Lun Masking And Mapping - HP StorageWorks 12000 - Virtual Library System EVA Gateway Manual

• HPUX: prevent utilities like mt from rewinding a tape that is in use by the backup application
by turning on the HPUX kernel tunable parameter st_san_safe (which disables tape device
special files that are rewind-on-close)

LUN Masking and Mapping

D2D devices with the Fibre Channel option do not require LUN masking/mapping because every
virtual device LUN (library or drive) has its own unique Fibre Channel WWPN (using what is called
N-port virtualization). This means you use SAN zoning to present each virtual device's WWPN to the
backup hosts (but you must use a SAN switch that supports this and it makes sharing virtual devices
across multiple media servers more difficult).
On VLS devices, by default all hosts on the SAN can access all the virtual devices (libraries and tape
drives) of the VLS and this mode is used to easily share virtual drives across multiple media servers.
(Enterprise backup applications support the ability to share tape drives across multiple servers).
• The backup applications take care of drive locking to ensure only one media server is using one
shared drive at a time.
• VLS software manages the LUNs assigned to the VLS shared virtual devices by default. The VLS
changes device LUN assignments as needed to make sure that each Fibre Channel host port has
a LUN0, that there are no duplicate LUNs, and that there are no gaps in the LUN numbering on
a Fibre Channel host port (these are operating system requirements).
• This is the easiest scheme for the backup administrator because they can simply configure all their
backup jobs across their media servers to use the shared library with shared drives and the backup
application takes care of deciding which drive is used for which backup job and automatically
scheduling all the jobs onto the available drives.
• One disadvantage of this scheme is that if any one media server goes wrong (for example, mixes
up its tape drives paths), then it can interfere with the backups running from the other media
servers to the shared drives and can be very difficult to diagnose when there are many media
servers.
The VLS has an option to enable LUN masking in the device which is used to assign dedicated virtual
libraries/drives on a VLS. LUN masking lets you restrict which hosts you want to be visible to particular
virtual devices so that if a media server has any problem it only affects the tape drives dedicated to
that media server; it will not affect any backups on other media servers. Because LUN masking prevents
a host from seeing selected virtual devices on the Fibre Channel host ports, a LUN-masked host will
not see the virtual devices to which it does not have access and to which it cannot write. This requires
the administrator to manually allocate specific virtual drives in the virtual library to specific media
servers to meet their performance requirements.
HP StorageWorks VLS and D2D Solutions Guide 49