using tar with DAT drives

Simon Clayton Simon at
Thu May 31 16:13:21 BST 2001

>> > I have a machine running 4.0 to which I have fitted a 12/24 DAT drive.
>> >
>> > Using tar, the drive is recognized and seems to work perfectly but
>> > I have recently found that despite only having around 3Gb of data on
>> > the system and trying to backup to a 12Gb tape - only some of the
>> > data is actually being archived.
>> Meaning what, exactly? Are you getting "end of tape detected"? Some other
>> error? Forgetting to back up all your partitions?

not forgetting to back it up - I did something like

	tar -c /etc /usr/home /usr/shared

which as far as I've read should backup everything to the standard tape
drive - which flashes lots and does list lots of files when I do

	tar -t > filelist.txt

but the file I am trying to get at seems to fall off the end of the tape,
yet when I do a "df" I see that only 3Gb of disk is used????

I also did one with -cv to watch the progress but that mysteriously seemed
to carry on showing loads of files being backed up that weren't really
on the tape.

I have to confess that I have only just started looking at this and haven't
been paying full attention to the whole thing yet so what I am saying might
be total bollocks (technical term) it's just that's how it appears to me
on first inspection.

>> > I have read the man page a number of times and been told that I can
>> > change block sizes or block counts but I don't actually know what
>> > the parameters should be.
>> It varies from drive to drive. Most DATs can handle 64K blocks, so you'll
>> want something like:
>> 	tar cbf 128 /dev/nrsa0 /data/to/backup
>> or:
>> 	dump a0bf 128 /dev/nrsa0 /partition/to/dump
>> Bigger block sizes means more of the tape is written with real data,
>> than being used for inter-block gaps, and the drive will generally eat
>> faster. The downside is that if you get an error on the tape, you'll lose
>> more data, but that usually isn't an issue. Also, I can't really see you
>> losing 9GB of tape even using small records, so it may be another
>> Ian.

Thanks - I'll try those.


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