[SunHELP] 12/24 GB on a tape?

[Phil Stracchino]

On Mon, May 17, 2004 at 07:12:35PM -0700, ert weerr wrote:
> Hi Guruz,
> 
> I always wanted to know how I can write compressed
> data onto our tape and what the 12/24 GB means on
> the labels?

It means the raw, uncompressed capacity of the tape is 12GB (actually
12 billion bytes, about 11.17GB the way we count gigabytes), and its
capacity with the drive's hardware compression turned on is 24 billion
bytes, ASSUMING the data you're writing to it is such that you're able
to average 2:1 compression.

Note that you will seldom get 2:1 compression from real-world data,
unless your data contains predominantly English text or some other form
of data that compresses very well.  1.5:1 is a more realistic estimate,
and depending on your data you may get as little as 5% to 10%
compression or even none at all.  If your data is already compressed,
turning on hardware compression may actually reduce the amount of data
you can get onto the tape.

> So how can I write compressed data by ufsdump onto
> our tapes?

Turn on hardware compression on the tape drive.  How you do this will
depend on your hardware and your software.  You may be able to set it
with mt, you may have to use some proprietary utility to enable
compression in the drive's firmware, or you may have to physically set a
DIPswitch on the drive.

> In most cases the compressed storage of the data
> would solve that problem that I have to change
> the tapes because the complete system backup doesn't
> fit on only one tape.
> I suspect that the 24 GB doesn't mean that I can write
> this amount of tar.gz files on the tapes...

Indeed.  In fact, depending on the compression algorithm, you may get
more gzipped files onto the tape with hardware compression turned off.
Trying to recompress already-compressed data can cause the data volume
to grow by as much as 5%.


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