linux-ext4 - Re: Reproducible block structure

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Date:	Tue, 10 Dec 2013 08:42:55 -0500
From:	Theodore Ts'o <tytso@....edu>
To:	Jitesh Shah <jitesh.1337@...il.com>
Cc:	linux-ext4@...r.kernel.org
Subject: Re: Reproducible block structure

On Mon, Dec 09, 2013 at 09:03:23PM -0800, Jitesh Shah wrote:
> 
> Now, if the script is ran in the SAME way for all 5 HDDs, is it
> guaranteed that these HDDs will be same at the block level too? (i.e.
> block allocation/deallocation will follow the same pattern). Assume
> single-core system with only one process modifying the HDD in
> predetermined order.

Nope, there's no way to guarantee this.  There are a few places where
the algorithms are non-deterministic by design.  It would be possible
to make some changes to guarantee this, but I'm not sure it's really
worth it --- in real life, assuming a single core system with a single
process which is also single threaded is generally not a realisstic
scenario.

> Why do I ask -> I am tinkering with the idea of block level
> verification of images. If the above guarantees can be provided, I can
> easily hash the raw HDD for verification purposes.

If you want to do a block level verification of the image, why not
also do block level update of the image as well?

Regards,

						- Ted
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