adding new devices to degraded raid1

[Eric Wong <e@80x24.org>]

I don't need to do it right away, but is it possible to add new
devices to a degraded raid1?

One thing I might do in the future is replace a broken big drive
with two small drives.  It may even be used to migrate to SSDs.

Since btrfs-replace only seems to do 1:1 replacements, and I
needed to physically remove an existing broken device to make
room for the replacements, could I do something like:

	mount -o degraded /mnt/foo
	btrfs device add small1 small2 /mnt/foo
	btrfs device remove broken /mnt/foo

?

Anyways, so far raid1 has been working great for me, but I have
some devices nearing 70K Power_On_Hours according to SMART

Thanks.