Slow backups with c1537a

[Jan Klier]

Rob,

The one thing to understand when performance is involved is streaming of a tape drive.

Unfortunately this produces a rather dramatic effect. You would assume that the speed at which you feed data to how fast your backup is running is a linear function. For the most part it is, except for one big kink in the graph, and that is when the data feed drops below the rate at which the tape drive can continue to stream.

Tape drives write to the media while the media moves. And the media is typically moving at a constant speed (some higher end tape drives, such as LTO can use variable speed within a certain range to adjust). That means the system has to send the data tot he tape drive fast enough for the tape drive to continue writing at that speed. If the data come in slower, the drive has (what would be the CD-Rom equivalent of a buffer underrun) - except that instead of making a coaster (in CD-Rom cases) the tape stops the motor, backs up to where it left of, waits for sufficient data to come in and then resumes writing. However, this process of stopping, backing up, and resuming is very time consuming because of its mechanical nature. As a result the performance of a tape drive now suddenly drops dramatically.

Thus it is very important to be able to push data fast enough to a tape drive if you require the full performance.

And because of this inherent non-linear function, it is sometimes amazing what happens when you cross that threshold. So I wouldn't be surprised if your system is able to stream with the mail volume, but just falls short of the threshold with the 50,000 files, probably because of fragmentation or filesystem overhead.

There is a way to verify this on a Windows system (no, unfortunately these tools don't exist yet for NetWare, but the procedure can be used by booting the system in Windows or moving the tape drive to a Windows machine).

The diagnostics tool HP Library & Tape Tools can detect if your drive is not streaming from the drive logs. It calculates the ratio of start/stops vs. tape pulling hours. Connect the drive to a Windows system, download HP L&TT (http://www.hp.com/support/tapetools) and generate a support ticket. In the device analysis section it talks about these issues.