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Re: linux readahead bug is back



D. Hugh Redelmeier wrote:
| From andre bwh harvard edu Fri Nov 23 00:33:13 2007

| My father just bought a F8 DVD, but it failed the mediacheck. It | turns out that the vendor failed to apply zero-padding to the ISO file
| before burning, and that the failure occurs about 40 KB before the end of
| the disc, presumably due to the readahead bug having returned.  This had
| been fixed when F7 was released, but apparently is back now in both the
| updated F7 and F8 kernels.  Anyone know when this regression happened, and
| if there are plans to fix it?

My theory, based on a few samples, is that the "readahead bug" was
never fixed.  It only manifests itself on some drives.  Older drives,
I think.

I'm pretty sure that it was fixed, or at least less likely to manifest. I was using the same computer, with the same DVD drive, when F7 came out, and found by going through a pile of old Fedora CDs that I burned without padding that all of them passed mediacheck anyway, though many of them failed earlier. Testing now with F8, I find that 3 out of 3 of them fail (I was convinced at that point and stopped checking). My father tested the F8 DVD while running F7 by reading the ISO image directly off the disk, and it failed at the end, which shouldn't have happened if the readahead bug had been fixed. So it exists also in the latest F7 kernels.
I've never had trouble actually installing from a disc+drive because
of the readahead bug.  (But a mediacheck will fail.)
I have had trouble, before learning how to pad, though that was with installing with CD sets. It's probably more likely to happen with multiple discs, since the error can happen at the end of any of the discs, instead of just the one DVD, plus the packages at the end of the one DVD are probably very unlikely to be chosen for install by the average user.
The only time that I've experienced the problem is when trying to
access the whole raw contents of the disk.  The only important reasons
to do so are for integrity checking or duplicating.  Perhaps you can do
those on another drive.

I had my father read off the DVD image to the hard drive, minus the last 40 KB, then use wget -c to finish the ISO file properly, which only takes a few seconds even on dialup. Now, if necessary, he can use your isopad script, which I gave him, to pad the file, then burn a good copy to DVD. I just don't think it's worth the trouble to risk a failed install when it's so easy to avoid the problem. But the disc vendors should apply the padding. Even if the bug was fixed in the latest kernels, it's still necessary to allow old ones to read the discs properly.

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