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3.7 Chapter Summary

Storage Networking Terminology

  • RAID provides performance and data redundancy.

  • RAID 0 stripes data blocks over multiple disks.

  • RAID 1 provides data duplication (mirroring) between two disks.

  • RAID 5 provides striping of data blocks and distributed parity for data reconstruction in the event of failure.

  • RAID levels may be combined—for example, RAID 0 + 1.

  • JBODs are more economical than RAID but have no inherent redundancy or mirroring capability.

  • To maximize performance, software RAID on host systems can be applied against JBOD targets.

  • Tape subsystems may be SCSI, Fibre Channel, or Gigabit Ethernet interfaces.

  • Bridge products can bring legacy SCSI devices into an IP-based SAN.

  • SAN host adapters may include Fibre Channel HBAs and IP storage NICs.

  • IP storage NICs may provide optimized logic for TCP off-loading.

Legacy SCSI Cabling

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

  • The maximal SCSI cabling is 25 m.

  • The maximal 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 files; SANs provide data blocks.

  • The NAS architecture is comprised of a thin server and attached storage.

  • NAS storage may be ATA, SCSI, or Fibre Channel.

  • NAS uses NFS or CIFS over IP for file access.

  • NAS products are typically marketed as appliances, requiring little configuration or management.

  • NAS and IP-based SAN traffic may share a common network infrastructure.

Fibre Channel

  • Fibre Channel is a standards-based, layered architecture.

  • FC-0 defines gigabit physical layer specifications.

  • FC-1 provides data encoding and link-level controls.

  • FC-2 defines segmentation and reassembly of data via frames, flow control, and classes of service.

  • FC-3 is being developed for common services such as encryption.

  • FC-4 is the upper layer protocol interface between Fibre Channel and IP, SCSI-3, and other protocols.

  • The most commonly used FC-4 protocol is FCP for serial SCSI-3.

  • FC-0 and FC-1 provide the foundation layers for Gigabit Ethernet.

  • The 8b/10b data encoding algorithm coverts 8-bit bytes into 10-bit characters.

  • Encoding is required to prevent sustained DC states on the gigabit link.

  • A proportional representation of ones and zeros is maintained via running disparity.

  • Ordered sets are 10-bit characters used for frame delimitation, signaling, and link change notification.

  • The maximal payload for a Fibre Channel frame is 2,112 bytes.

  • The most commonly used class of service for Fibre Channel is class 3, which is connectionless and requires no acknowledgment of frame receipt.

  • Fibre Channel has no standardized encryption or authentication methods.

  • FCP is responsible for mapping SCSI devices at the operating system level to Fibre Channel-attached storage resources.

  • Fibre Channel topologies include point to point, arbitrated loop, and fabric.

  • Fibre Channel fabrics use a subset of OSPF called FSPF for fabric routing.

  • The Fibre Channel fabric appears as one integral network.

  • Interoperability and management issues for fabric switches have retarded the deployment of Fibre Channel SANs.

Gigabit Ethernet

  • Gigabit Ethernet is a data link transport that borrows from both Fibre Channel and conventional 802.3 Ethernet.

  • Ethernet framing is used to transport TCP/IP data over Gigabit Ethernet networks.

  • 802.1Q VLAN tagging allows segregation of devices on the SAN.

  • 802.1p/Q frame prioritization enables mission-critical traffic to be assigned one of eight levels of priority for SAN transport.

  • 802.3x flow control provides reliable transport of storage data over connectionless protocols such as UDP/IP.

  • 802.3ad link aggregation allows scalability of IP-based SANs with no loss in performance.

  • Gigabit Ethernet's transmission rate of 1.25 Gbps provides slightly better performance than Fibre Channel.

  • Gigabit Ethernet cabling includes category 5 unshielded twisted pair copper cabling as well as standard multimode and single-mode fiber-optic cabling.

Assumptions for IP-Based SANs

  • Storage networks based on IP and Gigabit Ethernet can leverage new functionality for class of service, VLANs, flow control, and trunking provided by Ethernet standards.

  • An optimal storage over IP solution accommodates legacy SCSI, Fibre Channel, and native IP storage devices.

  • Management of storage networks requires the integration of transport management and storage management.

  • Interoperability is a key driver for market adoption.

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