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One of the best ways to generate a quick return on an investment in storage area network (SAN) technology is to implement LAN-free backup. While LAN-free backup solves a number of problems through brute force performance, you still need to sweat out the operational details. To get the most from your LAN-free backup SAN, it's important to understand all the performance capabilities and constraints and solve them in parallel. In this article, we look at the steps needed for a meaningful and realistic schedule for LAN-free backup.

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Model LAN-free backup schedule
It shows how schedules can be created for the model SAN - which could then be optimized achieve the desired performance, redundancy, scalability and cost goals.

  download excel spreadsheet

Building a SAN for LAN-free backup begins with knowing how much backup traffic each server generates. Of course, the amount typically varies according to the day of the week and number of free backup operations that are run. For example, backups running on weekends are usually full system backups; backups on other days are either incremental or differential. Analyzing several weeks' worth of backup logs will give you a reasonable handle on day-to-day backup workloads.

Tape device performance
The next step for a smooth LAN-free backup is knowing the maximum transfer rate of the system's tape devices. This is normally calculated as the device's streaming speed with a 2:1 compression ratio. The differences between streaming and non-streaming speeds can make a significant difference using helical scan tape technology. This isn't a problem for helical technologies with transfer rates on 6MB/s or less because the streaming speed is slower than the system's transfer rate. Higher speed helical tape technologies can only realize their performance potential if they are paired with systems having equal or better transfer rates. In general, linear tape technologies provide the most consistent performance for LAN-free backup and are recommended over helical scan technologies.

Data compression increases the transfer rate of a tape drive and its capacity. The performance boost from compression depends on the ability of the system to transfer data to the device fast enough. Unfortunately, system backup transfer rates are difficult to determine and depend on many variables, including I/O activities driven by other applications. The best backup performance is achieved by running cold backup when no other applications are running in the system.

For planning purposes, you should assume Intel-based systems (Windows and Linux) can support a maximum cold backup transfer rate of 10MB/s; non-Intel Unix systems can support cold backup transfer rates up to 25MB/s. This means that an Intel-based system probably can't take full advantage of compression on tape drives with non-compressed transfer rates of 10MB/s. When planning LAN-free backup for Intel systems, you shouldn't calculate backup data transfer rates of more than 10MB/s, even if the tape device can provide performance at twice that level. Remember, the actual backup transfer rates for Intel systems may be slower than 10MB/s and there's no good way to estimate until you test it on real hardware.

Before configuring the number of tape drives you need for a LAN-free backup SAN, you should test the backup transfer rates of your systems using a single tape drive connected to a SAN storage router or a high-speed SCSI bus, SCSI-2 or higher. When Intel and Unix InfiniBand systems are released, it's safe to assume the cold backup transfer rates will be much higher than tape, making it easier to predict maximum backup transfer rates.

Now it's time to consider the pros and cons of connecting the tape drives by Fibre Channel (FC) interfaces or directing the backup through storage routers. Native FC interfaces have obvious ease-of-installation advantages compared to connecting SCSI tape equipment through storage routers. However, the advantages of multiplexing backup data paths through fewer switch ports and the ability to control access to backup devices through LUN mapping and LUN masking in storage routers may outweigh the additional installation efforts.


How to model your SAN for LAN-free backup

Modeling backup on a storage area network (SAN) is an iterative process, involving the four layers of servers, switches, routers and backup devices. This diagram shows a rough starting point for a network with 50 servers; the numbers would be different for every environment. Refining the model requires specific analysis of how data flows would stress downstream components.

It's important to understand that the SAN model you create is a work-in-progress that will probably change as you start applying your backup schedules to the resources in the model. For instance, you may need to move some tape devices from one bus to another in a router or move them to another storage router. Likewise, you may decide to change the switch configuration in order to achieve more flexibility in backup paths.

There's nothing that says your SAN model needs to be a single multiswitch fabric SAN. The model you create may be several physical SANs, and can be different sizes. The model used in this article could be implemented as a single SAN or as many as eight SANs, depending on whether any of the switches are connected. In the model discussed here, it wasn't necessary to include interswitch links as a resource for scheduling because none of the potential interswitch links would carry backup traffic.

This was first published in September 2002

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