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2005 Jun 29 19:28

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Re: [vox-tech] [fwd] backup solutions for 3 people
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Re: [vox-tech] [fwd] backup solutions for 3 people



on Wed, Jun 29, 2005 at 09:32:41AM -0700, Micah J. Cowan (micah@cowan.name) wrote:
> On Wed, Jun 29, 2005 at 08:35:21AM -0700, Jonathan Stickel wrote:
> > Karsten M. Self wrote:
> > > 
> > > Tape.
> > > 
> > 
> > I've never really used tape drives, but my one experience was not good. 
> >   Someone else backed up data to a tape on a mid 90s unix machine, I 
> > think.  I needed the data about a year ago.  We were unable to access 
> > the data because we were clueless about how the tape was formatted, what 
> > software utility wrote to the tape, etc.  The computer that wrote to the 
> > tape was long gone.
> > 
> >  From this experience, it seemed that there is no standard when it comes 
> > to tape formatting, reading, and writing.  Has this changed?  If not, 
> > they don't seem that useful to me.
> 
> I have no experience with tape either, but I thought that's what tar &
> ar were for? AFAIK, no actual filesystem would be practical, so probably
> a tape's content consists of nothing more than a tar file.

Right.  Tape is a character (not block) device.  a/k/a sequential media.
You can't get to some point n+1 on the tape without advancing through n
first.  This is opposed to disk, where the time to reach any given point
is relatively fixed.   At worst it's seek + spin:  the time for the head
to seek to the right cylinder, and the time for the disk to spin to the
right sector.  For most current [S]ATA or SCSI drives, this is on the
order of microseconds.  For tape, it's a linear relationship to tape
position.

There are some tricks to speeding this up, including file markers and
the like, allowing you to scan quickly to a given file in a tape.
Backup systems can include a directory either on the tape or on disk
indicating where a given item is to be found.  But you've still got to
roll through the tape to get there.


Peace.

-- 
Karsten M. Self <kmself@ix.netcom.com>        http://kmself.home.netcom.com/
 What Part of "Gestalt" don't you understand?
    The pitcher will go to the well once too often.

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