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Date:      Thu, 10 Nov 2011 09:19:39 -0700
From:      Warner Losh <imp@bsdimp.com>
To:        perryh@pluto.rain.com
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
Message-ID:  <00E98C7D-8683-4318-AFCA-5782EB17EE0E@bsdimp.com>
In-Reply-To: <4ebbf731.37OBtp%2BPJFFB1Hsl%perryh@pluto.rain.com>
References:  <201110281426.00013.jhb@freebsd.org> <4EB2C9DD.9090606@FreeBSD.org> <20111104160319.GD6110@elvis.mu.org> <201111080800.32717.jhb@freebsd.org> <CAGE5yCoTvYNXNc37V0%2Bt783uh=6J-_J-dt_Km_4xbO7O2O2BUw@mail.gmail.com> <6E287E90-AA62-4776-A09D-394D69C9494F@kientzle.com> <1B4CA8AC-8798-40CD-9379-FA0F379558DE@bsdimp.com> <4ebbf731.37OBtp%2BPJFFB1Hsl%perryh@pluto.rain.com>

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On Nov 10, 2011, at 9:09 AM, perryh@pluto.rain.com wrote:

> 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.
>>=20
>> 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.
>=20
> 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.

Seek(2) never makes it down to the driver for character devices.  =
Character devices have no position.

VMS did implement forward and reverse seeking on at least mag tapes.

Warner




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