SAN Backbone Device Quality of Connection
With the SAN Quality of Connection classification
matrix defined, it becomes a straightforward task
to classify types of interconnect devices according
totheir QoC potential as SAN backbone devices.
Table 5 presents this classification matrix as
defined by Strategic Research Corporation in the
Strategic Profile referenced earlier. QoC class
and architecture descriptions in this matrix match
those in Table 4.
Since QoC Classes 2 and 3 rely on the redundancy
of paths and interconnect devices to achieve their
respective availability-of-connection targets,
fault-tolerant features in backbone devices are
not required for these classes. However, adding
fault-tolerant features such as redundant,
hot-swappable power and cooling will certainly
improve quality of service by increasing
connection availability and reducing
performance degradation. A single Director may
be used in lieu of redundant fabric switches to
achieve Class 3 service levels.
In order to reach Class 4 QoC, backbone devices
must be consistently upgradable and repairable
or replaceable within the 53-minute availability-
of-connection window guaranteed by Class 4. They
must also meet the Class 4 performance
degradation guarantee of 500 annual path-minutes
maximum. These guarantees are nearly impossible
to meet unless backbone devices are designed for
continuous operation. Redundant Directors achieve
this QoC class today, and future fabric switches
with enhanced designs may be able to achieve it
as well.
Class 5 QoC backbone devices must clearly be
capable of delivering continuous operation by
protecting all critical components with
redundancy, automatic failover, and hot
replacement. Port cards must have minimal
granularity and be hot-swappable to meet the
performance degradation specification.
In addition, code must be upgradable while the
device is in full operation. Only under these
conditions can a backbone device meet the
stringent availability and performance criteria
of Class 5 QoC. Currently, a Director is the
only interconnect device offering these
features.
Designing a SAN
As a rule, SANs should always be designed with
the application in mind. Specific service goals
should be set by the application's requirements
for availability, performance, and scalability,
rather than dictating one service level for the
entire infrastructure. If not, excessive costs
will likely be incurred for applications that
don't require the same class of service needed
by critical applications. Conversely,
undercapitalizing the SAN infrastructure will
lead to poor application service. Using SAN QoC
by application ensures the lowest cost and
greatest probability of success.
Once the requirements of the application are
determined, the QoC classification matrix shown
in Table 4 or 5 specifies the SAN architecture
required to achieve that quality of service. For
example, if an application needs Class 5 QoC, a
SAN employing redundant Directors and redundant
paths with path failover is required. No other
architecture meets this need.
Table 5 – SAN Backbone Device Quality of Connection
Source: Strategic Research Corporation
QoC Class
Architecture Description
Minimum Device Capability
Current Device Implementation
QoC Class 1
Architecture Description:
Failure sensitive
- no redundancy
Miminum Device Capability:
No critical components are redundant with automatic
failover
Current Device Implementation:
Single Hub
QoC Class 2
Architectural Description:
Failure resilient
- partially redundant paths
- partially redundant interconnects
Minimum Device Capability:
No critical components are redundant with automatic
failover
Current Device Implementation:
Redundant switched hubs, EPL switches or fabric switches
QoC Class 3
Architectural Description:
Failure resilient
- fully redundant paths
- fully redundant or fault tolerant interconnects
Minimum Device Capability:
If redundant interconnects, no critical
components are redundant with automatic
failover; otherwise, fault tolerant features
are required
Current Device Implementation:
Redundant fabric switches or single Director
QoC Class 4
Architectural Description:
Failure tolerant
- fully redundant paths
- fully redundant interconnects
- backbone interconnects fault tolerant
Minimum Device Capability:
All critical components are redundant with automatic failover
Current Device Implementation:
Redundant Directors
QoC Class 5
Architectural Description:
Fault tolerant
- fully redundant paths
- fully redundant interconnects
- all interconnects fault tolerant
Minimum Device Capability:
All critical components are redundant with automatic failover
Current Device Implementation:
Redundant Directors
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