> I am using Retrospect 4.3 for Mac with an Aiwa TD-8001 tape drive. The drive
> has dual heads, apparently so that it can do read after write, eliminating
> the need for the whole second verification run after the backup.
> Does Retrospect support this feature? Read after write capability would
> dramatically cut down on the backup time. I can't seem to find any mention
> of it, though.
As far as I know all tape drives that are used for DATA storage use read
after write. They have to as the tape media, just like disk media, isn't
perfect and there will be errors. Due to the very different physical
designs of the media they handle bad bits in different ways. Tape drives
typically tag the trailer of the block if the RAW detects an error and
then writes it again. Drives used to and may still even allow one or
more recorded bit errors per block as one as the errors were well within
the recovery rate of the ECC coding. This is done for a single block up
to some error limit before the media is reported as bad. There used to
be (10 years ago) some fee/sw utilities floating around that would allow
you to access the extended SCSI data areas and see how many times this
had happened and other interesting information. Drives have gone so far
up in reliability over the last decade that most of these tools have
disappeared. At least from my radar screen. But this is the process that
would cause what looked like two identical tapes to record vastly
different amounts of data if one was certified for data and the other
for audio. The audio might have to write the data anywhere from 110% to
500% over original capacity to get good blocks.
Now back to your point. This RAW isn't an application verification. It
just makes sure the bits presented to the write head appear to get put
on the media. And you have to understand that the bits written onto the
tape are fairly far removed from the actually data that you, the user,
wants on the tape drive. There are lots of ECC, preamble, postables,
encodings, and other stuff inserted to make sure that data can later be
extracted from a flimsy piece of plastic ribbon moving at the speed of a
car. The Retrospect verification phase makes sure that the data is there
end to end. IE you can get back the original information. It protects
you from bad SCSI cards, cables, computer ram, tape controllers, etc...
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