On 2006-07-05 09:18, Nicola Mauri wrote:

 > if some of your DLEs contain lots of data that is random
 > (compressed iso images, mp3 files, images, zip archives, ...)
 > they will expand when taped

So the advantage of using SW compression instead of HW compression is that we can selectively choose on which DLEs compression should be enabled and where it is useless (or even counter-productive). In situations where CPU utilization is not a concern, this may be the preferred approach, as we should expect a better tape space usage.

Is it correct?

Amanda works better when the numbers it has are accurate.
First Amanda makes some estimates how big the backups will be (level 0,
level N, and level N+1).  Then, together with the tape capacity value
that you give in the tapetype, it works out a plan.

When the tape drive does not do any compression, the tape capacity value
can be pretty accurate.  But when doing hardware compression with the
tape drive, you can only guess how much the data will compress.
It even depends on the data itself.

So Amanda could make a plan that is on some days way too pessimistic
(needing to delay some full dumps, while there is in reality no need to)
and on other days too optimistic (resulting in a tape full, and some
files left on holdingdisk to be flushed).  When writing a dumpfile to
tape, and Amanda bumps into end-of-tape, that complete file has to be
restarted on the next tape.  The incomplete part written at the end
of the tape is just wasted space and time.  Amanda would have tried
to avoid it (when using taperalgo largest-fit) if she knew, or had
more accurate values for tape capacity.  You cannot give these accurate
values when using hardware compression.


--
Paul Bijnens, xplanation Technology Services        Tel  +32 16 397.511
Technologielaan 21 bus 2, B-3001 Leuven, BELGIUM    Fax  +32 16 397.512
http://www.xplanation.com/          email:  Paul.Bijnens AT xplanation DOT com
***********************************************************************
* I think I've got the hang of it now:  exit, ^D, ^C, ^\, ^Z, ^Q, ^^, *
* F6, quit, ZZ, :q, :q!, M-Z, ^X^C, logoff, logout, close, bye, /bye, *
* stop, end, F3, ~., ^]c, +++ ATH, disconnect, halt,  abort,  hangup, *
* PF4, F20, ^X^X, :D::D, KJOB, F14-f-e, F8-e,  kill -1 $$,  shutdown, *
* init 0, kill -9 1, Alt-F4, Ctrl-Alt-Del, AltGr-NumLock, Stop-A, ... *
* ...  "Are you sure?"  ...   YES   ...   Phew ...   I'm out          *
***********************************************************************