Re: receive failing for incremental streams

[Graham Cobb <g.btrfs@cobb.uk.net>]

On 15/12/2021 20:27, Eric Levy wrote:
> Hello.
> 
> I have been experiencing very confusing problems with incremental
> streams.

There is no such thing as an incremental stream. Send sends all the
information necessary to create a subvolume. Some of that includes
instructions to share data in other subvolumes but it is not incremental.

> For a subvolume, I have a simple incremental backup created from two
> stages:
> 
> btrfs send old/@ > base.btrfs
> btrfs send new/@ -p old/@ > update.btrfs
> 
> The two source subvolumes are snapshots captured at separate times from
> the same actively mounted subvolume.
> 
> On the target, I attempt to restore:
> 
> btrfs receive ./ < base.btrfs
> btrfs receive ./ < update.btfs
> 
> The expectation is that the prior command would create a restored
> snapshot of the initial backup stage, 

Yes

> and that the latter would apply
> the updated stage.

No. Receive always creates a brand new subvolume - it doesn't update
anything. Of course, the new subvolume may include clones of data stored
in other subvolumes but it doesn't modify any existing subvolumes.

> 
> The prior command succeeds, but the latter fails:
> 
> ERROR: creating snapshot ./@ -> @ failed: File exists
> 
> Since it is obvious I cannot usefully apply the second stage to a
> target that does not exist, I am puzzled about why the process performs
> this check, as well as what is expected to have success applying the
> update.
> 
> How may I apply the update stage to the target generated from restoring
> the initial stage?

You don't. Receive will create a new subvolume - which will include
unchanged data from the initial stage and whatever changes have
happened. If you want, you can then snapshot that (read-only or
read-write as you wish) into any position you want in your destination
filesystem.

Graham