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Architectures
IBM
Named the IBM TS7700 Virtualization Engine, this new system is a significant change
from the previous architecture. IBM calls this new architecture its “Virtual Tape Grid”
and, while it continues to be based on outboard subsystems, the way nodes operate
and are managed is totally changed. At its heart, this grid is a move away from a
monolithic approach to a more distributed one. Unlike IBM’s old peer-to-peer
architecture, the individual TS7700 nodes are managed as an integrated whole.
Moreover, each node adds capacity to the local grid and sports new gigabit IP links for
inter-node traffic.
Another important architectural difference is that IBM now virtualizes the location and
number of virtual volumes. This permits access and recovery from any node and was a
key design goal. The idea is to completely divorce the user from having to know where
tape data actually resides and how many copies there are. Instead, users can specify
recovery policies that will ultimately dictate these. As we discuss below, this added
virtualization dimension is not only a big step in supporting business continuity with
advanced policy management; it is also a necessary step in content-based access for
tape data.
User interest in the new gigabit IP links has been strong as they look for simpler and
less expensive ways to replicate tape data. IBM led the market with synchronous
replication and has always had asynchronous replication.
Although IBM’s tape grid currently supports three nodes, it has been designed to
support at least eight nodes as well as to support future enhancements in cache
capacity, performance, content-based access and data de-duplication. It also leverages
IBM’s vast array of technologies for performance, encryption and futures.
Playing catch-up, IBM has finally added the capability to export a copy of the logical
volumes stored to the node for disaster recovery purposes. Also, IBM’s tape grid is not
compatible with its previous architecture, but a wealth of migration tools are available.
Sun
Sun continues to employ a hybrid approach where most of the virtual tape logic sits on
mainframe-based software while tape I/O and replication services are handled by a
relatively dumb external hardware subsystem based on Sun’s SVA array which is no
longer being actively marketed.
In addition, availability features are limited to two-node clusters with no more than four
nodes total (one local cluster and one remote cluster) often referred to as a “quadplex”.
Worse yet, only one set of channels must handle all I/O (host, tape drives, and remote
communication) making it difficult to balance changing workloads. In addition, without
gigabit IP links, expensive channel extenders or routers must be used to reach remote
sites.
