> Wow, 5-6 hours?  I didn't realize tape was that slow.  I knew it was
> slow, but not that slow!  I'll try it again with the verbose option
> then, thanks.

Well you bring up an interesting point, which is worthy of discussion
on this list.

I have used a couple of dozen SCSI tape drives, including some really
fast ones (20MB/sec transfer rate to the tape), over the years.  I have
found that commercial backup programs like Veritas and others do some
real optimizations to the bytestream they feed to the tape drive,
to avoid the "shoeshining" on the drive caused by insufficient buffering.

On the other hand, tar just writes blocks of data, and it is up to
the drive to get them written to the tape.  There is no optimization
that's done, and in fact it's quite slow.  I think on my 20MB/sec
drive I got 1 MB/s or so using tar, IIRC.  There is plenty of shoe-
shining on the drive because nobody is optimizing it.

Does anyone have any ideas how tar can be used but still achieve native
drive speeds on the backup subsystem?  I've seen various schemes for
buffering the tar input and output using dd (through the system RAM)
but I've tried that and was underwhelmed by the speed increase.

Part of the optimization on commercial backup programs is that they
stream several backups to the tape at the same time.  This means
that the software has to keep a catalog of what's on the tape and
where it is - otherwise it can be problematic to restore.

I like the simplicity of tar but the speed is atrocious.  It is
completely unsuitable for any kind of short backup window, unless you
just don't have much data.

Ideas, anyone?

--
Jim Ockers (ockers@ockers.net)
Contact info: please see http://www.ockers.net/

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