Kaushal Shriyan wrote:
I have a 40GB/80GB HP VS80 DLT Tape Drive. Now amanda is only able to recognize 40 GB only how do i make it enable to utilize the 80GB space on the tape. Is there any way out ???
40 Gbyte is the native capacity. 80 Gbyte is what the marketing department would like you to believe when you use hardware compression.
The nature of compression makes that some files compress very well, other almost nothing, and, depending on the algoritm used, some may even expand. Marketing likes you to believe that "on average" you can compress to 50% resulting in twice as much raw data on the tape.
That means it depends on _your_ data how much actually will fit on the tape. If you have an almost empty oracle database, then you may compress even to 80% or 90% (meaning you can put a few 100 GByte on such a tape). If on the other hand your disk is full of jpgs, gifs, mpegs mp3's, tar.gz-files etc, then most(*) tape drives with hardware compression enabled will actually expand your data (usually to about 120%), resulting in a tapecapacity around 36 Gbyte, instead of the expected 40 Gbyte.
The conclusion of this is that when using hardware compression, you don't know how much will actually fit on a tape. Amanda does not like this: she can't plan very well.
If you use software compression however, amanda pipes the data through gzip before writing to tape. This has two advantages: gzip does not suffer from the "expand already compressed data" syndrome, and amanda has an exact knowledge how much bytes to write on the tape. The added accuracy greatly improves the planning capacities of amanda. Amanda really likes to schedule using accurate numbers.
The report shows you how both amounts: before and after compression. From my daily backup:
|STATISTICS: | Total Full Daily | -------- -------- -------- |... |Output Size (meg) 30104.2 16097.0 14007.2 |Original Size (meg) 73802.4 32964.9 40837.5 |Avg Compressed Size (%) 40.8 48.8 34.3
Using software compression resulted in 73 Gbyte data compressing to 30 Gbyte. (I use AIT-1 tapes, 35 Gbyte native, actually amtapetype reports 33400 Mbyte: difference between marketing and reality).
Don't use software compression + hardware compression at the same time. The result is a reduced tapecapacity, as explained above, already compressed data will expand on tape. That's why "amtapetype" warns you about hardware compression.
(*) Really modern tapedrives -- actually, I know only one: the LTO-drives -- have an algorithm that expands data almost never.
Using such drives, you can mix hardware + software compression.
Do software compression to feed amanda with better statistics,
and do hardware compression only on those DLE's where you want
to spare CPU-cycles.
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