HP StorageWorks 6000-series Virtual Library System User Guide (AH809-96049, September 2010)

Restarting VLS device emulations changes the default LUN numbers as necessary to remove the gap
or to make the virtual tape drive LUN numbers consecutive in each library. This is done so that the
virtual device LUN numbering meets the operating system LUN requirements.
NOTE:
When a LUN has been unconfigured or otherwise deleted, it may still show up in the VLS user interface.
Use the VLS discovery utility to refresh the list of LUNs that are present.
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 “Troubleshoot-
ing” on page 309 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 (v3.x)” on page 153 or “LUN Mapping
(v2.x)” on page 158 for instructions.
LUN Masking
The method of LUN masking differs between VLS firmware versions 3.x and 2.x.
LUN Masking (v3.x)
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
(v3.x)” on page 153.
HP StorageWorks 6000–series Virtual Library System User Guide 151