Separating system databases from MDW SQL Server 2008 R2
Join via snapshots_internal to source_info_internal which has the server instance name, eg

SELECT du.*, sii.instance_name
FROM [snapshots].[disk_usage] du
    INNER JOIN core.[snapshots_internal] si ON du.snapshot_id = si.snapshot_id
        INNER JOIN [core].[source_info_internal] sii ON si.source_id = sii.source_id
WHERE du.database_name = 'tempdb'