Symmetrix SAN Architecture

This document will be using the EMC symmetrix configuration. There are a number of EMC Symmetrix configurations but they all use the same architecture.

Front End Director
A channel director (front end director) is a card that connects a host to the symmetrix, each card can have upto four ports.

Cache
Symmetrix cache memory buffers I/O transfers between the director channels and the storage devices. The cache is divided up into regions to eliminate contension.

Back End Director
A disk director (back end director) transfers data from disk to cache. Each back-end director can have upto four interfaces (C,D,E and F). Each back-end director interface can handle seven SCSI ids (0-6)

Disk Devices
The disk devices that are attached to the back-end directors could be either SCSI or FC-AL.

Interconnect
The direct matrix interconnect is a matrix of high speed connections to all componentswith bandwidth up to 64Gb/s

SAN Components

The are many components to a SAN Architecture. A host can connect to a SAN via direct connection or via a SAN switch.

Host HBA	Host bus adaptor cards are used to access SAN storage systems
SAN Cables	There are many types of cables and connectors: Types: Multimode (<500m),>500m) and copper Connectors: ST, SC (1Gb), LC (2Gb)
SAN Switches	The primary function of a switch is to provide a physical connection and logical routing of data frames between the attached devices. Support multiple protocols: Fibre channel, iSCSI, FCIP, iFCP Type of switch: Workgroup, Directors
SAN Zoning	Zoning is used to partition a fibre channel switched fabric into subsets of logical devices. Each zone contains a set of members that are permitted to access each other. Members are HBA's, switch ports and SAN ports. Types of zoning: hard, soft and mixed
Zone set s	This is a group of zones that relate to one another, only one zone set can be active at any one time.
Storage arrays	Storage array is were all the disk devices are located.
Volume access control	This is also know as LUN masking. The storage array maintains a database that contains a map of the storage volumes and WWN's that are allowed to access it. The VCM database in a symmetrix would contain the LUN masking information.

SAN Login

The below table documents the various proccesses that occur when a fibre channel device is connected to a SAN

Information/process	FLOGI (fabric login)	PLOGI (port login)	PRLI (process login)
What is need ?	- Link initialization - Cable - HBA and driver - Switch Port	- FLOGI - Zoning - Persistent binding - Driver setting	- PLOGI - Device masking (target) - Device mapping (initiator) - Driver setting (initiator)
What information is passed	- WWN - S_ID - Protocol - Class - Zoning	- WWN - S_ID - ULP - Class - BB Credit	- LUN
Who does the communication ?	- N_port to F_port	- N_port to N_port	- ULP( scsi-3 to scsi-3)
where to find the information ?	Unix - syslog - switch utilites Windows - Event viewer - Switch viewer	Unix - Syslog - Driver Ulitities Windows - Driver utilities	Unix - Syslog - Host based volume management Windows - Driver Utilities - Host based volume management - Device Manager

If any one of the above were to fail then the host will not be allowed to access the disks on the SAN.

VCM Database

The Symmetrix Volume Configuration Management (VCM) database stores access configurations that are used to grant host access to logical devices in a Symmetrix storage array.

The VCM database resides on a special system resource logical device, referred to as the VCMDB device, on each Symmetrix storage array.

Information stored in the VCM database includes, but is not limited to:

• Host and storage World Wide Names
• SID Lock and Volume Visibility settings
• Native logical device data, such as the front-end directors and storage ports to which they are mapped

Masking operations performed on Symmetrix storage devices result in modifications to the VCM database in the Symmetrix array. The VCM database can be backed up, restored, initialized and activated. The Symmetrix SDM Agent must be running in order to perform VCM database operations (except deleting backup files).

Switches

There are three models of switchs M-series (Mcdata), B-series (Brocade) and the MDS-series (Cisco). Each of the switch offer a web interface and a CLI. The following tasks can be set on most switches:

• Configure network params
• Configure fabric params (BB Credit, R_A_TOV, E_D_TOV, switch PID format, Domain ID)
• Enable/Disable ports
• Configure port speeds
• Configure Zoning