[DRBD-user] drbd sync killing machine with OOM

Siim Vahtre siim_ at mumak.ee
Fri May 4 20:03:04 CEST 2007

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

On Fri, 4 May 2007, Rene Mayrhofer wrote:

> Exactly. To enable XEN live migration, both nodes need to be primary - with
> one drbd8 volume for each domU, I can modify the migration scripts to go
> primary on both for the period of the migration and secondary on the "old"
> node after migration has finished.

Why only for the period of migration? For safety? If only one node is 
using the certain block range at one time, there shouldn't be problem 
(please correct me if I'm wrong!).

> This has, IMHO, two advantages:
> - live migration should work
> - in normal operation, only one node is primary, and thus the chances of going
> split-brain should be _very_ slim (the network would need to fail exactly
> during migration, which seems unlikely)

What do you mean?

AFAIK Xen guarantees that while migrating only one node is writing to the 
storage at time. So migration either happens or it does not.

> Therefore, and after reading of different setup-variants, I decided to go for
> this one. It seemed the most flexible of the "safe" setups (since e.g. ocfs2
> over drbd8 does not yet seem to be mature enough for production systems(.

> With LVM on top of drbd8, both would need to be primary for the whole drbd
> volume, although they would only ever write in different logical volumes,
> i.e. different parts of the drbd8 volume.

AFAIK you can have LVM2 active on two primary nodes at the same time but 
if you do ANY changes to volume group structure (change volume sizes etc) 
then you have to do 'vgchange -a n' in all but one node and THEN do the 
changes on only that one node that is still active. For my needs, this has 
been enough.

You could also use CLVM - then you should be able to run with LVM being 
active on both nodes. However, I think it needs at least some parts from 
the redhat cluster framework, so it might not be what you want.

> Does anybody on this list use LVM2 on top of drbd8? Have you ever encountered
> split-brain?

I use LVM2 on top of DRBD8, but in primary/slave configuration where LVM 
is always active on only one node. I use migrations in another level 
(network attatched storage) so it is not problem for me.

Disclaimer: I am still a newbie at this and just testing my cluster, so I 
might be completely wrong!

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