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? =A0That's one of the missing pieces. >> >> I thought that you couldn't seek(2) on tape drives. =A0You must read(2) = the data. =A0At least for this application, since a tar file would be just = one file on the tape. =A0If 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. =A0It 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. =A0But I do= n'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. --=20 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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