Fibre Channel (FC) is a transport protocol commonly used in storage networks. A common misunderstanding is that FC and fiber optic infrastructure such as host bus adaptor cards and fiber optic cables are the one-in-the-same. This is incorrect. FC is a protocol, like TCP/IP, and can be used over fiber optic cables or over copper. FC is commonly used to transport SCSI commands over fibre optic or copper cables in a Storage Area Network (SAN.)
A SAN is simply that: a storage network. A traditional LAN connects computers and devices via a switched infrastructure (typically over copper cables) and employs TCP/IP as a transport protocol for passing data between devices and services. In a SAN computers and storage devices are connected via copper or fiber optic cables and Fibre Channel is employed as a transport protocol for passing data between storage devices and computers.
Fibre Channel SAN Topologies :
The way that devices are connected to each other in a SAN is referred to as its topology. There are 3 topologies available in Fibre Channel:
* Point to Point
* Arbitrated Loop
* Switched Fabric
P2P: For example a workstation with an HBA in it is hooked by a fiber optic cable directly into a tape array.
Switched Fabric is the most complex and most common topology for fibre channel storage networks.
Common Fibre Channel, SCSI, and SAN Terms :
HBA : Host Bus Adaptor. An HBA can be likened to a NIC for a SAN. An HBA is a card with ports, typically fiber optic ports, that allow the system it is housed in to connect to
the SAN infrastructure. HBAs typically have multiple ports (2 or 4) to allow for multiple paths to the storage. Common HBA speeds are 2 gigabit, 4 gigabit, and 8 gigabit and the HBA speed must match the speed of the switch and fabric. Common HBA vendors are Qlogic and Emulex.
Each port on an HBA appears to the system as its own SCSI host. A 2 port card, for example, will result in the system seeing 2 SCSI hosts. You can view the information for each port under /sys/class/fc_transport/
FCoE : Fibre Channel over Ethernet. As fibre channel is a protocol, not a class of hardware, it can pass over any suitable medium. FCoE uses copper ethernet cables to transport FC data rather than fiber optical cables. FCoE cards are HBAs with ethernet ports as apposed to fiber optic ports. Just as with fibre optic HBAs FCoE ports appear to the system they are housed in as SCSI hosts. Though FCoE utilizes standard ethernet cabling FCoE switches and cards are still required to allow for operation as standard ethernet LAN equipment does not support SAN specific functionality such as zoning, masking, logins, etc
WWN : World Wide Name
WWID is a device identifier. WWIDs are the preferred device identifier to be used within the kernel.You can view the device identifier of a SCSI device by running "scsi_id -gus /block/sd
WWPN is a port identifier, WWPN for a port can be viewed at /sys/class/fc_host/host
WWNN : World Wide Node Name. The WWNN is a unique identifier, like a MAC address, for a "node" (read: card) on a SAN. A WWNN for an HBA can be viewed at /sys/class/fc_host/host
JBOD : Just a Bunch Of Disks. A JBOD is just that, a bunch of disks. A JBOD is an array of disks with no intelligence. While full featured arrays have the intelligence to control RAID levels, striping, redundancy, replication, and carving out logical storage units a JBOD may have few or none of those features.
SP : Storage Processor. An SP can be thought of as the "brain" of an array. It is the control unit that houses the intelligence that allows modern arrays to do advanced operations like LUN allocation, striping, redundancy, etc.
Target : A target is a SCSI concept, not a fibre channel concept. A target is a device that allows for incoming connections and storage access from initiators. The target side would be the array side in a SAN.
Initiator : An initiator is a SCSI concept, not a fibre channel concept. An initiator is a device that connects to a target to access storage on that target. The initiator side would be the HBA in a SAN.
LUN : Logical Unit. A LUN is a SCSI concept, not a fibre channel concept. A LUN is analogous to a Logical Volume in LVM. The array would be the Volume Group and the LUNs would be the Logical Volumes over that. A LUN is a logical storage device carved out of a larger storage pool of aggregated physical devices.
Path: A path is a single IO connection back to a LUN or storage pool. A path typically maps to a physical connection; however, zoning and mapping must be taken into account too. A path defines the route between the system and a device and consists of 4 numbers: H:B:T:L, host, bus, target, and lun.
SAN Issue Troubleshooting Tips :-
multipath -ll
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