How to safely remove rest of GTP?

[Ian Lepore]

On Fri, 2016-12-30 at 20:08 +0300, Andrey V. Elsukov wrote:
> On 30.12.2016 19:46, Slawa Olhovchenkov wrote:
> > 
> > On Fri, Dec 30, 2016 at 07:28:19PM +0300, Andrey V. Elsukov wrote:
> > 
> > > 
> > > On 30.12.2016 15:44, Slawa Olhovchenkov wrote:
> > > > 
> > > > How to safely remove rest of GPT?
> > > > Disk have actual data (part of ZFS), I am don't need to destroy
> > > > this
> > > > data.
> > > > 
> > > > GEOM: da6: the primary GPT table is corrupt or invalid.
> > > > GEOM: da6: using the secondary instead -- recovery strongly
> > > > advised.
> > > > GEOM: da22: the primary GPT table is corrupt or invalid.
> > > > GEOM: da22: using the secondary instead -- recovery strongly
> > > > advised.
> > > > GEOM: da6: the primary GPT table is corrupt or invalid.
> > > > GEOM: da6: using the secondary instead -- recovery strongly
> > > > advised.
> > > > GEOM: da22: the primary GPT table is corrupt or invalid.
> > > > GEOM: da22: using the secondary instead -- recovery strongly
> > > > advised.
> > > You may try attached patch, I did only basic tests, so first try
> > > somewhere :)
> > What purpose of this patch?
> In case when you have not valid primary header, `gpart destroy` will
> not 
> touch first two sectors. In you case you can wipe only last sector,
> like 
> Ian suggested, but use 'gpart destroy -F da6' instead of dd.
> 

If you gpart destroy -F da6 won't that also clear the valid MBR at the
start of the disk?  I think Slawa needs to preserve that.

I think it would be really useful if gpart recover could help with this
somehow.  There is a valid mbr, and a partially-valid gpt (only the
backup is valid).  So gpart recover should provide a way to recover the
gpt by refreshing the primary gpt and the pbmr, or recover the mbr by
wiping the backup gpt.  I'm not sure what the syntax would be to
specify that, though.

This didn't used to be a problem until the changes that made the backup
gpt get used automatically without any sysadmin intervention.

-- Ian