[chasil]

ZFS will come closest.

I have a ZFS mirror, where I have taken one disk out, added files to it elsewhere, returned it and reimported.

The pool immediately resilvered the new content onto the untouched drive.

Doing this on btrfs will require a rebalance, forcing all data on the disks to be rewritten.

[yencabulator]

> Doing this on btrfs will require a rebalance, forcing all data on the disks to be rewritten.

I believe btrfs replace will copy only the data that had a replica on the failing drive.

[chasil]

You wouldn't replace in this context.

You would mount degraded on the remote system and copy in the new files.

After the returning the new drive, you would mount normally, but getting the new content mirrored requires a rebalance.

Replace is for a blank drive, and it hasn't worked very well for me, as status/usage reported some data that was not mirrored; a rebalance fixed this.