Virtualization in the switch? Not so fast


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As virtualization spreads its tentacles to almost every part of the storage area network (SAN), a new generation of SAN switches is set to join the party and offer storage virtualization services as well. Here's how a switch with virtualization services plays in a multilayered virtualization scheme.

But before discussing switch virtualization applications, let's quickly review the basics of SAN switching in order to separate the new functions from the old. As you know, a SAN switch forwards storage I/O transmissions that arrive in one (ingress) port through a different (egress) port. The egress port is typically determined by the destination ID that's contained in the transmission's header information. Switches maintain tables indicating which ports can communicate with known destinations.

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Virtualization operator

The virtualization operator exports virtual devices to upstream I/O requestors.

Switches can forward transmissions between similar and different types of networks. For instance, a switch can forward transmissions from one Fibre Channel (FC) link to another FC link on the same network. Another type of switch may be able to forward transmissions from an FC link to a Gigabit Ethernet (GbE) link. Switching between different types of networks is typically referred to as routing, requiring the ability to correlate different addressing schemes within a single larger address system. This has been one of the hallmarks of IP networks for many years.

Some network technologies--such as FC--provide other services through switches. These services are made available to network clients through reserved addresses and provide consistent information to all clients helping maintain a reliable, secure network environment. The services provided by FC switches provide such things as fabric login, simple name management, time synchronization, state change notification services, SNMP management and traffic segregation (zoning). Generally, these FC switch services are communications services that don't provide storing or filing functionality.

Wiring, storing and filing
To understand storage applications in switches, it helps to have a clear distinction between the orthogonal functions of storage networking which can be broken down into three areas: wiring, storing and filing. Wiring is all of the software and hardware involved with low-level transport technologies, including bus and networking technologies. Wiring includes such things as switches, routers, routing algorithms and software, addressing schemes and QoS (quality of service) algorithms.

Storing is what many people refer to as block-level storage and includes device operations and control as well as most of the storage virtualization technologies such as RAID and volume management. Storage defines bulk storage addressing such as disk partitions, extents and volume address spaces, but doesn't determine or have an awareness of the data objects or their placement within storage volumes. Redundancy can be provided by storage functions through mirroring and RAID.

Filing is the function that determines how data objects are stored. This includes the ability to create, read, write, update and delete specific data files or objects. Filing uses the underlying wiring and storing technology to perform its work. The most common filing technologies are file systems, database systems--over raw partitions--and backup software.

Simple SAN configuration

A complete way to draw the I/O path is to identify all the filing, storage and wiring components.

The I/O path
The I/O path is another key concept that will help you better understand switch storage applications. The I/O path is the complete set of filing, storing and wiring entities existing between a software application and storage media where data is stored. The nature of the communications changes along the path, but the intent of the operation remains consistent.

For example, an application requests services from a filing entity using a file access interface. In turn, the filing entity generates a storing request acted on by one or more storing level functions. The storing function passes the request to a network function, which frames the requested information for transmission over the wiring (network) function. Usually, host bus adapters (HBAs)--also called initiators--have both storing and wiring functions implemented in hardware and software.

The wiring--or network--function transfers the information to an eventual target receiving node. There, the wiring information is removed and the data is passed up to a storing function. In the simplest cases, the storing function translates the request to a device where the data is read or written to storage media.

The I/O path can be analyzed as the network connections that exist between initiators and targets. However, doing so leaves out important storage and filing components from the overall picture. A more complete way to draw the I/O path is to identify all the filing, storage and wiring components in each of the systems, subsystems and network devices (see "Simple SAN configuration").

There are a couple things worth noticing in the "Simple SAN configuration" figure. First, there are two storing functions in the server system. One of them is a volume manager, and the other is a device driver for the HBA. Secondly, unlike data networks, the functional stacks for SAN communications aren't symmetrical.

As mentioned, the next generation of SAN switches will attempt to provide both storing and filing functions as part of a switch. We'll look at how these functions are likely to be integrated in SAN switches.

This was first published in February 2003

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