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VSM Best Practices
There is a very common misconception that having more than one LSM in any
VSM configuration improves reliability by increasing redundancy. In fact, it
does exactly the opposite in a VSM environment.
By increasing the number of LSMs that house RTDs attached to a single
VTSS, the chance of Virtual Tape Volume (VTV) recall failure increases.
This is because, in the event of a failure in a two-LSM, single ACS
configuration with simplexed VTVs, VTCS will usually be able to continue
migration—since there should be available MVCs in each LSM—but about half
of the VTVs will have become unavailable for recall back into the VSM buffer.
In the event of a failure in a two-LSM, single ACS configuration, VTCS will
usually be able to continue migration—again since there should be available
MVCs in each LSM—but about half of the VTVs will have become unavailable
even with duplexed VTVs. This is true because VTCS only guarantees that the
two VTV copies will be on separate MVCs, regardless of location in the
libraries. Named MVCPools in each LSM does not completely address this
issue because the MVCs can still be moved from one LSM to the other
depending on RTD allocation.
In a two-LSM, multi-ACS environment with duplexed, triplexed or quadplexed
VTVs, the customer would eliminate the exposure of a VTV being unavailable
due to an LSM or ACS failure by migrating one VTV copy to each ACS using
MIGpol or ACS-list (for duplexed only). Migrations and recalls would continue
to occur in the event of an ACS failure or media failure, and will result in
business continuity in the event the local site is destroyed.
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Considering VTCS Maintenance
Customers should ensure that they have all of the latest PTFs applied to
VTCS and HSC.
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Configuring the LSMs and RTDs
The SL8500 is a new Library architecture. This is not new news—but, it is
worth repeating because it means changing the way we currently think about
VSM configurations. There is a smaller capacity per logical LSM, less tape
drives per logical LSM, multiple storage elements available to consider, and
both vertical and horizontal pass-thru capability.
The ideal configuration for RTDs in an SL8500 library to achieve optimum
performance is to have all RTDs and all MVCs reside in one logical LSM, if
possible, and to duplex to a separate ACS. This would eliminate any pass-
through activity and allow for VTV redundancy and availability in the event of a
failure in one ACS. For a customer with one to four VTSSs, with a typical RTD
configuration of 8 RTDs per VTSS, this would mean occupying one logical
LSM in each of two ACSs. This would be accomplished by connecting four
RTDs to the local ACS and four RTDs to the remote, or second ACS, from
each VTSS.
58 SL8500: Best Practices • March 2007
Revision B • TM0017
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