[Nottingham] Partition problem

[Martin]

Duncan John Fyfe wrote:
> On Sat, 1 May 2004, Martin wrote:
[...]
>>
>>And "LBA" answers Duncan's disk concerns from earlier. It overcomes any
>>worries about differing disk geometries should you want to do a "dd"
>>directly from one disk to another.
>>
> 
> 
> No it doesn't.
> 
> Pick up 2 nGB disks from different manufacturers and you will
> almost certainly find they are both ~ nGB but not quite the same.

Yes, very likely.


> Disk 1 may have 2000010 blocks while disk 2 has 2000000.  Thus if you dd 1->2 you
> won't have anywhere to put those last 10 blocks and you could lose data.
> Also you've just dd'd over a partition table which says the disk is bigger than it
> actually is.

It is always wise (healthyly paranoid) to do a fsck before and after 
transfer. I usuall also do a "diff -q /dev/src /dev/dst" to check 
against the unexpected.


> This is also a problem if you are replacing disks in a RAID array.

Very much so. But then, unless you are using JBOD mode, the disks 
ideally should be identical for consistent performance.


> You have to make sure the new disks is >= the size of the old disk or you're
> asking for trouble.  This is quite difficult as most manufacturers advertise
> disks the nearest large unit (GB, MB etc) not actual size.

This is usually the case when doing upgrades!


> By the way, when you dd'd between your disks did you do anything about the badblocks list ?
> Certainly ext[23] and reiserfs maintain internal lists for these and you will have dd'd them
> along with everything else. Those blocks may not be bad on the second disk, but others
> underlying your newly copied data might be....

Good note. Hopefully with these new disks, there were no spontaneous bad 
blocks between my running a surface check and then cloning. In the time 
available, a fair miniscule risk.


... All this could do with being put into yet another mini-howto! It 
seems an oft-asked topic. Another article for:
http://nottingham.lug.org.uk/new/
?


Regards,
Martin


-- 
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Martin Lomas
martin at ml1.co.uk
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