raw fs backup/copy

Is there a reason one can not mount a btrfs and it's byte-for-byte copy in parallel, or is the driver just acting silly?

Mini example:

# dd if=/dev/zero of=fs1 bs=1 count=1 seek=$((1024*1024*1024-1))
# mkfs.btrfs fs1
# cp --sparse=always fs1 fs2
# mkdir 1 2
# mount fs1 1 -o loop
# mount fs2 2 -o loop
# touch 1/1
# ls 2
1
# cat /proc/mounts
/dev/loop0 /tmp/test/1 btrfs rw,relatime 0 0
/dev/loop1 /tmp/test/2 btrfs rw,relatime 0 0

--
ivan

)

m

The reason I ask is, that I had a mirror RAID (btrfs on LVM on LUKS on = RAID)
across 2 disks. Then I snatched one to take with me when going away for holidays, copied some files on to it and when I came back, I wanted to = get
to those files. The problem was, I couldn't mount the fs I took with me= ,
because a similar fs was already mounted. As it wasn't the root fs, I could unmount it and copy the files on some other fs (though one could use pivot_root, if it was the root fs).

It seems to me there are cases, where one would want to mount two copie= s of
originally the same filesystem - think of RAID, VMs, distributing fs im= ages
on a network, LVM snapshots, backups, etc.

--
ivan
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