I often talk about disk-based backup and virtual tape libraries (VTL) and wanted to discuss physical tape. While VTLs are popular these days, tape is still in widespread use. LTO tape, the market share leader, continues to highlight increased density and performance. Do not be fooled with these claims. In the real world faster tape often provides little or no improvement in backup and/or restore performance. Ironically, faster tape increases (not decreases) the need for high performance disk devices like VTLs. Let me explain.
Modern tape drives use a linear technology where the tape head is stationary and the tape moves at high speed above it. Through each generation of LTO, the tape speed is largely unchanged while tape density doubles. At the same time, LTO drives have not expanded their ability to vary the speed of tape. Thus if you go from LTO-3 to LTO-4, you have doubled the density of your tape and you must double the throughput of data handled by the drive to keep tape speed unchanged. Why does tape speed matter? Because LTO tapes have a limited ability to throttle tape speeds, your performance will suffer terribly if you cannot meet the drives minimum streaming requirement.
If you are unable to stream enough data to your tape drives as mentioned above, the tape drive will go into a condition called “shoe shining” where it is constantly stopping and starting. It will try to stop when its buffer empties, but the tape is moving so fast that it will overshoot its stopping point and need to slowly stop, rewind to where it stopped writing and begin writing again. The tape moves forward and backward like shoe shine cloth. This process causes a massive reduction in performance and excessive wear on the tape drives and media. The table below comes from a Quantum whitepaper entitled “When to Choose LTO-3” and highlights the real world performance requirements of LTO-2 and LTO-3. I have estimated LTO-4 requirements for completeness.
Note how the minimum usable performance approximately doubles with each tape generation. As you upgrade tape technology, the challenge of streaming becomes greater and greater. Clearly, if you can stream LTO-3 there is no guarantee that you will be able to stream LTO-4. The ultimate irony is that upgrading to the latest tape technology often reduces not improves backup performance.
The only way to gain the benefit of the new technology is to thoroughly review and perhaps even re-architect your backup environment for the new drives. This creates a substantial hidden cost. Bringing in a disk-based backup target can often simplify the upgrade process.
Disk-based backup provide tremendous benefits in tape environments. They act as a variable speed backup target and will accept data at any rate you can send it. This frees you from the challenge in the tape world of maintaining a minimum throughput. Additionally, a VTL solution like the SEPATON S2100-ES2 provides very fast performance and can stream to LTO-4 drives and eliminate shoe shining if tape copies are required. You can often reduce the re-architecture costs associated with a tape upgrade by including a disk-based backup solution. Additionally, you can also reduce the amount of tape hardware required since the VTL will become the primary backup target of your nightly backups and you only need enough tape hardware to complete the tape copy process, if required, within your window.