HP 9000 Virtual Library System - Manual (Page 110)

Operating System LUN Requirements and Restrictions
Most operating systems require that each VLS Fibre Channel host port connected to the SAN has
a virtual device with the LUN number LUN0 and no gaps in the LUN numbering (LUN0, LUN1,
LUN2, and so on). If the operating system does not see a LUN0 on a VLS Fibre Channel host port
when it is scanning for new hardware on the SAN, it will stop looking for LUNs on that port and
erroneously report that there are no LUNs (virtual devices) on that port. If the operating system sees
a LUN0, LUN1, and LUN2 but not a LUN3 on the port, it will stop looking for LUNs on that port
when the gap in the LUN numbering is encountered. Even though there may be more LUNs, such
as LUN4, the operating system will erroneously report that there are only three LUNs on the port.
In addition, operating systems are configured to only look for a maximum number of LUNs per
device and no more. Once the maximum number of LUNs is detected, the operating system stops
looking. In the case of the VLS, the operating system considers each Fibre Channel host port to be
one device with its own set of LUNs. So, if the maximum number of LUNs an operating system is
configured to see is eight, the operating system will only see LUN0 through LUN7 and will not see
LUN8, LUN9, and up on each Fibre Channel host port.
To get around the maximum LUNs per device restriction, you can either:
•
Increase the maximum LUNs per device setting for the operating system. See
“Troubleshooting” (page 222) for more information.
•
Enable LUN mapping on the VLS to restrict the number of virtual devices the host's operating
system sees on the VLS Fibre Channel host ports. See
“LUN Mapping” (page 110) for more
information.
LUN Masking
LUN masking restricts a host's access to the virtual devices (virtual libraries and/or tape drives) on
the VLS. You should use it when there are more virtual device LUNs mapped to an Fibre Channel
host port than the operating system LUN restrictions support. You can limit the LUNs the host's
operating system can see on the port, so that only the virtual devices the host needs to see are
visible.
For example, if a host's operating system is configured to only see up to eight LUNs per Fibre
Channel host port, the host will not be able to see the virtual devices numbered LUN8, LUN9, and
up. Suppose that you need the host to see LUN0 through LUN5, LUN8, and LUN9. You can enable
LUN mapping, thereby hiding all of the LUNs from the host. Then you can map LUN0 through
LUN5, LUN8, and LUN9 to the host. The VLS will automatically renumber the mapped LUNs so
that LUNs 8 and 9 become LUNs 6 and 7 and they are now within the eight the host can see. See
LUN Mapping .
In the VLS, LUN masking occurs automatically for all hosts when you enable LUN mapping using
the global enable/disable LUN mapping setting. See
“Setting the Default LUN Mapping” (page 111) .
LUN Mapping
LUN mapping is used to present only a subset of the VLS virtual devices to a host. If LUN mapping
is enabled, the VLS prevents all hosts connected to the VLS through the SAN from viewing any
virtual devices until they are explicitly mapped to the host. By disabling LUN mapping, the VLS
allows all hosts connected to the VLS to access all virtual devices configured on the VLS. By default,
LUN mapping is disabled. The setting you choose will apply to every new host that you add to the
VLS. See
Setting the Default LUN Mapping .
110 Configuration