Re: cloning a btrfs drive with send and receive: clone is bigger than the original?

[Andrei Borzenkov <arvidjaar@gmail.com>]

10.01.2021 16:21, Hugo Mills пишет:
> On Sun, Jan 10, 2021 at 01:06:44PM +0000, Graham Cobb wrote:
>> On 10/01/2021 07:41, Cedric.dewijs@eclipso.eu wrote:
>>> I've tested some more.
>>>
>>> Repeatedly sending the difference between two consecutive snapshots creates a structure on the target drive where all the snapshots share data. So 10 snapshots of 10 files of 100MB takes up 1GB, as expected.
>>>
>>> Repeatedly sending the difference between the first snapshot and each next snapshot creates a structure on the target drive where the snapshots are independent, so they don't share any data. How can that be avoided?
>>
>> If you send a snapshot B with a parent A, any files not present in A
>> will be created in the copy of B. The fact that you already happen to
>> have a copy of the files somewhere else on the target is not known to
>> either the sender or the receiver - how would it be?
>>
>> If you want the send process to take into account *other* snapshots that
>> have previously been sent, you need to tell send to also use those
>> snapshots as clone sources. That is what the -c option is for.
> 
>    And even then, it won't spot files that are identical but which
> don't share extents.
> 

It won't with "btrfs send -p" as well.

>> Alternatively, use a deduper on the destination after the receive has
>> finished and let it work out what can be shared.
> 
>    This is a viable approach.
> 
>    Hugo.
>