SANs and Standards


The problems with SANs are that there are too many standards, and very often the standards aren't standard at all. Many "standards" are proprietary or are limited to a few vendors, confusing, and don't play well with others. Storage networking's vocabulary is a veritable alphabet soup.

The standards problemor, really, the lack of real standardshas been recognized as the single biggest issue facing the storage industry. This has led to the formation of industry organizations and working groups to address the problem. Notable among them are the Storage Networking Industry Association (www.snia.org) and the Fibre Channel Industry Association (FCIA; www.fibrechannel.org), but there are many others. These associations try to standardize the different technologies by sponsoring "plugfests," running conferences (Storage Networking World, for example), and seeing that the work of their members is published and publicized. (A plugfest is a meeting of vendors where their equipment, hardware, and software are connected together to check their interoperability.)

Table 12.1 lists some of the most prominent storage industry trade associations and standards committees. Their websites are great places to learn about current and future storage and server networking technologies.

Table 12.1. Storage and Server Trade Industry Organizations and Associations

Organization

Website

Purpose

AIT Forum

www.atitape.com

Sets the standard for software and hardware vendors using this enterprise tape format.

Blade Systems Alliance

www.bladesystems.org

Is a trade organization for small form factor blade servers.

Blue-ray Disk Association

www.blu-raydisk.com

Sets standards for this type of high-capacity optical disc drives. See also www.blu-ray.com/info/ as the bladesystems.org is a German-language site.

DAFS Collaborative

N/A

Creates the standards for the Direct Access File System.

DAT Manufacturers Group

www.datmgm.com

Sets the standard for DDS/DAT tape.

DVD Forum

www.dvdforum.org

Promotes standards for optical disc drives and discs.

Fibre Channel Industry Association

www.fibrechannel.org

Sets standards associated with Fibre Channel networking and components.

HDSA

N/A

Promotes multidrive devices, connectivity, and their relationship to multimedia applications.

IBTA

www.infinibandta.org

Sets the standards for the InfiniBand high-speed bus.

LTO

www.lto.org

Sets standards for a high-density tape format that was created by Seagate, IBM, and Hewlett-Packard and that is a competitor to DLT formats.

MMCA

www.mmca.org

Promotes open-standard small-format removable storage devices.

Open Group

www.opengroup.org

Sets standards for software over InfiniBand.

OSTA

www.osta.org

Promotes rewritable optical disc technology.

PCIMG

www.picmg.com

Promotes PCI products.

PCI Special Interest Group

www.pcisig.com

Is a PCI trade association.

RAID Advisory Board

www.raid-advisory.com

Promotes RAID and has a number of useful resources on its site.

RapidIO

www.rapidio.org

Is developing the standards for backplanes and other circuit boards that will operate beyond the 10Gbps standards.

Recordable DVD Council

www.rdvdc.org

Promotes DVD products that are standardized by the DVD Forum and is a Japanese website.

SATA-IO

www.sata-io.org

The Serial ATA International Organization promotes the SATA bus standard. This URL takes you to the scita.org website mentioned below.

SCSI Trade Assocation

www.scsita.org

Is responsible for SCSI standards.

Serial Attached SCSI Working Group

www.serialattachedscsi.com

Is defining a serial point-to-point direct attached interface based on the SCSI command set.

Serial ATA Working Group

www.serialata.org

Creates standards for SATA disk drives and SATA interfaces.

SNIA

www.storageperformance.org

Defines storage networking standards, promotes interoperability among vendors, and creates benchmarks.

USB

www.usb.org

Promotes the USB standard and products.

VMEbus International Trade Organization

www.vita.com

Promotes VMEbus (a fast interconnect standard for storage and clustering) and related PCI products.

XFPMSA.org

www.xfpmsa.org

Develops 10Gbps Fibre Channel and Ethernet connections.


To discuss SANs intelligently and make some sense about what standards are really important, we need a framework to place the various storage networking components in context. In an effort to bring all kinds of storage technologies into a unified theoretical framework, SNIA has developed a storage networking model that is similar in approach to the seven-layer ISO/OSI networking model. Figure 12.3 shows the SNIA shared-storage networking architectural model, which was created by Wayne Rickard, John Wilkes, David Black, and Harald Skadal, along with input from other members of SNIA during a period from 2000 to 2003.

Figure 12.3. The SNIA shared storage networking architectural model provides a context in which to discuss various networking devices and standards. Used by permission of SNIA (www.snia.org).


The SNIA storage model defines a set of architectural layers, functions, and services that are used to network storage. These layers define a set of interfaces: Some of those interfaces are contained within the storage devices themselves, some are conceptual interfaces, and some are actual network interfaces. By using this model, it is possible to describe where services are necessary, pinpoint areas where interoperability is a problem (typically at common interfaces), and describe the advantages and disadvantages of any particular storage networking approach. The SNIA model gives us a vocabulary we can use to compare different approaches and a framework we can use to define hardware and software. We can't use this model to describe whether any particular architecture or product is better than any otheronly that if you install a certain device, you will need certain other kinds of hardware and software to complete the solution.

In the SNIA model, the storage domain is the hardware container that stores your data. At the top of the container is organized information in the form of files or information stored in organized containers: fields and records with the associated metadata needed to make sense of what's stored in them. The file/record layer's function is to provide the means to access information and the logic necessary to package information for storage. The logic required involves taking small units of data and associating that data with a naming space allocation scheme in order to construct larger data structures. At the file/record level in the storage model is placed a data cache, which provides a means for data retrieval of commonly or recently used information. Caching data enhances a storage system's performance. Finally, this layer also requires the logic to determine whether the information has been correctly transmitted out of storage or back inwhich is often called a system's coherency.

The bottom layer of the model shows how information is stored in a storage network. At the lowest level, storage takes the form of blocks that contain data. Blocks have no high-level informational content but are merely locations contained within storage devices, such as hard drives, solid-state devices, tape, or optical discs. Each of these devices must have a way of organizing the blocks it contains (that is, organized pointers to each block). Block aggregation, or block address mapping, allows you to organize information in blocks without regard to where the blocks are located.

The term aggregation is synonymous with the concept of virtualization, where the definition of information is abstracted from the actual physical storage. You saw a number of examples of aggregation in Chapter 11, in the form of RAID. Virtualization also includes the concept of logical units (LUNs) and volumes, and the software you use to manage them, called volume managers.

See "Introduction to RAID," p. 600.


Because you can attain a performance advantage by caching commonly or recently used blocks, caching is also part of the block layer. On-board cache is the reason that modern enterprise disks ship with 8MB of memory or more.

The third part of the SNIA storage model is the access paths that you see as arrow-headed lines leading from the application layer to the storage devices. Each of the eight possible paths that can be defined represents a different method of data storage and retrieval.

If you examine Figure 12.3 closely, you see that there are four defined networks or interfaces:

  • Application/operating system This interface is composed of an API that connects the two layers.

  • Operating system/file layer This interface also consists of an API.

  • File layer/block layer The file/block layer is the storage network layer.

  • Block layer/storage device Blocks and their device storage involve a bus interface such as PCI.

When you hear the word interface, you should picture a duck dropping from the ceiling and saying the two magic words: trouble and opportunity. Interfaces are troublesome (and make IT folk grouchy) because they are where most of the interoperability issues crop up. Interfaces also offer an opportunity because when they are open standards, they allow you to pick different devices above and below them, offer horizontal scaling opportunities, and provide a measure of supplier independence. Thus, if you have storage software written on an industry-standard API, any device that can talk to that API can connect to your SAN at that point in the topology. Thus, it is a widespread trend in the industry to openly publish central APIs, even when (as is the case with EMC, for example) the API is highly proprietary because it allows other vendors to write to that API and extend its value. An important consideration in building any SAN, regardless of the size, is to consider these four interfaces as fundamental building blocks and choose accordingly.




Upgrading and Repairing Servers
Upgrading and Repairing Servers
ISBN: 078972815X
EAN: 2147483647
Year: 2006
Pages: 240

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