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What's so great about tape?



 
 
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  #1  
Old September 3rd 03, 08:13 PM
Anton Rang
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Default What's so great about tape?

Eric Lee Green writes:
In article , Malcolm Weir ruminated:
Tape's failure modes tend to be less catastrophic than disk's. E.g.


That is not my experience. In general, when a section of tape becomes
unreadable, every bit of tape after that section is no longer
accessible.


I've never seen a drive which behaved that way. Are you sure it's not
the driver on your system refusing to skip past the bad block?

-- Anton
  #2  
Old September 3rd 03, 08:55 PM
Eric Lee Green
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In article , Boll Weevil ruminated:
First of all, nobody makes a robotic hard drive changer. We go through
about 500 tapes a day using 20 to 40 tape drives concurrently all
managed by automated robotic tape libraries. I can't imagine to trying


Let me get this straight. You back up 50 terabytes per day? Or are you
using older/smaller capacity technology, let's say DLT1, and backing up,
say, 10 terabytes per day?

10 terabytes per day = 3650 terabytes per year. You're saying that your
installation is pushing 3650 terabytes of data per year through your
systems? Or are you saying that, due to the inefficiencies of current
tape backup solutions (which operate upon a whole-file basis rather
than on a differential block basis), you need 3650 terabytes of tape storage
to store, say, 365 terabytes of changed data?

--
Eric Lee Green
Linux/Unix Software Engineer seeks employment
see http://badtux.org for resume


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  #3  
Old September 3rd 03, 09:25 PM
Rob Turk
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"Anton Rang" wrote in message
...
"Rob Turk" writes:
"Marcin Dobrucki" wrote in message
...
August 2003 issue of SysAdmin Magazine (www.sysadminmag.com) has an
article just for you. "Tapes: A Modern History, Trends", by Henry
Newman, p. 43

/Marcin


This article is so full of semi-technical nonsense that it ain't funny
anymore. I agree with the overall conclusion (tape is here to stay) but

this
guy has definitely not been doing his homework on helical scan

recording.
This stuff is directly copied out of 10-year old Quantum DLT sales

pitches.
Yuck...


I'm curious, what do you disagree with in the article? I don't know
enough about helical scan vs. linear to make a strong argument one
way or the other, but I haven't run into anyone with several hundreds
of terabytes stored on helical scan tapes yet.

Henry's company (www.instrumental.com) has set up a fair number of
multi-petabyte tape sites, so I tend to give them some trust.


The section that describes the 'differences' between linear and helical scan
is totally out of wack. The claim that 'a small defect on tape would create
data corruption in a full buffer' is so blatantly wrong, it's just sad this
shows up in such an article. The stuff about tape passes, tape longevity,
mechanical stability et all, this is the stuff that linear and helical
vendors would throw at each other 15 years ago. None of it is based on
facts, especially with most of today's implementations

In my opinion, helical scan and linear technology are just two different
ways of accomplishing the same goal; create sufficient head-to-tape speed to
get the desired transfer rate and control it well enough to allow the
desired data density. Neither is better per se, they can both be implemented
right and wrong.

Rob


  #4  
Old September 3rd 03, 10:00 PM
David Magda
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(Peter da Silva) writes:
[...]
With Amanda it's always one tape per drive per night, so there's no
other messing around necessary.


Bacula supposedly allows for multi-volume backups:

http://www.bacula.org/

YMMV.

--
David Magda dmagda at ee.ryerson.ca, http://www.magda.ca/
Because the innovator has for enemies all those who have done well under
the old conditions, and lukewarm defenders in those who may do well
under the new. -- Niccolo Machiavelli, _The Prince_, Chapter VI
  #5  
Old September 4th 03, 03:21 AM
Malcolm Weir
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On 2 Sep 2003 20:15:27 -0700, (idunno) wrote:

On Tue, 02 Sep 2003 13:17:01 -0700, Malcolm Weir
wrote:

Tape's failure modes tend to be less catastrophic than disk's. E.g.
there are many disk failures that result in the entire disk being
totally unusable, and all data on it lost. While that can (and does)
happen with tape, more often you get a situation where part of the
tape can be read, so only part of the data is lost.

-Keith



My experience with tape is very limited and very different.


[ Snip a Travan horror story ]

You've intermixed an implementation issue with an architectural one.
My statement about tapes and disks is true for basically all tapes and
all contemporary disks.

[ Snip ]

I watched a lot of kinds of removeable media and tape standards come
and go since that time. During that time I've also been less than
blown away by my sony DAT audio deck. All this time I've been
wondering what a reliable tape system is like and whether tape in
general would be altogether replaced by something different. Most of
the time I retire or RMA a disk when it is having problems even though
it can spin up. As long as it can spin, I can usually get everything
I want off it. I've also become quite handy at resurrecting badly
scratched CD's.


Yet the most prevalent home video technologies have been predominantly
tape based (VCRs, camcorders, etc.). These days recordable optical
media (CD|DVD-R|RW) is making an appearance, but as yet the
penetration is low.

So if you want to look at what a reliable tape system looks like, look
at a VCR. Granted, the data rate is low and the tolerance to data
loss is high, but it generally does what it's designed to do.

Malc.
  #6  
Old September 4th 03, 03:28 AM
Malcolm Weir
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On 3 Sep 2003 11:09:35 -0500, Eric Lee Green wrote:

In article , Malcolm Weir ruminated:
Tape's failure modes tend to be less catastrophic than disk's. E.g.


That is not my experience. In general, when a section of tape becomes
unreadable, every bit of tape after that section is no longer
accessible.


It is, *in general*, "accessible". Many drives lack a command along
the lines of "keep going until you see something you understand", but
that's because of (reasonable) issues involving "what if there's an
EOD in that bad spot"?

Still, what you've experienced shows the benefit of writing lots of
file marks. Classic Unix (and Unix-like) things (tar, etc.) want to
write *one* tape file, and if part of that file is corrupted, then
it's all lost. But if you write a filemark every (say) 100MB, then
the chances of being able to seek past the bad spot improve
dramatically.

Whereas with disks in a RAID-5 orientation, failure of a
disk is not a disaster.


With tapes in a RAID-x orientation, failure of a tape is not a
disaster....

Even with hard drives in a non-RAID
orientation, generally the system will "oops!" on the file whose
underlying blocks are no longer accessible, and you can move that file
off to a "dead zone" and recover the rest of the files to another hard
drive. Very few hard drives outright "die" nowdays.


That's not true. Motor failures are significant causes of disk
failure, and are catastrophic. Servo problems, likewise.

Malc.
  #7  
Old September 4th 03, 03:34 AM
Malcolm Weir
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On 03 Sep 2003 14:12:35 -0500, Anton Rang wrote:

This article is so full of semi-technical nonsense that it ain't funny
anymore. I agree with the overall conclusion (tape is here to stay) but this
guy has definitely not been doing his homework on helical scan recording.
This stuff is directly copied out of 10-year old Quantum DLT sales pitches.
Yuck...


I'm curious, what do you disagree with in the article? I don't know
enough about helical scan vs. linear to make a strong argument one
way or the other, but I haven't run into anyone with several hundreds
of terabytes stored on helical scan tapes yet.


TV stations have that, as do certain well-known superpower
governments...

Henry's company (www.instrumental.com) has set up a fair number of
multi-petabyte tape sites, so I tend to give them some trust.


Sure. But all that proves is that you can do that with technology X,
not that you cannot do it with technology Y.

-- Anton


Malc.
  #8  
Old September 4th 03, 04:38 AM
Paul Rubin
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Malcolm Weir writes:
Still, what you've experienced shows the benefit of writing lots of
file marks. Classic Unix (and Unix-like) things (tar, etc.) want to
write *one* tape file, and if part of that file is corrupted, then
it's all lost. But if you write a filemark every (say) 100MB, then
the chances of being able to seek past the bad spot improve
dramatically.


I've heard that if you write two consecutive EOF marks on a DDS or 8mm
tape, that's interpreted as end-of-tape and there's absolutely no way
to read past it. It's not like the old days of 9 track tape.
  #9  
Old September 4th 03, 04:50 AM
Peter da Silva
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In article ,
idunno wrote:
My experience with tape is very limited and very different. Around
1995 I got a Conner Travan drive that had a floppy interface (I used


Stop right there.

The Travan drives, particularly the floppy based ones, were foul beyond
belief. If DAT is the "jalopy" of tape drives, Travan is the "Barbie Ferrari"
of tape drives. It looks like a drive, and you can even kinds use it as one,
slowly, in small pieces. But it's little better than a toy by comparison
even with traditionally "poor" drives like (say) TU58.

I'm not kidding. The quality of the Travan drives really was that far
behind even inexpensive professional drives. If someone tried to sell a
car as badly made as the Travan tape drive they'd be put in jail, even in
Eastern Europe they'd have been put in jail, back in the depths of the
Warsaw Pact. They're that bad.

I haven't touched tape since.


No wonder. That's like being taken to a greasy spoon and getting salmonella
and assuming that's a typical American dining experience.

--
I've seen things you people can't imagine. Chimneysweeps on fire over the roofs
of London. I've watched kite-strings glitter in the sun at Hyde Park Gate. All
these things will be lost in time, like chalk-paintings in the rain. `-_-'
Time for your nap. | Peter da Silva | Har du kramat din varg, idag? 'U`
 




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