On 15.12.2021 23:27, Eric Levy wrote:

Hello.

I have been experiencing very confusing problems with incremental
streams.

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, and that the latter would apply
the updated stage.

The prior command succeeds, but the latter fails:

ERROR: creating snapshot ./@ -> @ failed: File exists

You need to restore it in different directory. Each send stream defines
subvolume and you cannot have two subvolumes with the same name in the
same directory. 

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 misunderstand what happens. btrfs receive does not update existing subvlume.
It always creates new subvolume by cloning parent replica and applying changes
to this clone. Parent remains in its original state and read-only.