On 07/10/2012 10:52 AM, Arnd Hannemann wrote:

Hi,

Am 10.07.2012 05:30, schrieb Christian Robert:

quoted

I agree with you, but you should never mount a snapshot of a btrfs filesystem at the same time the original is,
because both the original and the snapshot had same "device fsid 5c3e8ca2-da56-4ade-9fef-103a6a8a70c2"

I think that the kernel should be smarter in this regard.

At kernel level, as golden rule it should be not possible to add a
duplicate fsid if the previous one is mounted. "btrfs dev scan" MUST
return an error in this case. This in any case is an error.

Today it seems that if a device with the same fsid is already
registered, the new one overwrites the old one (or almost the name is
overwritten). This could be acceptable if the filesystem is unmounted. I
think that it is a serious error otherwise.

quoted

the kernel will tkink twice and fold back to the same device.

If that is correct the bug is that the kernel lets me mount the same device fsid on different devices twice.

quoted

btrsf does not behave like other filesystems, you can't snapshot a btrfs filesystem
and hope to mount the snapshot somewhere else.

quoted

snapsoot also duplicate lots of things internally that have no sence in a snapshot (like raid level, single or multiple devies ...)

I see. However, I expect a "simple" btrfs to just work or fail gracefully.

Best regards,
Arnd

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