If you plan to form a shadowed system disk, you must add system parameters to the SYS$SYSTEM:MODPARAMS.DAT file. Add the following lines to the bottom of the MODPARAMS.DAT file:

SHADOWING=2 !Enable volume shadowing
SHADOW_SYS_DISK=1 !Enable shadowing of the system disk
SHADOW_SYS_UNIT=n !Optional: default is 0, which creates DSA0
SHADOW_MAX_COPY=4 !Allow up to 4 shadow copies or merges going on at the same time
ALLOCLASS=x !This number must be non-zero;
!it must be used if local non-FC devices are going to be
!shadow set members

If a nonzero ALLOCLASS value is already in use for your system, do not change the ALLOCLASS value. For more information about these and other system parameters you can set for volume shadowing, see the HP Volume Shadowing for OpenVMS manual. For more information about setting ALLOCLASS for clusters, see the HP OpenVMS Cluster Systems manual.