AIT-3 Capacity

[DKS]

I have a Sony LIB-81/A3 library using 100GB AIT-3 tapes. I am using Retrospect 6.5. I cannot seem to get beyond 100GB on a tape. I have hardware compression enable, I was expecting to get over 100GB per tape. It seems to be acting like I am getting compression for AIT-2 tapes. On several pages concerning capacity I do not see any mention of AIT-3, just 1 and 2 I am using the correct tapes, and have a different set of tapes for each day of the week on a 2 week rotation. Any help would be appreciated.

[mcswgn]

This is one of those topics that keeps coming up. 100GB is the native capacity of the media. Whether you exceed that and by how much will depend on a couple of factors. First is the compression ratio you can achieve with your data. The media manufacturers claim a "compressed capacity" of either 200GB or 260GB for the same media! That's a compression ratio of 2x or 2.6x, but many files are already fully compressed and will be unable to be compressed at all. For example, .mp3, .mpg, .jpg files all use compression as part of their file structure. Attempting to further compress files of this type will actually increase their size slightly. Binary system files also tend to have very low compression ratios. However, I've seen compression ratios as high as 10x for ASCII postscript files. The typical Word document will be somewhere in between. So the overall compression ratio you actually see is not going to be the mythical ratio claimed by the manufacturer and will be highly dependent on your particular mix of files. Second, capacity of a tape drive is only optimal when data is streaming continuously to the drive. An AIT-3 drive takes data at a phenomenal rate. LaCie quotes transfer rates of I think 12MB/sec. That's 720MB/min. In order to optimize capacity you would need an *after* compression supply of data at this rate. If your mix of files is such that they are actually capable of being compressed at a rate of 2x, you would need a continuous supply of data at at a rate of 2x720 = 1440MB/min. If you don't meet that rate, then the drive buffers keeping running out of data and every time they do the drive will start writing nulls to the tape until more data arrives. This really messes with tape capacity! They do this because it is very bad for a tape to keep starting, stopping, re-seeking, starting again (called "polishing the heads"). It is much better to waste some tape capacity than to wear out both the media and drive prematurely.