[CentOS] OT: Fortunate clueless dd chum - lvm recovery
jpyeron at pdinc.us
Fri Aug 14 15:38:44 UTC 2009
> -----Original Message-----
> From: centos-bounces at centos.org
> [mailto:centos-bounces at centos.org] On Behalf Of Ross Walker
> Sent: Friday, August 14, 2009 10:30
> To: CentOS mailing list
> Subject: Re: [CentOS] OT: Fortunate clueless dd chum - lvm recovery
> On Fri, Aug 14, 2009 at 5:00 AM, Chan Chung Hang
> Christopher<christopher.chan at bradbury.edu.hk> wrote:
> >> 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
> > 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
> > 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?
> Well if he dd'd 64 sectors instead of 63 then the first
> sector of the first partition is going to be zero'd too.
> Backup the data while the stale partition table is still in memory!
> A reboot will make it inaccessible.
> Try a:
> # sfdisk -d /dev/sdX >/root/sdX.save
ROTFLMAO, you should make sure, before copying and pasting that /root/sdX.save
does not exist.
> Look at it and see if it contains a valid partition table, if
> it does then do a:
> # sfdisk --no-reread /dev/sdX </root/sdX.save
> Question now is, was the first sector of partition 1 damaged (was it
> 63 or 64 sectors dd'd)?
> If so it will require a more tricky procedure to fix.
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> CentOS at centos.org
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