Fedora Core 5 common issues and bugs
Rob van Nieuwkerk
robn at berrymount.nl
Wed Mar 29 23:50:17 UTC 2006
On Wed, 29 Mar 2006 14:26:26 -0500
Peter Jones <pjones at redhat.com> wrote:
> On Wed, 2006-03-29 at 19:51 +0200, Rob van Nieuwkerk wrote:
>
> > Sure. Or something like this:
> >
> > dd conv=idirect if=/dev/cdrom count=right_number | sha1sum
> >
> > And if the dd used on the FC ISO does not support the O_DIRECT feature,
> > just add it. Or write a completely trivial 10 line C program that
> > does the same.
>
> This will almost always get you the wrong result. At the very least,
> you need 'bs=2048 count="$(($(isosize /dev/cdrom) / 2048))"' with that,
> or you wind up doing md5sum on completely bogus data with some media.
>
> But as it turns out, that fails the exact same way that mediacheck does.
>
> > I really never understood why there never has been any trivial
> > work-around on the FC images for this very annoying problem.
>
> dd does read(2) just like mediacheck does. There isn't some spooky
> magic here. It fails in exactly the same ways for exactly the same
> reasons.
Hi Peter,
Did you miss the O_DIRECT part ?
A read(2) on an fd opened with O_DIRECT should never lead to any readahead
by the kernel. On the device level *only* the blocks requested by the
userspace read() are read. Nothing more.
greetings,
Rob van Nieuwkerk
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