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Mysterious failures
It is completely routine for a tape that was backed up
and verified, to simply be unreadable.
Why does this happen? Who knows. Certainly not the tape hardware
or software vendors. Their answer is always "oh well, too bad so
sad." Or they suggest "running 2 tapes for important backup jobs."
Did you know that Veritas has recently introduced a feature that
lets you "clone" a tape? It is specifically designed to write 2
tapes for every job, to increase your chances of not getting one of
these Mystery Failures.
50 uses per tape.
Tape vendors claim that tapes can be used 100,000 times. Having
worked with tape backup, experience has never proven this to be
true. Customers are often recommended to rotate their tapes out
after a year. Assuming that each tape gets written to once a week,
this comes out to about 50 uses.
This figure of less than 50 uses applies to *any* tape
technology used in a real-world setting. Why is there this
tremendous disparity between what they claim and reality? A guess
might be that they are "proving" the 100,000 number in a sterile
room somewhere. They have perfect climate control and a robotic
device that runs the a 1KB backup job over and over. This must be
the case, because the newer tape types haven't even been around long
enough to have real world tested them 100,000 times. By
calculations, this would take about 2000 years of weekly backups.
If you live in an artificially created environment where tapes
last forever, then great, buy yourself a tape drive. If you live in
the real world, talk to any IT person who uses tape regularly and
this less than 50 number comes up time and again.
Sequential-access
Tape is sequential, as opposed to random access. If you want to
restore a single file from a tape, and that file is located
somewhere in the middle, the tape drive has to scan over the whole
tape in a linear fashion until it finds it. This makes restoring
files take much longer than a random access situation. Backup to a
hard drive and the device can pull a single file straight from a
particular spot very quickly.
One of the major problems with this scheme, is if a part of the
tape data becomes corrupt, then everything after it on the tape is
often inaccessible. On Hard Drives, a particular sector can go bad
without affecting the other sectors.
Sensitivity to dirt and other
contaminants.
Because tape must pass over a read head, this means that tape
cartridges are "open to the air". There is a mechanism on tape
cartridges that exposes the tape directly to the head when it is in
use. This means it is very easy for environmental contaminants to
get onto the tape or into the tape drive. Even a little bit of dust
can cause a tape to become unreadable.
Ever wonder why tape drives need constant cleaning? It's because
their basic design allows them to get dirty.
Hard drives are "hermetically sealed". This means that they are
airtight, there is no chance of dirt getting onto the platters and
screwing with your ability to read data from them. The interface
between a computer and a hard drive is purely electronic.
Browsing tape contents
With most removable media, it is a simple procedure to view them
in Explorer and check out what is on them. You can do this
regardless of the circumstances under which they were originally
written. You can't do this with tape media.
With tape, first you have to have the same software it was
written with. Then, if you need to search a stack of tapes for a
particular version of a particular file, you get to "catalog" each
tape. Cataloging can take as much as an hour per tape.
Portability issues
Not portability in the sense of carrying tapes to your car,
rather portability in the sense of your ability to take a tape and
read it on another tape drive. You can have problems reading a tape
on another drive, even if it is the exact same model, firmware rev,
etc.
This is because the alignment of the heads in your tape drive can
drift over time. You could wind up in a situation where tapes
written on your drive are only readable by that drive. Better hope
that drive doesn't go bad. |