[libvirt] [PATCH v2 3/3] disk: Force clearing of the first 2048 bytes of device prior to mklabel

[Daniel P. Berrange]

On Fri, Apr 07, 2017 at 04:27:40PM +0200, Peter Krempa wrote:
> On Fri, Apr 07, 2017 at 09:52:09 -0400, John Ferlan wrote:
> > https://bugzilla.redhat.com/show_bug.cgi?id=1439132
> > 
> > During 'matrix' testing of all possible combinations I found that if
> > device is formated with "gpt" first, then an attempt is made to format
> > using "mac", a startup will fail.  By adding a clearing of the first
> > 2048 bytes of the device (similar to the logical pool code), the issue
> > is resolved.
> 
> I'm not a fan of this and neither thing this is robust enough:
> 
> The problem here is that apparently the "mac" table fits into the first
> block on the disk. Since the GPT disklabel is stored at LBA address 1
> it is not overwritten at all. Thus it's apparent that the detection tool
> then prefers GPT over a older disklabel.
> 
> The GPT disklabel has also a secondary copy at the last LBA of the disk.
> 
> As for robustness: SECTOR_SIZE is defined as 512 in our code, thus this
> code nukes 2k of data. For devices with 4k sectors which are already
> available this won't help at all, since LBA 0 will still contain the
> protective MBR, and the 3,5k left of LBA 0 will be unused. GPT disklabel
> still will start at LBA 1.

Personally I'd forget about sectors and just nuke 1 MB of data at start
and end of the device. The time for 2k vs 1 MB is negligible.



Regards,
Daniel
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