Backup on DDS-4 tapes

[Alex Zbyslaw]

Ludo Koren wrote:

>It doesn't help either... The result is the same.
>  
>
Just to check I'm understanding your problem correctly -- you're 
expecting to write much more data to the tape than is actually being 
written.

If that's correct, then there's a couple things I can think of:

1) Your tape drive isn't doing hardware compression.  Check the manual 
and see if there are any dip switches you need to set.  (Make a note of 
how they're set before you change anything, so you can go back to what 
you had originally!).

When you say the result is the same, if it used exactly the same number 
of tapes (down to the decimal point) then that definitely suggests that 
your tape drive is not compressing.

2) The data you're writing to the tape is already mostly compressed, so 
you won't fit as much as you might if it were uncompressed data.

Also, the 40Gb per tape that you quote is, I think, the MAXIMUM amount 
of data the tape will take.  It's only 20Gb native.  40Gb is how much 
will fit at optimum compression, which you never get.

It's unlikely to be a FreeBSD problem because I regularly fit 6-7Gb on a 
DDS-2, which has a native size of 4Gb.  I use dump options like the ones 
in my last message.

--Alex