dm-7

so I conclude all disks are successfully assigned to the raw pool for my 'archive' volume.

Quoted 6 lines

You're not comparing the right numbers here. "btrfs fi show" shows the raw available unallocated space that the filesystem has to play with. "btrfs fi df" shows only what it's allocated so far, and how much of the atllocation it has used -- in this case, because you've added new disks, there's quite a bit of free space unallocated still, so the numbers below won't add up to anything like 3TB.

So how is the available space in the raw pool finally allocated to the usable area? Must I manually enlarge the filesystem by issuing a 'btrfs fi resize max /mountpoint' (like assigning space of a VG to a logical volume in LVM) or is the space allocated automatically when the filesystem gets filled with data?

Quoted 5 lines · Alexander Koch

So I added the two new disks to my existing filesystem

=C2=A0 =C2=A0 $ btrfs device add /dev/sde1 /dev/sdf1 /mnt/archive

and as the capacity reported by 'btrfs filesystem df' did not increa=

=C2=A0 It won't -- "btrfs fi df" reports what's been allocated out of=

raw pool. To check that the disks have been added, you need "btrfs fi show" (no parameters).

Quoted 3 lines

I got two questions now:

1.) Is there really no difference between btrfs-raid1 and btrfs-raid=

Quoted 1 line

=C2=A0 =C2=A0 in my case (2 x 2TiB, 2 x 1TiB disks)? Same degree of =

Quoted 1 line

=C2=A0 =C2=A0 tolerance?

=C2=A0 There's the same degree of fault tolerance -- you're guarantee=

be able to lose one disk from the array and still have all your data.

=C2=A0 The data will be laid out in a different way on the disks, tho=

In your case, with four unevenly-sized disks, you will get the best usage out of the filesystem with RAID-1. With only 4 disks, RAID-10 will run out of space when the smallest disk is full. (So, in your configuration, you'd still have only 2TB of space usable, rather defeating the point of having the new disks in the first place).