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Subject: [Bug 212139] r298900 introduced a fatal failure case for >2TB disk
 size reporting bugs
Date: Thu, 25 Aug 2016 20:18:05 +0000
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https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=3D212139

--- Comment #1 from Peter Wemm <peter@FreeBSD.org> ---
I'm adding another thought experiment hack.

The problem is that we can't trust the bios media size reporting mixed with
readahead causing read-beyond-end-of-disk.

Might it be sufficient to simply de-fang the readahead when crossing 2TB
aliases of the end of disk?  That should restore behavior comparable to the=
 old
loader.

In other words, suppose a 3TB disk incorrectly reported as 1TB.  Instead of
assuming the disk is actually 1TB, instead we assume it could be 1TB, 3TB, =
5TB,
... for the purposes of preventing readahead.  This would prevent readahead=
 of
reading the last sector of a 3TB disk misreported as 1TB.

I've probably done the math wrong in the hack, but the idea should be clear
enough to show the idea.  It just truncates the end-of-disk math to cause i=
t to
calculate using aliases.

I think it would be sufficient, and would be compatible with old 10.x and
earlier behavior.  It should stop the 10.3 -> 11.0 -> brick upgrade path.

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