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List:       drbd-user
Subject:    Re: [DRBD-user] MD5-Hash of DRBD Resource
From:       Arnold Krille <arnold () arnoldarts ! de>
Date:       2013-11-19 21:20:27
Message-ID: 20131119222027.26541f0e () xingu ! arnoldarts ! de
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On Tue, 19 Nov 2013 14:29:32 +0100 "FDS | Forensik Data Services"
<office@fds.at> wrote:
> Just clarifying my understanding of an UpToDate-Status of a DRBD
> Resource in Dual Primary Mode on 2 nodes.
> Does a MD5-Hash calculation of both resources lead to equal  MD5
> values?

That depends. Prerequisites:
 - Both primaries are unmounted.
 - Both primaries are completely synced from the os/fs point of view.
   (That is "sync" has been run and thus all write-buffers are cleared.)

Then an md5-hash of the drbd-device on both machines should return the
same. Also md5-hashes of all the underlaying devices should(*) return
the same.

(*) With internal meta-data, there could be differences in the
meta-data. With external meta-data the md5-hash of the data partitions
used by drbd should all be equal.

But be aware that with md5 it seems as if there is a slight chance of
hash-collisions, it might be the hashes are the same but the data on
your disks is actually different. Better do your tests with a sha-sum:-P

Have fun,

Arnold

PS: Afaik if your devices aren't unmounted and synced, the data on disk
can be different for a variety of valid reasons.

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