[CentOS] OT: Fortunate clueless dd chum - lvm recovery

Ian Murray murrayie at yahoo.co.uk
Fri Aug 14 09:20:49 UTC 2009

----- Original Message ----
> From: Chan Chung Hang Christopher <christopher.chan at bradbury.edu.hk>
> To: CentOS mailing list <centos at centos.org>
> Sent: Friday, 14 August, 2009 10:00:41
> Subject: Re: [CentOS] OT: Fortunate clueless dd chum - lvm recovery
> >
> > First of all, I would dd a copy of the whole drive off to another drive, so 
> you can have a few goes at this.
> >
> > How do you know only those bits where lost?
> >  
> The dd command zeros the first 64 sectors, that is, the mbr and then the 
> next 63 sectors which would the bootsector of the first partition and 
> the next 62 sectors following that. The first partition on both disks 
> belong to the md device that is the basis for the physical volume for 
> the system. And if it had not, it would have belonged to the md device 
> for the /boot partition which is not a great loss. Default Redhat layout 
> this.
> The box is still live and I am glad he was not clueless enough to say 
> yes to the mkefs2 command he was following from whatever howto he had 
> been looking at. It looks like the lvm survived having the first 62 
> sectors being zeroed. Apparently lvm uses the first 255 sectors/blocks 
> for lvm config data. No alarms/warnings in the logs. Certainly no panics 
> otherwise I would not even be able to get in.
> I get to learn something new at his expense, (which is now just a scare) 
> nice successor eh? :-D
> Absolutely zero data loss. What can I say?
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I think if you can access the data at the moment, I would back it up and set the LVM stuff up and mbr up again, unless you are really sure you can fix the zero'd info. Either way, if the data is accessible, back it up now!


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