[dm-crypt] some questions and FAQ suggestions

Christoph Anton Mitterer calestyo at scientia.net
Mon Jul 8 22:41:27 CEST 2013


On Mon, 2013-07-08 at 19:48 +0200, Arno Wagner wrote:
> My intuition is that performance questions are too volatile 
> to put into the FAQ.
Well AFAICS that would apply only to Q3 then... and well... I didn't
mean to give any real world values but rather telling people about some
caveats like that placing MD above dmcrypt is not so good for RAID 4,5,6
(Neil wrote over at linux-raid, that the issues described by Milan are
no longer true for levels 1 and 10).
I mean the situation is now that all these legacy rumors and information
is floating around in dozens of howtos... so either people have no idea
what they're doing (and thus suffer performance)... or we should tell
em.


> And to dependent on the actual details
> of the target system, i.e. most people will actually have to
> benchmark on their own set-up to find out what _they_ get.
Sure but that doesn't apply to general principles or stuff.



> One thing is for sure, more layers do not make things better.
> Hence I do not have any LVM set-up. A second problem with LVM
> is that it complicates things vastly in case something goes 
> wrong and you need to do data-revocery. KISS applies. 
> (Personally, I think LVM is a complicated, intransparent 
>  monster that adds complexity where it is rarely needed...)
Well the only alternative (for the scenarios in which one wants to use
LVM) would be to create partitions on top of dmcrypt... is that possible
at all?


> As to MD, I still use superblock format 0.90, because
> assembling an array ist clearly the controllers job (i.e.
> the kernels), hence I use it with kernel-level autodetection.
Well you can easily screw your RAID with that... and I think it's
generally deprecated...
I do not even know whether it would e.g. work with GPT... and IIRC it
does (obviously) not work with the RAID modules not compiled into the
kernel... not to talk about other limitations of the v0 superblock.


> With 0.90 at least I can find
> the data on disk manually if something breaks without 
> understanding some convoluted line of reasoning
I guess you mean "mount" with find... which is obviously only possible
with RAID1,... even that can be done with the v1 superblocks (out of the
box with 1.0)... and if you just set an offset... also with the others.

And directly mounting a RAID1 is really a dangerous thing, when one
doesn't know what one's doing or when it happens accidentally (which is
easy with 0.9 and 1.0 superblocks)... your RAID1 can get dirty without
it ever noticing it (thus likely data corruption).



Cheers,
Chris.
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