Date: Fri, 11 Nov 2011 16:18:48 -0800 From: Peter Wemm <peter@wemm.org> To: Tim Kientzle <tim@kientzle.com> Cc: arch@freebsd.org, "arch@" <freebsd-arch@freebsd.org> Subject: Re: [PATCH] fadvise(2) system call Message-ID: <CAGE5yCp1OKeTFLgKDttJcAZ0Q_j9UsuJ=%2BLBzCmOruFDkQ%2BSnQ@mail.gmail.com> In-Reply-To: <4698F60B-CBF7-4D80-9368-CC6FBD893C0B@kientzle.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> <4698F60B-CBF7-4D80-9368-CC6FBD893C0B@kientzle.com>
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On Fri, Nov 11, 2011 at 4:08 PM, Tim Kientzle <tim@kientzle.com> wrote: > On Nov 9, 2011, at 10:46 PM, Warner Losh 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. > > Yes, seek(2) is badly broken on tape drives. It does nothing and doesn't return an error (unlike seek(2) on pipes, which does return an error). > > I've seen references to ioctls that are supported by some drivers and/or hardware that allow skipping forward by some number of records. But I don't know the details. > > I doubt it's a major performance optimization in most cases, but it would still be nice to avoid copying the data all the way down just to ignore it. > > Tim Honestly, I think we've got bigger problems to worry about than whether lseek() works on magnetic tape drives in a form that tar/libarchive can use. -- Peter Wemm - peter@wemm.org; peter@FreeBSD.org; peter@yahoo-inc.com; KI6FJV "All of this is for nothing if we don't go to the stars" - JMS/B5 "If Java had true garbage collection, most programs would delete themselves upon execution." -- Robert Sewell
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