2.6 Chapter Summary

Networking in front of the Server

• SANs bring new networking concepts to the server/storage model.

• Moving data serially allows higher speeds and longer distances between servers and storage.

• Access to the physical transport involves low-level protocols to ensure data integrity.

• You must use network addressing to uniquely identify each participant and to properly route data across the infrastructure.

• Large blocks of data must be segmented into packets or frames for orderly shipment through the network.

• Routers and switches route data packets between network segments.

• Networking facilitates movement of data whose source and destination are ultimately upper-level application interfaces.

The SCSI Architecture

• Applications view storage resources as abstract representations of the underlying physical devices.

• I/O requests are generated by applications, file systems, and operating systems.

• Logical storage identifiers (such as E: drive) are mapped to the SCSI addressing scheme of bus/target/LUN.

• The supporting SCSI infrastructure may also provide additional mapping between the conventional bus/target/LUN addressing and the address scheme required for that topology.

• The SCSI Architectural Model (SAM-2) separates SCSI commands from SCSI delivery protocols and underlying interconnections.

• SAM-2 defines SCSI relationships in a client/server model.

• Initiators are host systems that generate I/O requests.

• Targets are storage devices that process those requests.

• An application client resides in the initiator and is generated for every I/O operation.

• The device client resides in the target device and serves requests.

• Multiple instances of the application client may be running concurrently to support multiple outstanding I/Os.

• I/O is performed through a command/response sequence.

• SCSI commands and parameters are contained in the command descriptor block.

• The command descriptor block is encapsulated within the Fibre Channel Protocol information unit or iSCSI Protocol data unit.

• Multiple data delivery requests and acknowledgments can be exchanged within a single command/response pair.

• The efficiency of the SCSI protocol depends on the stability of the supporting interconnect infrastructure.

The Parallel SCSI Bus

• SCSI cabling provides parallel wires for simultaneous transfer of data bits.

• The maximum SCSI cabling distance is 25 meters.

• The maximum device population for SCSI cabling is 15 devices on a string.

• Skew refers to the window of time required to capture all data bits in a parallel transmission.

Network-Attached Storage

• NAS serves storage data as files; SANs serve storage data as blocks.

• NAS resources are shared using NFS or CIFS file transport protocols.

• NAS enables cross-platform access to common data.

• The NAS architecture is based on a thin server processor with attached storage resources.

• NAS products may use Fibre Channel or any SCSI interconnect as a back-end architecture for higher-speed access to disks.

Networking behind the Server

• SANs are a new type of network optimized for storage.

• SANs may use Fibre Channel, Gigabit Ethernet, or a combination of both.

• SANs enable new storage models such as server clustering and LAN-free tape backup.

• SANs require new management tools to supervise new storage relationships.