Re: how to clone a btrfs filesystem

[Kai Krakow <hurikhan77@gmail.com>]

Kai Krakow <hurikhan77@gmail.com> schrieb:

> Christoph Anton Mitterer <calestyo@scientia.net> schrieb:
> 
>> Hey.
>> 
>> I've seen that this has been asked some times before, and there are
>> stackoverflow/etc. questions on that, but none with a really good
>> answer.
>> 
>> How can I best copy one btrfs filesystem (with snapshots and subvolumes)
>> into another, especially with keeping the CoW/reflink status of all
>> files?
>> And ideally incrementally upgrade it later (again with all
>> snapshots/subvols, and again not loosing the shared blocks between these
>> files).
>> 
>> send/receive apparently also works for just one subvolume,... and
>> documentation is quite sparse :-/
> 
> You could simply "btrfs device add" the new device, then "btrfs device
> del" the old device...
> 
> It won't create a 1:1 clone, if that is your intention. But it will
> migrate all your data over to the new device (even a bigger/smaller one),
> keeping shared extents (CoW/reflink status), generation numbers, file
> system uid, label, each subvolume, etc... And it can be done while the
> system is running.
> 
> It looks like this way fulfills all your requirements. That same way you
> can later incrementally update it again.

BTW: I've done that when migrating my 3-device btrfs pool to bcache. I 
simply removed one device, reformatted with bcache, then added it. I 
repeated those steps for each device. It took a while (like 24 hours) but it 
worked. In the end I just did a balance to rebalance all 3 devices. That 
last step is not needed if you are migrating only one device.

As a safety measurement I did a backup first, then ran "btrfs check" to 
ensure file system integrity, and did start into the rescue shell to have 
only a minimum set of processes running. Due to a bug in the Intel graphics 
stack the system froze sometimes, and I didn't want that to happen during 
data migration.

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