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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 should have mentioned that both I and my father are using a Sony DRU-120C DVD RW drive (reported by dmesg as a model Sony DW-G120A), both purchased about 11 months ago. So it's not just older drives.

I've never had trouble actually installing from a disc+drive because
of the readahead bug.  (But a mediacheck will fail.)

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.


Which raises the question - is there a way to find out which files/packages are physically located at the end of the ISO file, where the readahead bug prevents reading? It's probably only when one of those packages are chosen during install that a problem occurs - and this is much less likely with the DVD install only using a single disc, but knowing the specific packages would make it much easier to reproduce.

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