If you do the arithmetic, you will see that continuous reading or writing
of 4K blocks at 4Gbits / sec is about 6 months.
If your disk is not that fast (likely), or if you turn the machine off
sometimes (horrors!) it will be at least several years, probably around
the lifetime of the disk, or 5 years.
However, eventually every disk WILL fail. So it is always necessary to
keep copies on two separate physical disks to be absolutely safe.
In real life, it more likely to lose data because of some system problem,
or a mistake, or something catastrophic like a lightning strike or
a power failure, so the whole system can be hosed. Dual copies of the
backups are safest.
It is best to use two different machines.
Incremental backups don't require all that much additional space for each
one, and of course they are much faster than full backups.
I think it is safe to say that you can go for months and months without
filling up a 1 or 2 TB disk. Then you can always do a full backup, or
merge the incrementals in order to delete old stuff.
Lex
On Thu, 30 Sep 2010, Matthias Johnson wrote:
I know several people must be involved in backups so I thought I might pose
this question. What backup media do you / would you use for a business? I
know there has been some that have stopped using tape media and now do
backups to disk but is this a smart approach. To quote the O'Reilly book PC
Hardware in a Nutshell by Robert Bruce Thompson and Barbara Fritchman
Thompson.
"Uncorrectable error rate is the minimum number of bits the drive is
expected to read successfully before encountering an unrecoverable error.
An unrecoverable error is one for which the drive cannot reconstruct the
data from ECC data. When that happens, the data is irretrievably lost. For
example, a typical tape drive has an unrecoverable error rate of 10-15.
That means that the drive on average reads 1015 bits (more than 100,000 GB)
before it encounters a read error that cannot be corrected using the ECC
data stored on the tape. For comparison, typical hard drives have an
uncorrectable error rate of 10-14 and typical CD writers 10-12. That means
in the course of reading the same amount of data the hard drive generates 10
times as many uncorrectable errors as the tape drive, and the CD writer
1,000 times as many."
http://books.google.com/books?id=kG8LcWfruOAC&pg=PA285&lpg=PA285&dq=tape+drive+vs+hard+drive+uncorrectable+error&source=bl&ots=xi35kbg1iG&sig=vyMd67nlcW7LuuFj7jQQpSqSmUs&hl=en&ei=FnukTKn5LIOB8galo_mEAg&sa=X&oi=book_result&ct=result&resnum=7&ved=0CDAQ6AEwBg#v=onepage&q&f=false
If tape is the best option, what tape drives and tape media have you found
to be the best?
--
Matthias A. Johnson
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