Mounting multiple regular files as a filesystem

Mounting multiple regular files as a filesystem

[Ravi Pinjala]

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

Re: Mounting multiple regular files as a filesystem

[Yan Zheng]

2009/6/15 Ravi Pinjala <ravi@p-static.net>:
> 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
>
>

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