[DRBD-user] DRBD failover between datacenters if one's network fails

Andreas Kurz andreas at hastexo.com
Thu Dec 15 23:58:21 CET 2011

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


Hello,

On 12/15/2011 06:55 PM, Trey Dockendorf wrote:
> 
> On Dec 15, 2011 10:22 AM, "Felix Frank" <ff at mpexnet.de
> <mailto:ff at mpexnet.de>> wrote:
>>
>> Hi,
>>
>> On 12/15/2011 05:09 PM, Trey Dockendorf wrote:
>> > Thanks for the input.  Your right in that 2 days is too little time to
>> > do this, so I'm going to manual route of shutting one server down at a
>> > time, migrating the virtual disks then bringing it back up on the remote
>> > site.
>>
>> I had thought the QCow images were on one disk. If there are indeed
>> several disks you can sync, yes, you can take that route.
>>
>> > To avoid more downtime of manual migration once this is all over with, I
>> > think I will first attempt just getting a DRBD resource up and running
>> > to sync my servers back to the primary datacenter.  Can a DRBD resource
>> > on an existing LVM be done without effecting the data ?  Also since I
>>
>> Yes, provided you can a) enlarge the LV a bit to use internal meta data
>> or b) have some extra space on both machines to create an external meta
>> data disk.
>>
>> > don't plain to have automatic failover, any precautions I should take if
>> > the network connection is lost between the two datacenters ?  Ideally
>> > this would allow me to have minimal downtime while the nodes re-sync.
>>
>> Resyncing does not require downtime. Migrating the VMs to the other DRBD
>> peer needs downtime, and it's always brief.
>>
>> I cannot think of any required precautions.
>>
>> So the actual plan is to migrate the VMs before the connection is lost?
>> Great, this way you get away with an (arbitrarily long) quicksync once
>> the link returns and once that's finished, you can migrate back at your
>> leisure.
>>
>> Cheers,
>> Felix
> 
> All the qcow images are in pools located on the same logical volume.
> 
> Your correct.  The plan is to migrate before the fiber repair and
> network outage then sync them back with DRBD.
> 
> The meta space, does it have to be stored separate from the replicated
> LVM?  I have a few 100 GBs left on that device.

meta-data is located at the end of a device (internal) or on an extra
device. About 32MB per 1TB is needed ... exact calculation:

http://www.drbd.org/users-guide/ch-internals.html#s-meta-data-size

Regards,
Andreas

-- 
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http://www.hastexo.com/now

> 
> Thanks
> - Trey
> 
> 
> 
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