Storage Nitty-Gritty 2 of 5 – SAN

In direct attached storage (DAS), storage is server centric and the host owns the storage. The storage is fully dedicated to the server that owns it.

With DAS, storage is server-centric

Storage has evolved into information centric model. In this model, when a new server is deployed in the environment, storage is assigned from the same shared storage pool to the new server.

Network based centralized storage solution

A network-based storage solution is the centralized storage pool. No single host owns the entire storage pool. The storage solution consists of two categories based on the interface with the host: SAN (storage area network) and NAS (network attached storage). To a client OS on the host, SAN typically appears as a local disk, allowing block-level access from the client OS, and therefore is more suited for structured workload such as database storage. It operates on its own storage network independent of the host network. NAS on the other hand, typically appears as a file share to the client OS, identified by an IP address and path. This is because NAS operates on the same TCP/IP network where the hosts are operated on. The client has file level access to storage, therefore NAS is better for unstructured data such as video and medical images. It is very important to understand the difference between SAN and NAS.

SAN deployment consists of two categories based on the connection technology. FC SAN is based on Fibre Channel network; and IP SAN is based on Internet protocol (iSCSI, FCIP, FCoE).

Fibre Channel SAN

  • Cable types: MMF (multimode fibre, usually for short distance within data centre because of signal attenuation due to modal dispersion) and SMF (single mode fibre, carries a single ray of light, used for long-distance cable runs;
  • Connector: standard connector (SC), lucent connector (LC) and straight tip connector (ST);
  • Interconnect device
    • FC hub – for FC-AL implementation, but no longer in use
    • FC switch – directly route data from one physical port to another (more intelligent than hub)
    • Director – high end switches with redundant components to provide high availability
  • FC connectivity
    • point-to-point: two devices connected directly to each other;
    • arbitrated loop (FC-AL): devices are attached to a shared loop;
FC- AL (rarely used today)
  • switched fabric (FC-SW): uses switches that can switch data traffic between nodes directly through switch ports. Frames are routed between source and destination by the fabric
Fibre Channel Switched Fabric
  • Protocol: Fibre Channel Protocol (FCP): defines protocol stack (five layers, FC-0 through FC-4), addressing, identification (world wide name), frame, data structure, flow control, fabric services
Fibre Channel Protocol stack
FC frame
  • Topology: mesh topology and core-edge fabric topology
  • Block-level virtualization: aggregates block storage devices (LUNs) and enables provisioning of virtual storage volumes, independent of underlying physical storage. The virtualization layer maps the virtual volumes to the LUNs on the individual arrays. Block-level storage virtualization not only enables extending the storage volumes online; it consolidates heterogeneous storage arrays and enables transparent volume access. It also provides the advantage of non-disruptive data migration, where the virtualization layer handles the back-end migration of data, which enables the LUNs to remain online during migration.
Block-level virtualization (classic)
Federation of block storage across data centers (new generation)
  • Virtual SAN (VSAN, aka virtual fabric) – a logical fabric on an FC SAN, which enables communication among a group of nodes regardless of physical location in the fabric.

IP SAN

iSCI (one of the IP SAN protocols) – an IP based protocol that establishes and manages connections between host and storage over IP. iSCSI encapsulates SCSI commands and data into an IP packet and transport them using TCP/IP. It is relatively inexpensive and easy to implement so widespread in environments without FC SAN.

  • Topology
    • Native connectivity (without FC components)
    • Bridged connectivity (including FC components in the configuration)
    • Combined connectivity (most common because a storage array usually comes with both FC and iSCSI ports)
iSCSI topologies
  • Protocol
    • stack: SCSI is the command protocol that works at the application layer of OSI model; iSCSI is session-layer protocol that initiates a reliable session between devices that recognize SCSI commands and TCP/IP. The iSCSI session-layer interface is responsible for handling login, authentication, target discovery, and session management.
iSCSI protocol stack
  • iSCSI session and PDU encapsulation
PDU encapsulation
  • iSCSI discovery – an initiator must discover the location of its targets on the network and the names of the targets available to it before session establishment. Two types are SendTargets discovery and internet Storage Name Service
  • iSCSI names:
    • IQN, iSCSI Qualified Name such as iqn.2008-02.com.example:optional_string
    • EUI, extended unique identifier such as eui.0300732A32598D26
  • iSCSI command sequencing – A command sequence may generate multiple PDUs. A command sequence number (CmdSN) within an iSCSI session is used for numbering all initiator-to-target command PDUs belonging to the session. This number ensures that every command is delivered in the same order in which it is transmitted, regardless of the TCP connection that carries the command in the session.

FCIP (one of the IP SAN protocols) – transporting FC block data over the IP infrastructure.

  • protocol stack and packet encapsulation
FCIP protocol stack
FCIP encapsulation
  • Topology (FCIP gateway involved):
FCIP topology

FCoE (one of the IP SAN protocols) – consolidation of LAN and SAN traffic over a single physical interface infrastructure. FCoE helps organizations address the challenges of having multiple discrete network infrastructures.

  • CNA (converged network adapters) replaces both HBAs and NICs in the server and consolidates both the IP and FC traffic
  • Special requirement on cables and switches
  • protocol stack and encapsulation
FCoE field mapping
  • CEE (converged enhanced Ethernet, or lossless Ethernet) provides new specification to existing Ethernet standard that eliminates the lossy nature of Ethernet. This makes 10Gb Ethernet a viable storage networking option, similar to FC. It features the following functionalities as part of IEEE 802.1:
    • PFC (priority-based flow control)
    • ETS (enhanced transmission selection)
    • CN (congestion notification)

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