[Debburn-devel] Repeatable images

[Steve McIntyre]

On Tue, Feb 10, 2009 at 01:59:59PM -0800, G. D. Chase wrote:
>On Mon, Feb 09, 2009, Steve McIntyre <steve at einval.com> wrote:
>>
>> What exactly changes from one the next, then? I'd only expect things
>> like timestamps in the ISO header, and we have patches submitted to
>> allow you to specify those on the command line. If that's all, then we
>> can easily give you consistent images. :-)
>
>$ diff --text firstimage secondimage | strings
>makes it appear that there are time stamp differences, so I'm guessing
>your expectation is right on.
>
>$ mkdir mydir
>$ echo filecontents > mydir/myfile
>$ tar --create mydir > first.tar
>$ tar --create mydir > second.tar
>$ diff --brief --report-identical-files first.tar second.tar
>Files first.tar and second.tar are identical
>
>If tar is consistant, why isn't genisoimage?  I don't know a whole
>lot about the ISO 9660 format . . .  Why is there a timestamp?  Is it
>required (by the format)?  What ramifications would there be for
>leaving it out?  If one were to leave it out, would the space where
>it would have gone have to be padded with nulls, or is the format a
>tagged format instead of a fixed-position one?

ISO9660 specifies the four timestamps, as you've found below. The
default is for genisoimage (and most tools) to fill them in according
to current time. You can fill them all with zeroes if you wish, but
the expectation is that they will contain meaningful values. If you
want to know more, the ISO9660 is available for purchase (sigh!), or
you can cheat and look at ECMA 119 (the standard that ISO9660 is based
on) for free [1].

>I found the patches you referenced
>http://bugs.debian.org/cgi-bin/bugreport.cgi?bug=498151
>which mentiones four timestamps: creation_date, modification_date,
>expiration_date, and effective_date.  Could you point me to some
>documentation which describes these timestamps?
>
>Also, I just discovered isoinfo.  Does anyone think that it would be
>good to include these timestamps in the 'isoinfo -d' output?

Yup, sounds like it might be a useful thing to do. Patches welcome... :-)

[1] http://www.ecma-international.org/publications/standards/Ecma-119.htm

-- 
Steve McIntyre, Cambridge, UK.                                steve at einval.com
"I can't ever sleep on planes ... call it irrational if you like, but I'm
 afraid I'll miss my stop" -- Vivek Dasmohapatra