SAN Design: March 29, 2001 3:18 pm
53-0001575-01 BROCADE Technical Note Page: 4 of 31
Tier Architecture Designs
•Typically three layers of switches, a host layer, core layer, and storage layer
•Natural extension to star design (in some cases, Tier designs are Stars)
•Core switches are used to provide connectivity between host and storage layer switches, includes redundant switching elements
•Each layer can be scaled independently
•Cores can be simple to more complex, easily replaced with higher port count switches
•Can still use knowledge of data locality in placing devices in fabric
•Allows for bandwidth improvement by using multiple ISLs where needed
•Multiple paths in fabric allowing for redundant path selection should a failure occur
2.1 REDUNDANCY AND FAILOVER
One of the major benefits of a Fibre Channel fabric is the ability of the fabric to recover from failures of individual fabric elements as long
as the design includes redundant paths. The BROCADE SilkWorm design supports auto re-configuration of the fabric when switches are
added or removed from the fabric. This allows for auto discovery of alternate routes between fabric nodes with the routing algorithm deter-
mining the most efficient route between nodes based on the currently available switches. Obviously to take advantage of this feature the
basic fabric design should have built in redundancy to allow for alternate paths to end node devices. A single switch fabric will have no
alternate paths between devices should the switch fail. However, a simple two switch fabric can be designed, along with redundant ele-
ments in the host and storage nodes, to allow for failure of a single switch and to use a route through an alternate switch.
A number of factors should be considered when designing a fabric and there is no one answer or single topology that addresses all prob-
lems. Each user will have unique system elements and design needs that will need to be factored into the fabric design. The later portion of
this document provides for a number of design topologies that can be used as templates for fabric designs. Key elements to consider are:
•How much redundancy is required? Hosts with key applications should have redundant paths to required storage (via the fabric),
meaning multiple HBA’s per host should be considered so a single HBA failure will not result in loss of host access to data
•Storage considerations. RAID devices provide for more reliability and resilience to failure of a single drive and allow for auto-
recovery on failure. HIgh availability designs should use RAID storage devices as the building blocks for storage -- these devices
have built in recovery when using RAID 1(mirroring) or RAID 5 (striping with parity disk). A greater level of reliability can be achieved
by mirroring the storage device remotely using the switch support for devices at 10Km distance (or more using devices that support
extended distance optical signaling). Some RAID subsystems include the ability to mirror writes to another disk system as a feature
of the disk controller; software support for this feature (e.g. Veritas) also exist. A critical storage node can be mirrored locally within
a fabric or mirrored across an extended fabric link. BROCADE provides a licensed software option (Extended Switch, available in
release 2.1.3) that allows for increasing the E-port buffer credits for extended links. [Buffer credits allow for the sending device to
continue to send data up to the credit limit without having to wait for acknowledgment, improving performance. More credits allows
for a greater pipeline of data on a link, particularly useful when transmitting over extended distances.] The extended fabric option is