From: Luca Berra <bluca@comedia.it>
To: linux-raid@vger.kernel.org
Subject: Re: Bug or not? Same array reports different/transformed UUID depending on check-method used.
Date: Tue, 21 Jun 2011 07:42:58 +0200 [thread overview]
Message-ID: <20110621054258.GA22391@maude.comedia.it> (raw)
In-Reply-To: <1308328189.28028.1464311105@webmail.messagingengine.com>
On Fri, Jun 17, 2011 at 09:29:49AM -0700, jeffs_linux@123mail.org wrote:
>I suppose I should split this into its own thread rather than burying it
>in my other.
>
>Question first:
>
> I have two arrays attached to my Linux box. Two methods of checking
> for arrays UUIDs give different results. Why, and can I reply on
> these arrays?
>
>Details follow:
>
>Checking with,
>
> mdadm --incremental --rebuild-map
I don't think this is a valid method for checking array uuid.
> cat /dev/.mdadm/map
> md126 0.90 52f5b43c:e83f7e2a:be6ad32e:0536ab0e /dev/md/0_0
> md127 1.2 79fb7ad4:289bfae5:86c535ff:202960f2 /dev/md127
>
>Staring at those UUIDs, I notice that one array's UUIDs match exactly
>for the two methods of checking,
>
> /dev/.mdadm/map /dev/md/0_0 52f5b43c:e83f7e2a:be6ad32e:0536ab0e
> mdadm --detail --scan /dev/md/0_0 52f5b43c:e83f7e2a:be6ad32e:0536ab0e
>
>but the OTHER array's two UUIDs
>
> /dev/.mdadm/map /dev/md127 79fb7ad4:289bfae5:86c535ff:202960f2
> mdadm --detail --scan /dev/md127 d47afb79:e5fa9b28:ff35c586:f2602920
>
>are 'transforms' of one another; e.g.,
...
>Why are /dev/md127's UUIDs, unlike /dev/md/0_0's, reporting mis-matched
>& 'transformed'?
>
mdadm internally stores an uuid as an array of four 32bit integers
on disk it is different.
superblock version 0 (md0_0) stores 4 32bit values in host endian order(*)
(little-endian on x86)
superblock version 1 (md127) stores 16 8bit values in human readable
format (humans at least in western world countries are big endian)
The map_file, on the contrary, is always read and written as 4 32bit
values in host endian order, so on x86 machines you will find it
swapped.
Summary: the mapfile is for mdadm internal use only, use mdadm commands
(--detail, --examine) to obtain data.
(*) http://en.wikipedia.org/wiki/Endianness
L.
--
Luca Berra -- bluca@comedia.it
next prev parent reply other threads:[~2011-06-21 5:42 UTC|newest]
Thread overview: 3+ messages / expand[flat|nested] mbox.gz Atom feed top
2011-06-17 16:29 Bug or not? Same array reports different/transformed UUID depending on check-method used jeffs_linux
2011-06-21 5:42 ` Luca Berra [this message]
2011-06-21 12:50 ` jeffs_linux
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