Re: [dm-crypt] WD20EARS (4KB sector size) lost Luks header after reboot

[Ma Begaj <derliebegott@gmail.com>]

2010/9/15 Arno Wagner <arno@wagner.name>:
> On Wed, Sep 15, 2010 at 01:02:25AM +0200, Ma Begaj wrote:
>> >> > This confirms that you did create the LUKS container at the
>> >> > right place, namely the LUKS header is at 64 x 512 B offset.
>> >>
>> >> 64x512 =32768
>> >> 32768 ?!= ?32256
>> >>
>> >> 512 bytes is partition table? should it be 32780?
>> >> I am probably missing something.
>> >
>> > No, you see what I missed. Your LUKS sector is at 63 sectors
>> > offset, not 64. However the partition starts at 64 and therefore
>> > the LUKS sector is not at its start.
>> >
>> > I don't know what went wrong here, but partitions 1-4 are special in
>> > that you can change them without any changes to the disk except
>> > in the first sector (for partitions >= a chained partition sector
>> > is writte to disk and that can kill a LUKS header and do other dmage).
>> >
>> > This smeans you can just delete the partiton with fdisk and create
>> > a new one, which will the start at sector 63 and should have
>> > your LUKS container in it. It will not be 4kB alingned, though.
>> > You should unplug and replug the disk after the partition creation.
>> >
>> > If this goes wrong, you still have the first 100MB as backup.
>>
>> ;-)). you are the man :).
>>
>> Working like a charm.  Deleted partition, created new starting at 63,
>> unplug/plug and luks partition is there. Thanks.
>>
>> I owe you at least a drink.
>
> You are welcome. Made my now day too ;-)
>
>> >> I don't know enough about partition tables and headers but it looks like
>> >> /dev/sdm1 is starting little bit too far and LUKS header stays before the
>> >> beginning of /dev/sdm1 (first partition).
>> >
>> > Indeed.
>> >
>> >> Only obvious reason (at lease for me) ?for this could be in this 4Kb
>> >> sector size.
>> >
>> > I think something has gone wrong when you did the alignment.
>> > Did you first create a not-4kB aligned partition and delete
>> > that again? If so the kernel would could remembered the partition
>> > but not that it got deleted and replaced (that needs a disk unplug).
>> > The reason is that the kernel sees partition changes only under some
>> > circumstances.
>>
>> Wow great assumption. I did exactly that. I was doing this remotely
>> (I was not able to unplug) and I forgot first time to set 4kB sector size.
>> I deleted partition (which already was luksFormated too) and recreated
>> it again but this time in 4kB format.
>
> Its easy to have the idea when you already messed that up yourself
> once ;-)

Well, I will not forget my experience either ;-).


>
>> Thanks a lot.
>>
>>
>> One more question because this is going pretty well now :) :
>>
>> Do you have an idea how could I recreate 4kB aligned partition
>> without deleting data from the disk? This is more facultative question,
>> because I know have my data.
>
> Difficult. You would need to shift the wole partitoon by
> one sector.
>
>> I thought about shrinking this partition, creating a second parition
>>  with 500gb to copy all data on this new partition, creating first paritition
>> in 4kB aligned format, copying all data back to this partition etc.
>
> That would work if you can shrink. At least my standard
> tool (Gnu parted) cannot do much with XFS.
>
> Best option would be if you could temporary store all the date
> somewhere else on the machine and repartition.
>
> The other option is to live with it, USB2 is very slow anyways,
> sector alignment may not even matter that much.

Well, I was "pre-thinking" (this word sounds nice ;-) ) and I bought a USB3
case for the HDD although my old server still only have USB2 ports.



>> That could maybe  even work if XFS would support shrinking. But it
>> does not as I can remember ;-). Could partition start be moved from
>> sector 63 to sector 64 and keep working filesystem (of couse at the same
>> time risking to destroy some of the data)? Or is this (if possible) would
>> destroy filesystem totally?
>
> Well, you could either use dd_rescue or write a program in C that
> moves the raw partition. From your fdisk -ul output, I conclude
> that there is no space left at the end of the disk, so you would
> have to move the start from sector 63 to sector 60. That
> should still work. Another option is to do away with partitioning
> and move the start from sector 63 to 0. Your filesystem is then
> on /dev/sdm.
>
> A possible commandline for dd_rescue would be something like this
> (moving the partition to the start of the disk)
>
>  dd_rescue -s 63 -S 0 -b /dev/sdm /dev/sdm
>
> Looks good, but better assume a 99% probability of complete data
> loss. Connection problems, read errors, all can kill the data.
> And this will take something like > 10 days for 2TB on USB2.

Good that I was not patient enough ;-). I backed up everything, deleted
and recreated partition again.


Thanks.

M.

p.s.: I should contact you for a drink when I visit which city? ;-)