You can specify a custom volume for the temporary backups to go into, see Issues with export ('io: read/write on closed pipe') - #5 by tomp
But as LXD is a server it cannot create the backup directly on the destination specified by the client, and when adding files to the tarball we need to know the size up front and that isn’t available for optimized exports. Additionally we don’t want to end up buffering the tarball in memory while its streamed out.