Note: "permalinks" may not be as permanent as we would like,
direct links of old sources may well be a few messages off.
> On Thu, Dec 18, 2008 at 08:11:28PM +0100, Christoph Adomeit wrote: >> Hi, >> >> I would love the idea to live migrate xen-domU's in rare cases during >> maintenance. >> >> But I hate the idea to generally allow dual primary mode for those drbd >> resources > > I don't think it is even necessary. > But I have to double check that... AFAICS, yes it is. Xen wants to fire up the DRBD device (i.e. in the case of the block-drbd helper, make it Primary) on the migration target even before it starts the migration. So you need two Primaries for the entire duration of live migration. >> because of the risk to accidently mount a filesystem >> twice and destroy it. >> >> Is it possible to switch drbd resources to dual-primary mode at runtime >> and later switch them back ? > > disconnect both nodes (both go to StandAlone) > > reconnect both with the two-primaries setting toggled. > (should do a very brief re-sync, and then be Connected again) Unfortunately not easily scriptable with the Xen VBD helper infrastructure. If anyone's got a better idea on how to do this, I'm all open for suggestions. And of course, the fact that you _allow_ dual-primary mode does not mean that the device in fact _operates_ in dual-primary mode the whole time. Indeed the block-drbd helper makes sure that the device is Primary on both nodes _only_ during migration. So I don't really see the added security in scripting - enable dual-primary mode - make primary - migrate - make secondary - disable dual-primary mode versus the existing - make primary - migrate - make secondary. Then again, there is of course the option of using file-backed VBDs on GFS or OCFS2. That way you'll at least have a lock manager to coordinate access. And since the blktap Xen driver uses direct I/O these days, it should be pretty much equivalent performance wise, compared to using the block devices directly. Just steer clear of sparse files. :-) Just my two cents. Cheers, Florian