[DRBD-user] Question on ext3 and dual primaries mode

Mark Watts m.watts at eris.qinetiq.com
Mon Sep 14 10:09:19 CEST 2009

Note: "permalinks" may not be as permanent as we would like,
direct links of old sources may well be a few messages off.

On Mon, 2009-09-14 at 09:26 +0200, Florian Haas wrote:
> Using ext3 on top of dual primary DRBD is a sure fire way of destroying
> data. Does that answer your question?

I have seen (and used) allow-two-primaries with ext3, when using the
drbd block device driver for Xen to allow live migration of a DomU
between two nodes where the DomU is given a full dedicated drbd block
device for its backing store.

However, for 99.999% of the time DRBD is running in Primary/Secondary
mode and it is Xen which controls promoting and demoting each side as a
migration. Note that I believe Xen is using this in order to prove to
itself that the remote node is capable of accepting disk-writes and is
thus safe to migrate to; it does not, I believe, allow writes to happen
on both nodes at the same time in the general case.

This is the _only_ case I know of where allow-two-primaries and ext3 can
co-exist, if only for a very short and controlled time. For anything
else Florian is right - kiss goodbye to your data.


Mark Watts BSc RHCE MBCS
Senior Systems Engineer, Managed Services Manpower
QinetiQ - Delivering customer-focused solutions
GPG Key: http://www.linux-corner.info/mwatts.gpg
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