[DRBD-user] DRBD from inside a VM?

Miles Fidelman mfidelman at meetinghouse.net
Thu Jul 24 00:23:50 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.

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):

VM (Linux DomU) --- auto failover to node 2 ----
DRBD ---- replicated to node 2 ----
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)

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, 
- 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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