On Thu, 23 Jun 2005, Joshua Baker-LePain wrote:
Something else I heard on another mailing list: "However AIT-4 (unlike AIT-1 til -3) appears to write fill bytes onto the tape if it's not fed with data quickly enough, thus wasting lots of capacity." The person said they heard it somewhere and hadn't seen it confirmed. If true, though... yuck.
Well, maybe, maybe not. DLT8000 did that, too. Lots of people griped bitterly about "poor performance" with their DLT4000's and 7000's. Problem was usually that their 1-pass backup software kept starving the write buffer and the drives had to shoe-shine in order to deal with it. The DLT8000 had variable speed write, which meant the tape kept streaming and the data was laid down as fast as it came in, even if that was slower than what the tape could handle. I always had a good chuckle at conferences listening to vendors trying to explain this problem to all the folks griping about their expensive tape drives that would only write at a fraction of their advertised speed. In general the vendors would do everything they could to avoid pointing the blame at the expensive commercial backup software products because they usually sold that to the customers, too. The good news for amanda users is that when you stage your dumps to holding disk, you eliminate the most frequent cause of the data starvation problem, which is the backup program scouring the partition looking for which files to backup today. Once you have your data on the holding disk you're unlikely to starve the tape drive and it can stream the data at full speed onto the tape. If it can't, you have a h/w problem that is typically easy to fix. -Mitch
