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