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Chapter 4 - SAN Solution Strategies
SAN Configuration Not Supported
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SAN Configuration Not Supported
In the Active-Active:Dual Port configuration with two independent hubs or the
internal hubs, two single-ported hosts is
not supported. The issue with this
configuration is that if a controller fails, each host loses access to one of the LUNs.
For example, if Controller 1 fails, Host 1 can no longer access LUN 2, because
Port 2 on Controller 2 has changed from presenting its own LUNs to presenting
Controller 1’s LUNs. A need for this SAN configuration can arise if multiple
single-ported hosts need access to LUNs from both controllers, and two hubs are
desired to improve performance by decreasing loop access contention.
Note that this SAN is only partially fault-tolerant because the hubs and controllers
have redundant hardware, but the hosts do not. The problem with this
configuration can be overcome by using a single switch at the cost of replacing
the two hubs with a switch. If loop access contention is not an issue, then the
single hub configuration works as well.
True LUN Sharing
When considering the presentation of LUNs for Active-Active controllers, a good
question is: “Why not just present all LUN on all ports?” The answer is that LUN
sharing greatly increases complexity and sacrifices performance. In order to do
LUN sharing between two controllers, the following must happen:
1
Some common data sharing method must be developed so that access to
resources shared by both controllers can be administered. Because of this
arbitration for resources, the performance of both controllers suffers as
compared to two controllers that must only control their own resources.
2 Read cache on one controller must be invalidated if the data is modified by a
write to the other controller.
3 Writes to disk must be controlled to ensure that simultaneous updates to the
same region do not occur by both controllers.
4 RAID utility functionality (parity verification and expansion, for example)
must be controlled to ensure that collisions between utilities and data
accesses on the two controllers don’t occur.
The Active-Active strategy chosen allows a host to access two separate Fibre Channel
ports (one from each controller) which are connected together via a switch or hub,
in order to support access to LUNs owned by different controllers. This is simpler
and provides higher performance than so-called “true LUN sharing” schemes.
4100FS.book Page 80 Tuesday, July 8, 2003 4:35 PM