Les Mikesell wrote:
Rodrigo Barbosa wrote:
On Thu, Mar 29, 2007 at 04:31:45AM -0000, Mark Schoonover wrote:
If you rely solely on your tape software verification to tell you
your tapes
are 100%, there will come a day, you'll be in for a surprise. The
only 100%
sure fire way to determine if your tapes are good is to actually
restore
from them back to a drive, and open them with the applications that
created
those files. Only after that kind of testing, can you be sure your
tapes are
good. I've had many tapes verified from the days of Arcserve, through
Brightstor, Arkeia, CTAR and BRU. All of these backup software
systems ran,
and verified flawlessly, then a disaster struck only to find out my
verified
tapes actually had problems with them.
And how is that any different than any other media, including disks ?
An online backup system that uses rsync with the --ignore-times option
(as, for example backuppc during full runs), will be reading your
existing files frequently and re-copying any mismatches detected with
the rsync algorithm. Also, the system may offer the option to archive
to some other media at convenient times which gives you another chance
at it. A problem I've seen with older tape drives was that the heads
would lose alignment so that that tapes would only work in the drive
that wrote them. You might verify a tape and send it offsite only to
find after a disaster that another drive would not read it. I don't
know if newer designs have eliminated this problem or not. If not,
you really need a 2nd drive to do the verify/read test.
>>You might verify a tape and send it offsite only to find after a
disaster that another drive would not read it
This is a very real problem, tape drives with larger drums and wider
tapes will suffer less wear and friction on the tape, and to that end
will suffer less from azimuth problems due to age.
DLT tapes or LTO tape drives will provide better reliability over 4mm
and 8mm formats in this area, However modern tape drives, LTO and DLT
included have auto azimuth feature in them to overcome these issue.
The acid test is as wisely spoken above is to do a restore. Online
media such as disks, generally don't have replaceable media, which means
on sealed disk drives, the head that wrote it is the one that reads it,
so in that respect it is very different to removable media such as
tapes. I bet everyone here has seen a DVD-RW or CDRW that reads and
writes ok in the optical drive that wrote it, only to have it choke in a
another drive, this is the same problem and its about wear and tolerences...
Regards
Pete
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