Hi Jon,

> LTO is unusual.  When hwc is enabled each input block is compared
> with and without compression.  The smaller of the two is recorded.
> As amtapetype feeds "random" data, data that is not compressible,
> the original input block is taped.  Thus you see the same results
> with or without hwc enabled.  And amtapetype can't tell whether
> hwc is enabled or disabled.

Ah, I see. So I don't have to worry about that anymore. Good.


> If some or all of your DLEs are not software compressed and you are
> using hwc, there is no single value that is accurate every day.  The
> reason is that different data compress different amounts.  Varying
> widely, like not at all to 90 percent.  As what you backup each
> day varies, the degree of hwc will vary too.

> So pick a value and watch the daily reports.  See how much data is
> actually written to your tapes.  You will soon get a feel for whether
> it is too low or high.  If you never get a tape overflow then how
> accurate your estimate is doesn't matter.  If you regularly get
> tape overflow the reports will tell you how much amanda was able
> to write up to the end of tape.  Compare several to approximate
> the tapelength.  Note, this will primarily affect amanda's estimate
> and planning phase.

Sure, I will have to find a "good" value for the scaling factor within
the first test runs.
I was just confused because of the examples at zmanda-Wiki that used
IDENTICAL values for the tape length in both cases and in that case HWC
wouldn't make that much sense at all imho.


That was really a good and helpful answer. Thanks a lot!

        Ralf