Freeing the LAN from backup traffic

Network backup is one of network management's greatest headaches. Frequently difficult to manage and tedious to implement, backup can choke the LAN, slowing down critical user applications.

Now a technique known as LAN-free backup - one of the first of a new breed of storage-area network (SAN) applications - promises to bring an end to LAN backup hassles through a combination of technological and topological advances.

The need for better backup

But having more data can be a double-edged sword, particularly when backing up that data involves travel across the production LAN, as it does in nearly all of today's business organizations.

For users, performing backup over the LAN can be excruciatingly disruptive - and slow. A daily backup can suddenly introduce gigabytes of data into the normal LAN traffic.

In a shared Fast Ethernet LAN, backup can cause momentary slowdowns; in a shared 10M bit/sec LAN, it can bring response times to a crawl, lowering productivity and raising frustration. To the network manager, it can mean an extra dose of effort, and perhaps even risk, to prevent backups from disrupting production work by scheduling them around the normal workday routines.

The solution to LAN backup problems may be found in the form of new topologies based on dedicated SAN architectures.

The new SANs - for which LAN-free backup represents a potential "killer app" - represent a departure from conventional LAN backup schemes, which employ storage devices that are directly attached via SCSI to the LAN's servers. In such systems, backup traffic must flood the LAN to get to the designated backup server or servers.

Now, rather than using SCSI, newer SANs employ Fibre Channel. They also use star-wired hubs in combination with storage management software that can mediate contention and synchronize data - activities necessary for moving backup data from multiple servers to multiple storage devices.

In a LAN-free backup architecture, a Fibre Channel adapter is installed in each server. These are connected via the server's PCI bus to the server's operating system and applications. And because Fibre Channel's transport-level protocol wraps easily around SCSI frames, the adapter appears to be an SCSI device.

The adapters are connected to a single Fibre Channel hub, running over fiber-optic cable or copper coaxial cable. Category 5 cable, the high-end twisted pair rated for Fast Ethernet and 155M bit/sec ATM, will be a future option.

Attached to the hub via Fibre Channel is some type of automated tape library. This machine typically includes a mechanism capable of feeding data to multiple tape drives and may be bundled with a front-end Fibre Channel controller. Existing SCSI-based tape drives can be used also through the addition of a Fibre Channel-to-SCSI bridge.

Storage management software running in the servers performs contention management by communicating with other servers via a control protocol to synchronize access to the tape library. The control protocol maintains a master index and uses data maps and time stamps to establish the server-to-hub connections. Currently, control protocols are specific to the software vendors. Eventually, the storage industry will likely standardize on one of the several protocols now in proposal status before the Storage Network Industry Association.

From the hub, a standard Fibre Channel protocol, Fibre Channel-Arbitrated Loop (FC-AL), functions similarly to token ring to ensure collision-free data transfers to the storage devices. The hub also contains an embedded SNMP agent for reporting to network management software.

With this architecture, the hub is able to multiplex connections between servers and storage devices at the 100M byte/sec transfer rates of Fibre Channel.

While the architecture is simple to describe, it is best implemented by vendors with understandings of - and partners in - the various component hardware and software technologies, from the Fibre Channel adapters and managed hubs to the server software.

It's a good idea to begin with simple installations, as described above. Over time, standards and technologies will mature, and a move to more complex configurations, such as switch-based architectures, will be more feasible.

Also over time, users can expect to find their SAN applications expanding beyond LAN-free backup. On the horizon will be other applications, from remote mirroring to clustering and disaster recovery, that are also best performed away from the production LAN.

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