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Most backup architectures still contain a sizable tape infrastructure, but hitting the performance benchmarks for these drives becomes a challenge. Consider this: LTO-4 drives, at 120MB/sec native (up to 240MB/sec at 2:1 compression) are faster than a single gigabit Fibre Channel (FC) connection. While most SAN environments are moving toward 4Gb/sec or 8Gb/sec FC, many are still running 2Gb/sec, especially on older backup servers. In those environments, deploying LTO-4 drives will hurt performance unless special attention is paid to the fan-in ratio (the number of drives per HBA). To determine the appropriate fan-in ratio for your environment, it's critical to understand the speeds and feeds of the components. For instance, a 2Gb/sec HBA has a maximum throughput of 256MB/sec, which is equal to a single LTO-4 drive with compression. Based on that statistic, you would need a 2Gb/sec HBA for each LTO-4 drive you deploy. Even with 4Gb/sec HBAs, the fan-in ratio should not exceed 3:1 (drives:HBAs).
Many purchasing departments cringe when they see a server order that contains numerous 4Gb/sec HBAs just for tape plus another set of cards for disk connectivity. But if you want your new tape drives to function at their intended speeds, you must provide sufficient connectivity. The customer discussed earlier was planning to purchase 20 LTO-4 drives for its media servers, but hadn't planned to upgrade
| the server or the HBAs. As a result, the media servers would have had a fan-in ratio of 10:1 on 2Gb/sec ports. When data was written to tape, the media servers would be unable to keep the drives streaming, resulting in what's known as the shoeshine effect. By upgrading to 4Gb/sec and reducing the number of drives on each HBA, that customer was able to maximize the performance of the new drives.
Performance tuning is an art, but identifying infrastructure bottlenecks is more of a strict mathematical exercise once you know the important numbers. Understanding the source of existing and potential bottlenecks makes it easier to find and remove them. Sure, it may require an investment in hardware to remediate them, but improving the speed of backups in an IT environment that's growing and changing is critical to meet the needs of the business. Do the math, and start hitting your backup windows. Then you won't have to worry so much when your phone rings.
This was first published in September 2008