[mythtvnz] HPA - more information

[Mark van Dijk]

ok - I've done some more reading of HPA and know a little more about how it
works.  As usual, Wikipedia is pretty good:
    http://en.wikipedia.org/wiki/Host_protected_area

The OS is perfectly capable of detecting when HPA is in use - indeed the
dmesg log will indicate if HPA is enabled for any particular drive.  Also
[sudo hdparm -N /dev/sdX] will give details.

The problem with Gigabyte motherboards is that on startup, they apparently
look at the first disk initialised (not necessily the disk on the first
port) to see if a copy of the BIOS is saved on the end of the disk.  If
not, then the BIOS will enable HPA, steal the last few megabytes of the
disk, and overwrite it with a copy of the BIOS.  This is obviously bad and
could very easily corrupt any existing the filesystem on that disk.

Apparently newer Gigabyte motherboards have this feature disabled by
default.  As mentioned in my last post, looks like some older motherboards
had a bug where instead of reserving a few megabytes off the end of the
disk, it actually reserved a full 1TB.  There is no way to disable this
feature on older Gigabyte motherboards.  There are a large number of posts
and articles on the internet talking about corrupted filesystems and RAID
arrays caused by this bug.  General consensus seems to be to avoid older
Gigabyte motherboards completely.

I'm now wondering if the kernal version had nothing to do with the issue.
Maybe the system drive on my Myth box was just a bit slow starting up and
meant that the BIOS decided to use my 3TB drive for the BIOS backup?  I did
reboot several times while trying to fix the issue, but still...

Also, assuming that my motherboard did in fact write a BIOS copy to the end
of the disk - why didn't xfs_repair find any issues once I got my partition
back?
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