[PATCH] fadvise(2) system call

[Peter Wemm]

On Fri, Nov 11, 2011 at 4:08 PM, Tim Kientzle <tim at 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 at wemm.org; peter at FreeBSD.org; peter at 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