Mounting multiple regular files as a filesystem

I'm trying to create a multi-device filesystem on top of regular files (not actual disks), and mount that to a loopback device. For a filesystem created on a single file, it works fine, but for a filesystem across multiple files, it doesn't.

dd if=/dev/zero of=img1 bs=4096 count=65536
dd if=/dev/zero of=img2 bs=4096 count=65536
dd if=/dev/zero of=img3 bs=4096 count=65536
dd if=/dev/zero of=img4 bs=4096 count=65536

mkfs.btrfs img1
mount -o loop -t btrfs img1 /mnt/test # works

mkfs.btrfs img1 img2 img3 img4
mount -o loop -t btrfs img1 /mnt/test # fails

When I try to mount with multiple files, it also prints a message to syslog:

Jun 15 00:32:10 3vil device fsid a147d6b950f66dca-3e2e7f52011c93b0 devid
4 transid 9 /dev/loop/0
Jun 15 00:32:10 3vil btrfs: failed to read chunk tree on loop0
Jun 15 00:32:10 3vil btrfs: open_ctree failed

Am I doing something wrong, or is this use case broken right now?

--Ravi

2009/6/15 Ravi Pinjala <[email protected]>:

Please setup loop devices for the image files manually. In this case, 'mount -o loop' can't do it for you.

For example:
mkfs.btrfs img1 img2 img3 img4
for i in seq 1 4; do losetup /dev/loop$ img$i; done btrfsctl -a
mount -t btrfs /dev/loop1 /mnt/test

See below for more information:
http://btrfs.wiki.kernel.org/index.php/Using_Btrfs_with_Multiple_Devices

Yan, Zheng