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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. |
