[SunHELP] 12/24 GB on a tape?

Peter Stokes peter at ashlyn.co.uk
Tue May 18 03:27:21 CDT 2004


On Tue, 2004-05-18 at 08:06, Phil Stracchino wrote:
> 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?

As far as Sun is concerned with Sparc Solaris (no idea on X86 version).
There a whole bunch of devices you can use to write to the tape drive.
They are all in /dev/rmt and your tape drive will probably start with a
'0'. The different devices control how the driver will talk to the tape
drive. For this to work correctly you will need to have either a fully
supported Sun tape drive or a correct entry in /kernel/drv/st.conf. See
man st for the full info. So to use compression select 

/dev/rmt/0c  - rewind device
/dev/rmt/0cn - no rewind device


> 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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