When you installed Windows Server 2003 Domain Controller and used iSCSI mounted drives for your SYSVOL and Active Direcotry Database, you recognized that this doesn’t work because the iSCSI LUNs are not mounted when Active Directory Services expect it.
So you decide to use iSCSI volumes for system volume and additional volumes for SYSVOL and Active Directory Database too.
If you think this works for Windows Server 2008 R2 Domain Controller, you will fail. The Active Directory Installation Wizard does not support saving the Active Directory Database and the SYSVOL folder on to an iSCSI volume. Microsoft release a MSKB article 977184 about this issue.
But the article also described an alternative and the necessary steps to enable the Active Directory Installation Wizard to save Active Directory Database and SYSVOL folder on iSCSI volume.
But this is only possible if the iSCSI volume is the system volume.
The operating system cannot start if you save the Active Directory Database to a non-system volume that is not available during the startup process. Therefore, do not put the Active directory database on an iSCSI volume that may not be available during the startup process.