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Storage Area Networks (SANs) based on Fibre Channel architecture represent a radical departure from traditional server/ storage configurations. Traditional parallel SCSI cabling is a thick umbilical cord that ties each group of disks to ownership by a single server. By severing this connection, Fibre Channel SANs allow storage to stand on its own. This enables at truly datacentric worldview, which was unachievable as long as each server was the exclusive owner of its own data, Servers also gain from this new freedom, since each now has access via the storage network to any disk resource.
Designing Storage Area Networks thus requires a new way of thinking about the server-storage relationship. If servers, disk arrays, and tape subsystems can be networked, entirely new possibilities emerge for optimizing and managing data access, data integrity, and data management.
DATA ACCESS
As illustrated in Fig 1, a Fibre Channel SAN may include a number of building blocks for redefining the connectivity between servers and storage. In this example, a Fibre Channel fabric switch forms the backbone of the SAN. Each fabric port provides 100MB/sec of bandwidth, which allows high-speed access between servers, disks, and a tape backup subsystem.
The Arbitrated Loop hub represents a shared 100MB/sec segment with two servers (B and C) attached. If both servers were equally active, the bandwidth available to each would be slightly less than 50MB/sec.The design criteria for including a shared loop hub are application-driven. If neither server engenders traffic patterns above 50MB/sec, they would waste bandwidth and hardware investment if attached directly to the fabric switch. The per-port cost delta between fabric switches and loop hubs encourages use of shared segments whenever possible. Casual database queries or OnLine Transaction Processing (OLTP) applications, for example, may allow 20 to 30 servers to be configured on a single Arbitrated Loop segment at a significant savings vis-a-vis fabric ports.
Server A, by contrast, may be an Intranet server supporting larger graphic or multimedia transactions. Even without the bandwidth requirement of sustained full motion video, retrieving or writing large files may engender bursty data traffic that justifies a dedicated 100MB/sec port on the switch.
Fibre Channel Host Bus Adapters (HBAs) provide the connection between the server bus and the Fibre Channel network. Selecting the appropriate HBA...