Default LUN numbering
The VLS automat
ically assigns a logical unit number (LUN) to each virtual library and tape drive created
on the VLS in the order in which they are created by you, starting with LUN0 and increasing incrementally
by one as each new virtual library or tape drive is created on an FC host port (LUN1, LUN2 and so on).
The first virtua
l device port m apped to any of the FC host ports is assigned the LUN numb er LUN0. The
second virtual device port mapped to an FC host port is assigned the LUN number LUN1, and so on.
The default LUN numbers are changed by the VLS software when the VLS device emulations are restarted
if:
• A virtual device has been deleted since the last restart, cre ating a gap in the LUN numbering, or
• A virtual tap
e drive has been added to a library since the last restart and the default LUN number
it was assigned is not consecutive with the other virtual tape drives in the same library.
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 ea ch library. This is done so that the virtual
device LUN nu
mbering meets the operating system LUN requirements.
NOTE:
When a LUN has been unconfigured or otherwise deleted, it may still sho w up in the VLS user interface.
Use the VLS disc overy utility to refresh the l ist of LUNs that are present.
Operating system LUN requirements and restrictions
Most operating systems require that each VLS FC host port connected to the SAN has a virtual device
with the LUN number LUN 0 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 FC host port when it is scanning for new hardware
on the SAN, it will stop looking for LU N s 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 p ort, it will stop looking for LUNs on that port when the gap in the LUN numbering is encountered.
Even though there m ay 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. O nce the ma ximum number of LUNs is detected, the operating system stops looking. In the case
of the VLS, the operating system considers each FC host port to be one device with its own set of LUNs.
So, if the maximum number of LUNs a n operating system is con figured to see is eight, the op erating system
will only see LUN0, LUN1, ..., LUN7 and will not see LUN8, LUN9 , ..., and up on each FC 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
for more information.
• Use the VLS’s LUN masking feature to restrict the number of virtual devices the host’s op erating
system sees on the VLS FC host port(s). Then, use the VLS’s LUN mapping feature to assign LUNs
to the virtual devices the host can see, such that the LUN numbers on each FC host port include a
LUN0 and no gaps in the LUN numbering. See LUN masking and LUN m apping for instructions.
LUN masking
By default all hosts on the SA N can access all the virtual libraries on the VLS. You can restrict a host’s
access to the virtual libraries and/or tape drives (virtual devices) on the VLS by performing LUN masking.
LUN masking should also be used when there are more virtual devices (LUNs) port mapped to a FC
host por t than the operating system LUN restrictions s upport. This prevents the host’s operating system
from seeing the virtual devices it needs to see on the port. For example, if a host’s operating system is
configured to only see up to eight LUNs per FC host port, the host will not be able to see the virtual
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