4692 storage networking pt1

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StorageNetworking, Part 1:SANs and Fibre Channel

an Storage eBook


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1

contents

This content was adapted from Internet.com'sEnterprise Networking Planet Web site and waswritten by Charlie Schluting.

Storage Networking, Part 1: SANs and Fibre Channel, an Internet.com Storage eBook.

2008, Jupitermedia Corp.

2Understanding SANsand Storage

4 Understanding Fibre Channel

6 Understanding the FibreChannel Protocol

9 Understanding FibreChannel Domains

11 Understanding FibreChannel Zones

4

2

6

9 11

Storage Networking, Part 1: SANs and Fibre Channel

[ ]


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Astorage network is any network that's designed totransport block-level storage protocols. Hosts(servers), disk arrays, tape libraries, and just about

anything else can connect to a SAN. Generally, one would

use a SAN switch to connect all devices, and then configurethe switch to allow friendly devices to pair up. The entireconcept is about flexibility: in a SAN environment you canmove storage between hosts, vir-tualize your storage at the SANlevel, and obtain a higher level ofredundancy than was ever possi-ble with direct-attached storage.

An FC-SAN, or Fibre ChannelSAN, is a SAN comprised ofthe Fibre Channel protocol.

Think of Fibre Channel (FC) asan Ethernet replacement. Infact, Fibre Channel can trans-port other protocols, like IP, butit's mostly used for transportingSCSI traffic. Don't worry aboutthe FC protocol itself for now;we'll cover that later.

A fairly new type of SAN is the IP-SAN: an IP networkthat's been designated as a storage network. Instead ofusing FC, an IP-SAN uses Ethernet with IP and TCP to

transport iSCSI data. There's nothing to stop you fromshipping iSCSI data over your existing network, but anIP-SAN typically means that you're using plumbing

dedicated for the storage packets. Operating systemsupport for the iSCSI protocol has been less than stel-lar, but the state of iSCSI is slowly improving.

Another term you'll frequently see thrown around isNAS. Network Attached Storage doesn't really haveanything to do with SANs it's just file servers. A NAS

device runs something likeLinux, and serves files usingNFS or CIFS over your existingIP network. Nothing fancy tosee here; move along.

There is one important take-away from the NAS world,however, and that's the differ-

ence between block-level stor-age protocols and file-levelprotocols. A block-level proto-col is SCSI or ATA, whereas fileprotocols can be anything fromNFS or CIFS to HTTP. Blockprotocols ship an entire disk

block at once, and it gets written to disk as a wholeblock. File-level protocols could ship one byte at atime, and depend on the lower-level block protocol toassemble the bytes into disk blocks.

Block-Level ProtocolsA protocol always defines a method by which twodevices communicate. Block storage protocols are no

2 Storage Networking, Part 1: SANs and Fibre Channel, an Internet.com Storage eBook. 2008, Jupitermedia Corp.

Storage Networking, Part 1: SANs and Fibre Channel[ ]

UnderstandingSANs and Storage

By Charlie Schluting

A protocol always defines a method by which two devices communicate. Block storageprotocols are no different: they define how storage interacts with storage controllers.

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different: they define how storage interacts with stor-age controllers. There are two main block protocolsused today: SCSI and ATA.

ATA operates in a bus topology, and allows for twodevices on each bus. Your IDE disk drive and CD ROMare, you guessed it, using the ATA protocol. There aremany different ATA standards, but we'll cover just theimportant ones here. ATA-2 was also known as EIDE, orenhanced IDE. It was the first of the ATA protocol weknow today. ATA-4 introducedATAPI, or the ATA PacketInterface, which allows for CD-ROM devices to speak SCSI-likeon the same bus as a regular

ATA device.

The neat thing about ATA is thatthe controllers are integrated.The only "traffic" sent over theATA bus is plain electrical sig-nals. The host operating systemis actually responsible for imple-menting the ATA protocol, insoftware. This means that ATA devices will never, everbe as fast as SCSI, because the CPU has to do so muchwork to just talk to these devices. As far as SANs areconcerned, ATA isn't that important. There are someATA-based devices that allow you to connect cheapdisks, but they translate operations into SCSI beforesending them out to the SAN.

SCSI, on the other hand, is very confusing. SCSI-1 andSCSI-2 devices were connected via a parallel interfaceto a bus that could support 8 or 16 devices, dependingon the bus width. Don't worry about the details unlessyou're unfortunate enough to have some older SCSIgear lying around.

SCSI-3 separated the device-specific commands into a

different category. The primary SCSI-3 command setincludes the standard commands that every SCSI-3device speaks, but the device-specific commands can

be anything. This opened up a whole new world forSCSI, and it has been used to support many strangeand wonderful new devices.

SCSI controllers normally contain a storage processor,and the commands are processed on-board so that thehost operating system doesn't become burdened to do

so, as with ATA. Such a SCSIcontroller is called a Host BusAdapter. In the SAN world, theFC card is always called an HBA.

The main thing to know aboutSCSI is that it operates in a pro-ducer/consumer manner. OneSCSI device (the initiator) will ini-tiate the communication withanother device, which is knownas the target. The roles can bereversed. Most people call this acommand/response protocol,

because the initiator sends a command to a target, andawaits a response, but not always. In asynchronousmode, the host (initiator) can simply blast the targetwith data until it's done. The SCSI bus, parallel innature, can only support a single communication at atime, so subsequent sessions must wait their turn. SAS,or Serial Attached SCSI, does away with this limitationby automatically switching back and forth.

SCSI is tremendously more complex, but that's the gistof it.

We need to understand SCSI to know how our storagenetwork is going to ship data. The SCSI protocol playsan enormous role in storage networking, so you mayeven want to look at it more in-depth. I



The main thing to know aboutSCSI is that it operates in a

producer/consumer manner.One SCSI device (the initiator)will initiate the communicationwith another device, which is

known as the target.


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As we dive deeper into SAN technology, it's FibreChannel's turn to be examined. Fibre Channel, orFC, is the underpinning of all SAN technologies

these days, as it won the protocol war roughly 25 years ago.

FC wouldn't be much use without something on top ofit, namely SCSI. FC is the low-leveltransport that ships data, but hostsare normally communicating viaSCSI as far as they're concerned.The hubs, switches, and HBAs in aSAN all speak FC, while the appli-cations that use SAN storage con-tinue to use familiar protocols, likeSCSI.

The idea behind FC was to createa high-throughput, low-latency,reliable, and scalable protocol.Ethernet wouldn't quite cut it forhighly available storage needs. FCcan currently operate at speeds upto 10Gb/s (10GFC) for uplinks,and 4Gb for standard host connections. FC also pro-vides small connectors. As silly as it sounds, SCSIcables become unruly after time, and small strands offiber are certainly easier to manage. The equipmentrequired to connect to a FC SAN (multiple HBAs for

each host, fiber, and switches) is extremely expensive,and was the main reason SAN technologies took solong to become widely adopted.

TopologiesIn reality, two different protocols, or topologies, makeup the FC protocol. FC supports all topologies, but the

behavior of the protocol changes depending on thetopology. The following three types of topologies aresupported:

PTP (point to point): normallyused for DAS configurations. FC-AL (FC Arbitrated Loop):Fabric Loop ports, or FL ports on aswitch, and NL_Ports (node loop)on an HBA, support loop opera-tions. FC-SW (FC Switched): the mode

when operating on a switchedSAN.

FC-AL operation has its share ofproblems, but sometimes a devicedoesn't support FC-SW operations,and there's no choice. A hub has

no choice but to operate in FC-AL mode, and thereforeattached hosts must do so as well. When a device joinsan FC-AL, or when there's any type of error or reset,the loop must reinitialize. All communication is tem-porarily halted during this process, so it can causeproblems for some applications. FC-AL is limited to 127nodes due to the addressing mechanism, in theory, butin reality closer to 20. FC-AL is mostly relegated to



UnderstandingFibre Channel

The idea behind FC was to create a high-throughput, low-latency, reliable, and scalableprotocol. Ethernet wouldn't quite cut it for highly available storage needs.

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niche uses now, including but not limited to internaldisk array communications and internal storage forhigh-end servers.

FC switches can be connected any way you please,since the FC protocol avoids the possibility of a loop bynature. Ethernet isn't so lucky. The addressing schemeused does impose a limit of 239 switches though. FCswitches use FSPF, a link-state protocol like OSPF in theIP world, to ensure loop-free and efficient connectivity.

FC networks are generally designed in one of twoways: either one big star, or one big star with edgeswitches hanging off it. These are commonly known as"core-only" and "core-edge" configurations. Normally

a SAN will contain two of these networks, and eachhost's HBA or storage device's controller will attach toeach. Keeping these networks separate isn't as neces-sary as it is with FC-AL topologies, but even with FC-SW setups it still provides complete isolation and assur-ance that a problem in one fabric won't impact theother. An FSPF recalculation, for example, could causea brief interruption in service.

PortsAs previously mentioned, there are different port typesin a SAN, and it can get confusing. Let's try to clear up

some of that terminology:

N_Port: Node Port; the node connection point;end points for FC traffic F_Port: Fabric Port; a switch-connected port, that isa "middle point" connection for two N_Ports NL_Port: Node Loop Port; connects to others viatheir NL_Ports, or to a switched fabric via a singleFL_Port; or NL_port to F_Port to F_Port to N_Port(through a switch) FL_Port: Fabric Loop Port; a shared point of entryinto a fabric for AL devices; example: NL_Port to

FL_Port to F_Port to N_Port E_Port: Expansion Port; used to connect multipleswitches together via ISL (inter-switch links)

G_Port: Generic Port; can switch between F_Portand E_Port operation depending on how it's con-nected

TE_Port: Trunked Expansion Port; link aggregationof multiple E_Ports for higher throughput

You'll generally only see F_Ports and FL_Ports whenlooking at a single SAN switch, and knowing the differ-ence helps. FL means that you're talking FC-AL, andthere's a device attached that is either a hub, some-thing that can't do anything but FC-AL, or somethingstrange. Ports will automatically configure themselvesas an FL_Port if the attached device is Loop-only, other-wise it will be an F_Port. It's also worth noting thatsome brands of FC switches don't allow you to have an

E_Port unless you pay a higher licensing fee. It's some-thing to think about if you ever plan to connect multi-ple switches together.

FC LayersFC has its own layers, so in fact, calling it "likeEthernet" isn't quite accurate, even if it helps for under-standing. They are:

FC-0: The interface to the physical media; cables,etc. FC-1: Transmission protocol or data-link layer,

encodes and decodes signals FC-2: Network Layer; the core of FC FC-3: Common services, like hunt groups FC-4: Everything! Protocol mapping for SCSI, iSCSI,FCP, IP, and others

The bulk of FC is really in FC-2. FC-PH refers to FC-0through FC-2, which are strangely dubbed "the physi-cal layers."

FC also supports its own naming and addressing mech-anism, which sheds light on the previously mentioned

limitations in FC-AL and FC-SW topologies. I




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2008 Hewlett-Packard Development Company, L.P. The information contained herein is subject to change without notice. *Source: Edison Group.

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Understanding the guts of the Fibre Channel (FC) pro-tocol itself, including the naming format andaddressing scheme, allows one to better understand

what's happening on a SAN. Quickly glancing at a problem

and knowing what's wrong requires thorough knowledge ofall the protocols involved. While it's possible to operate aSAN with only point-and-click GUIs and limited knowledge,it certainly isn't recommended. So let'slearn about the FC protocol.

To reiterate: Fibre Channel is not areplacement for SCSI; SCSI generallyrides on top of Fibre Channel. Nowthat we have that out of the way,let's get to work.

FC generally refers to the FC-PHYlayers: FC0-FC2, which were brieflydiscussed earlier. The term FCP,Fibre Channel Protocol, refers to theinterface protocol for SCSI, or theFC-4 mapping. We're talking aboutthe inner-workings of FC here, notFCP.

FC data units are called Frames. FCis mostly a layer 2 protocol, even though it has its ownlayers. The maximum size for a FC frame is 2148 bytes,

and the header FC frame itself is a bit strange, at leastwhen compared to Ethernet with IP and TCP. FC usesone frame format for many purposes, and at many lay-

ers. The function of the frame determines the format,which is strange and wonderful, compared to ournotions in the IP world.

FC frames begin with a start-of-frame (SOF) marker fol-lowed by the frame header, which will be described ina moment. The data, or FC content, comes next, fol-

lowed by an EOF. The reason for theencapsulation is so that FC can becarried over other protocols, such asTCP if desired.

The FC frame itself, the general for-mat that is, varies in size quite a bit.In Figure 1 (next page) you can seethe SOF and EOF markers we men-

tioned before. The strange part aboutFC headers is that they are word-ori-ented, and an FC word is 4 bytes. Upto 537 words are allowed, whichgives us our 2148-byte capacity.

The components of the header, withall the optional items listed, are:

SOF (1 word): The start of a frame. Frame Header (24 bytes): The header that specifieswhat protocol is being used, as well as the source

and destination address. Varies depending on theprotocol in question. Optional ESP Header (8 bytes): Provides encryp-



Understanding theFibre Channel Protocol

Jupiterimages

Understanding the guts of the Fibre Channel (FC) protocol itself, including the naming for-mat and addressing scheme, allows one to better understand what's happening on a SAN.
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tion; includes the SPI and ESP sequence number. Optional Network Header (16 bytes): So that youcan connect an FC-SAN to non-FC networks. Optional Association Header (32 bytes): Not usedby FCP, but can be used to identify processes withina node. Optional Device Header (up to 64 bytes): Not usedby FCP, and is application specific. Payload: The data, up to 2048 bytes. Optional Fill Bytes (variable): Used to ensure thevariable-length payload ends on a word boundary. Optional ESP Trailer (variable): Contains check val-

ues for ESP. CRC (4 bytes): A CRC of the header and FC datafields. End of Frame (4 bytes): Ends the frame, and sayswhether or not it's the last in a sequence.

The FC frame format includes FC-specific information,including the source and destination, among others.Hopefully it is clear now why FC is so flexible, whichalso explains why there's so many FC protocols avail-able to give you a headache.

The actual FC Header, depicted in Figure 2 includesthe following fields:

Routing Control (1 byte): The routing portion says ifthis is a data frame or a link-control frame (either anACK or a Link_Response), and the information por-tion indicates the type of data. Destination ID (3 bytes): The FC address of the des-tination. Class Specific Control/Priority (1 byte): Essentially,



Storage companies began rolling out their first 8

Gigabit per second Fibre Channel products during

the summer of 2007, but customers won't get their

hands on the devices until sometime this year, and

complete systems composed of host bus adapters

(HBAs), switches, and storage arrays will take a lot

longer than that.

One reason for the long lag is the rigorous process

imposed on new products before they reach users,

said Tam Dell'Oro, founder and president of

Dell'Oro Group. "The testing process typically takes

six months or more," she said. "It's lengthy and thor-

ough."

"This equipment has to be highly robust super,

super reliable," Dell'Oro explained, "and it has to be

able to operate with a bunch of other stuff."

As a result, adoption of new technology like 8Gbps

Fibre Channel can take years. For example, accord-

ing to Dell'Oro, switches and HBAs incorporatingthe technology's predecessor, 4Gbps, began falling

into users' hands in 2004, but it hasn't been until

this year that it has begun to dominate shipments of

new equipment. In 2007, 97 percent of Fibre

Channel switches and 80 percent of HBAs will use 4

Gbps technology, she said.

Storage arrays, she added, are usually slower than

other system components when it comes to falling

in line with an evolving Fibre Channel technology.

"We didn't see the first four-gig storage arrays come

to market until the end of 2006," she said, "and at

that time, Hewlett Packard, which is a pretty signifi-cant manufacturer of storage equipment, still did

not have a four-gig product out."

Historically, new generations of Fibre Channel tech-

nology have been shipped every three to four years.

"That's the cycle we're on again," observed Scott

McIntyre, vice president for software marketing at

Emulex, which announced several new 8Gbps prod-

ucts last summer, including a family of HBAs,

8-Gig Fibre Channel

Arrives ... SlowlyBy John P. Mello Jr.

Figure 1
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Quality of Service. Source ID (3 bytes): The FC address of the originat-

ing node. Type (1 byte): Indicates the next protocol (what's inthe Payload), unless R_CTL indicates a control frame. Frame Control (3 bytes): Various crazy FC options,such as sequencing information and what to do incase of a problem. Sequence ID (1 byte): A sequence number, just likeIP. Data Field Control (1 byte): Indicates the presenceof optional headers, and the size. Sequence Count (2 bytes): The number of framesthat have been transmitted in a sequence.

Originator Exchange ID (2 bytes): Assigned by theinitiator, used to group related sequences. Responder Exchange ID (2 bytes): Same as theOX_ID, but assigned by a target node. Parameter (4 bytes): Mostly used as a "relative off-set" in sequences, much like IP's offset.

Yes, it is confusing, and there's a lot of new terminolo-gy, compared to the IP world. We'll continue to referback to these headers as we continue, so hopefully thefields and their purposes will become second natureafter some real-world examples.

The next important concept to grasp is the way FCassigns names. Notice that the D_ID and S_ID fields inthe FC Frame Header only allow for 24 bits. Each HBAis assigned a WWN, and each port on it is assigned aPort WWN, or PWWN. These WWNs are 64-bits inlength, which are larger than the 24 bits in FC. TheANSI T11 Address Identifier Format says that the FCIDis made up of three parts, which are the Domain_ID,the Area_ID, and the Port_ID.


Storage Networking, Part 1: SANs and Fibre Channel[ ]8-Gig Fibre Channel continued

custom mezzanine cards for server blades, and an

embedded I/O controller. Emulex's main competitor,

QLogic, has also rolled out 8-gig components, and

Brocade has unveiled 8-gig blades for its 48000

Director.

McIntyre noted that the ramp up for 4Gbps was the

fastest in the history of Fibre Channel. "That indi-

cates that there's a strong and consistently growing

demand for I/O throughput," he said.

One of the drivers of that throughput hunger is the

spread of virtualization technology. "What we're see-

ing is very strong adoption of server virtualizationtechnologies by our enterprise customers,"

McIntyre said. "That means they're stacking up

more and more virtual machines and more and

more applications on a single server, and in many

cases driving them to larger servers to accommo-

date many more virtual machines, and that's obvi-

ously creating a higher demand for I/O throughput

on each server." I

FC networks are broken up into hierarchies, dynamical-ly. The Domain_ID is assigned to each switch when a

fabric comes online using a Domain_ID distributionprocess. Normally the Domain_ID is administrativelyconfigured. The Domain_ID, along with the Area_ID, asecond hierarchical level, are combined with a Port_ID(assigned by the switch) to identify each FC node in afabric. So the WWN doesn't really mean anything as faras SAN routing goes.

Domain_IDs are distributed by a Principal Switch, whichensures that everyone has the correct information. Inshort, an FCID will be completely random the first timea node connects, which is generally fine, unless an

administrator manually configures it. Some Domain_IDsare reserved for multicast and other purposes, but thedetails are a bit outside our scope here. Refer to theANSI T11 FC-SW-3 specification for more details. I

Figure 2
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a change of a Domain_ID results in everyone sendingan EFP frame with the updated information.

Configuring the Domain_ID is important, becausemerging fabrics can be disruptive if conflictingDomain_IDs are present. When you have a singleswitch, and want to extend the fabric by connecting thetwo together, everything goes fine unless they're bothDomain_ID 1, as some vendors set by default. Everynew switch that's brought online needs to be config-ured with a unique Domain_ID before connecting it tothe fabric.

Conflicting Domain_IDs frequently happen when usingVSANs. A VSAN is the same as a VLAN, but for FC net-

works. You can configure a VSAN-capable switch (usual-ly a Cisco) to segment ports into separate fabrics. Onenode connected to switch port 1 may be in fabric 322,while the node right next to it lives in fabric 4; twocompletely separate fabrics. Each fabric may have adomain 31, for example. For the most part, excludingsome fanciness implemented by a few vendors, there isno inter-fabric routing, so nodes in different fabricswon't be able to talk to each other. This is wonderful,but often times it's necessary to merge two fabricstogether.

Merging two fabrics is normally accomplished by con-necting multiple switches together. If a "core" switchalready had a link to two switches, and suddenlydecides to merge the fabrics by placing them in thesame VSAN, those switches better have uniqueDomain_IDs. If not, traffic will suddenly be spotty, sincethe FCIDs include the Domain_ID. Furthermore, eachPS in a domain runs its own name server containinginformation about N_Ports, and when receiving aframe, a switch will not know which way to send it if ithas conflicting information.

Just like VLANs, a VSAN can be used to implement

arbitrary boundaries, in ways that make administration

much more tolerable, compared to manually movingwiring. The Cisco VSAN technology is gaining wide-spread adoption since ANSI blessed its implementa-

tion, calling it "Virtual Fabrics." The neat thing about aVSAN is that it's more capable than the Ethernet'sVLANs.

The Virtual Fabric model takes virtualization to the nextlevel. It is possible to configure a zone server, so that allfabric-attached nodes know how to reach it. FC servic-es run on a switch, unlike the IP world where serviceslike DHCP and DNS normally run on a host. In a VSANenvironment, the switch actually runs each service mul-tiple times, once in each fabric.

Speaking of fabric services, there are a few well-knownFC addresses associated with SAN services. The brieflist is:

0xFF FF F5: Multicast server 0xFF FF F6: Clock Sync server 0xFF FF F7: KDC (key distribution) 0xFF FF F8: Alias server (for multicast, or huntgroups) 0xFF FF F9: QoS information 0xFF FF FA: Management server 0xFF FF FB: Time server 0xFF FF FC: Directory server 0xFF FF FD: Fabric Controller 0xFF FF FE: Fabric Login server

FC addresses (the FCID) aren't actually necessary forSCSI over FC operation. Unicast FC frames are sent toand from the WWN of the node, so the FC address isreally only needed in two cases: during link initializa-tion, or when sending IP over FC. When sending IPover FC, and IP address needs to be turned into theFCID. Very similar to the Ethernet world, ARP is used inFC land. Either "ARP over FC" or FARP, which are twodistinct protocols, is possible, depending on what thedevices support. And you wondered why FC has so

many interoperability issues? I


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