Re: Problems with dumps

On Sun, Aug 01, 2004 at 09:17:53PM -0300, Fábio Mendonça Albuquerque Cunha 
wrote:
> On Fri, 30 Jul 2004 23:24:21 -0500, Frank Smith <fsmith> escreveu:
> 
> > De: Frank Smith <fsmith>
> > Data: Fri, 30 Jul 2004 23:24:21 -0500
> > Para: Fábio Mendonça Albuquerque Cunha <fmacunha>,        amanda-users AT 
> > amanda DOT org
> > Assunto: Re: Problems with dumps
> > 
> > --On Friday, July 30, 2004 18:20:08 -0300 Fábio Mendonça Albuquerque Cunha 
> > <fmacunha> wrote:
> > 
> > > Hello guys,
> > > 
> > > I am trying to do a full backup of 03 filesystems (127 GB), and my tape 
> > > is an AIT 50/130 GB tape driver.
> > > 

You must differentiate between YOUR data size and the amount
of actual data that can be written to a tape.  Your vendor's
claim of 50/130 GB means the tape itself can hold about 50GB
worth of actual written data.  With compression, that may
represent, according to the vendor, 130GB of YOUR data.

If your data are HIGHLY compressible, the vendor MAY be right.

A lot of my data can not be compressed at all.  Others can be
compressed as little as 20% of the original size. Your data are
not my data so until you try it, you will not know how compressible
your data are.  The more random your data are (music, video,
encrypted, compiled programs, already compressed zip files, ...)
the less it will compress.

In fact, random data has a habit of "expanding" when passed
through some compression schemes.  Thus, your tape could hold
less than 50GB of YOUR data if compression expands it.  One
form of highly randomized data is already compressed data.
If you use amanda's gzip compression AND hardware compression
both, your tape may seem smaller than it really is.

This may be the explanation for the following situation.

> > >> NOTES:
> > >>   planner: tapecycle (5) <= runspercycle (7)
> > >>   planner: Adding new disk servarq.estatica.com.br:/projeto2.
> > >>   planner: Adding new disk servarq.estatica.com.br:/projeto3.
> > >>   planner: Adding new disk servarq.estatica.com.br:/projeto1.
> > >>   taper: tape fita03 kb 43281440 fm 2 writing file: No space left on
> > > device
> > 
> > Amanda wrote 43GB when it hit the end of tape.
> 
> I don't understand this ... How works AIT tape hardware compression ?
> > 


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Jon H. LaBadie                  jon AT jgcomp DOT com
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