[dm-crypt] (More) Questions about LUKS / LVM

Arno Wagner arno at wagner.name
Tue Sep 20 16:14:31 CEST 2011


On Tue, Sep 20, 2011 at 03:13:24PM +0200, Milan Broz wrote:
> On 09/20/2011 01:47 PM, Arno Wagner wrote:
> > The encryption can be established as long as the header and
> > at least on ekeyslot are intact. If you cut the power just in the
> > microsecond while a keyslot is written you would damage that
> > keyslot. If it was your only one and you do not have a header backup,
> > then you would have total data loss. That is the only scenario
> > I can think of. In normal operation, the header is not written.
> 
> Yes, for open and normal activation/deactivation nothing is written
> to LUKS header.
> 
> And all keyslot operation are done through sync|direct io path
> (avoids cache) so it should hit hw immediately.
>
> > Keep in mind that LVM adds to the complexity when you have to do
> > data recovery when something went wrong. Other that that it sounds
> > like a good approach.
> 
> You can say that the same for MD.

Indeed. Especially with the incredible mess that MD superblock
positioning is. I only use superblock format 0.9 for that
reason. Then I at least know it is at the end and that the 
kernel can auto-detect. They should have let it stay 
there. That would have been massively better than the insanity 
of having 3 possible positions.
 
> Btw LVM has much better recovery abilities than other systems.
> Just people are not so familiar with them.
> 
> I tried to show it some time ago in some talk,
> you can check how easy is to recover complete disaster
> (slides are not perfect, missing most of the comments)
> http://mbroz.fedorapeople.org/talks/LinuxAlt2009_2/ 

Nice nonetheless!
 
> But use of lvm2 is completely optional.
> What is complex is incredible complicated lvm2 user interface (CLI),
> here I fully agree. But even for notebook, pvmove or online resize
> is useful sometimes.

But strictly not necessary. I usually test my backup and restore 
procedure when having to resize something. (Yes, I do 2 current 
backups and a very careful verify before.)

> >> (I keep daily backups of $HOME and of essential system settings, the
> >> rest can be reinstalled if needed, but I'd prefer not to have to spend a
> >> few days recovering everything if I had a hard reset or something like
> >> that.)
> > 
> > You will not damage the encrypted data in normal operation.
> > All the header-damages reported were done during installation,
> > repartitioning, moving partitions, etc. 
> 
> These days is my favorite to LUKS damage bad MD resync
> (usually mistake in partition change or MD metadata format change).
> (No idea why such problems come in batches to lists :)

Probably because default metadata format for mdadm is now 1.2
(places superblock at 4k from start). Personally I have never 
damaged anything with MD resync but I only use metadata 0.90. 

Seems to me the kernel folks are not happy woth the > 0.90 
formats either or autodetection would work for them.

No idea what I am going to to when I hit the 2TB size 
restriction on the 0.90 metadata format, but that will still
be a while. 

Arno
-- 
Arno Wagner, Dr. sc. techn., Dipl. Inform., CISSP -- Email: arno at wagner.name 
GnuPG:  ID: 1E25338F  FP: 0C30 5782 9D93 F785 E79C  0296 797F 6B50 1E25 338F
----
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If it's in the news, don't worry about it.  The very definition of 
"news" is "something that hardly ever happens." -- Bruce Schneier 


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