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Cc: bruce@cran.org.uk, ed@80386.nl, jilles@stack.nl, tim@kientzle.com,
	alfred@freebsd.org, freebsd-arch@freebsd.org
Subject: Re: [PATCH] fadvise(2) system call
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Warner Losh <imp@bsdimp.com> wrote:
> On Nov 9, 2011, at 11:03 PM, Tim Kientzle wrote:
> > Anyone know how to properly request a "skip forward"
> > on tape drives?  That's one of the missing pieces.
>
> I thought that you couldn't seek(2) on tape drives.  You must
> read(2) the data.  At least for this application, since a tar file
> would be just one file on the tape.  If you want to see to the
> next file mark, you need to use an ioctl to get there, or read a
> lot.

AFAIK you can't seek(2) backward unless using something along the
lines of a DECtape (remember those?), but there's no reason in
principle why a forward seek(2) could not be implemented in the
driver -- even without any help from the hardware.  It would save
some DMA, interrupt, and context-switch traffic, but even when
searching for a filemark the drive won't move the tape any faster
than when reading.