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

[Hugo Mills <hugo@carfax.org.uk>]

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.

> 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.

-- 
Hugo Mills             | The last man on Earth sat in a room. Suddenly, there
hugo@... carfax.org.uk | was a knock at the door.
http://carfax.org.uk/  |
PGP: E2AB1DE4          |                                        Frederic Brown