[DRBD-user] DRBD from inside a VM?

Miles Fidelman mfidelman at meetinghouse.net
Thu Jul 24 00:40:40 CEST 2014

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

Cool!  DRBD over ZVOLs looks like it could be a really sweet, and 
easier-to-manage HA environment.

Thanks for the advise!


Simon Ironside wrote:
> Hi Miles,
> Yes - I run DRBD inside KVM virtual machines in production just fine. 
> There were no performance issues or real gotchas that I've encountered.
> I was just careful to not base the VM's storage on DRBD *outside* the 
> VM in case I accidentally had both VMs running on the same physical 
> server ;) I was also careful to maintain separate front and back 
> networks by bridging two separate virtual NICs in the VM to two 
> separate physical NICs on two separate networks on the physical host 
> just as I do with DRBD on "real" servers.
> HTH,
> Simon.
> On 23/07/14 23:23, Miles Fidelman wrote:
>> Hi Folks,
>> I'm wondering if something is doable - comments please:
>> I have a very specific situation where I'm trying to maintain a legacy
>> high-availability application running in a Xen VM (Linux DomU).
>> Current stack (per node, in a 2-node HA cluster):
>> Applications
>> VM (Linux DomU) --- auto failover to node 2 ----
>> DRBD ---- replicated to node 2 ----
>> LVM
>> md raid
>> Xen (Linux Dom0)
>> We're looking at migrating our infrastructure to an OpenSolaris base
>> (probably SmartOS), which will end up looking like this:
>> VM (Linux KVM or Zone) ------------auto failover ot node 2 -----
>> <synchronous replication> ----- to node 2 ----
>> ZFS (exporting a ZVOL)
>> KVM (or Zones)
>> SmartOS
>> The kicker, is that there is no available equivalent to DRBD for the
>> illumos ecosystem - hence, my thought is to mount the ZVOL block device
>> inside the 2 virtual machines, and use DRBD running on Linux inside the
>> pair of VMs to achieve storage replication.
>> Will DRBD run inside a virtual machine?  Will it give any kind of
>> reasonable performance?  Are there any serious gotchas to watch out for?
>> Three notes re. motivation and thinking:
>> - for all our other applications, we're doing application layer
>> replication (e.g., database sync) - but for smtp and list processing
>> this doesn't seem easily doable (too many queues and messages in flight,
>> asynchronously)
>> - looking to move from Linux for a number of reasons, but 
>> particularly ZFS
>> - looked at BSD, which has HAST as an equivalent of - virtualization is
>> a mixed bag (Xen on NetBSD, but not FreeBSD; KVM sort of, a new
>> hypervisor for FreeBSD), and ZFS is only stable on FreeBSD - can do some
>> things on FreeBSD, some on NetBSD, but not all on the same one - and
>> right now the illumos world seems to be gaining  lot of momentum as the
>> alternative open-source Unix
>> Thanks very much,
>> Miles Fidelman

In theory, there is no difference between theory and practice.
In practice, there is.   .... Yogi Berra

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