Re: [Xen-API] redundant xcp server with drbd

[Denis Cardon <denis.cardon@xxxxxxxxxxxxxxxxxxxxxx>]

Hi Georges,

> We've tried that for internal laboratory. Problem: sometime drbd do
> additional IO and making storage out of sync. We've spend few weeks on
> this set up (primary/primary drbd, and iscsi export with LVMoISCSI SR),
> but it getting unstable after some time, so we gave up.
Actually I have not encountered instability in my test setup (the SR was 
define directly over /dev/drbd0, no iscsi in between). During the test, 
I was running iozone in a loop in VMs that were switch from host every 
few minutes in a loop (linux and windows VMs) for a whole 48 hours.
I have'nt encountered any issues during that stress test (set aside the 
global warming). But when I went ahead with disaster recovery scenario, 
it was a no go : severing the primary-primary drbd link left me with a 
brain split to solve by hand.
I was thinking to use the more conservative approach with the following 
scenario. It adds the iscsi latency to the io path, but  :
  XCP1 in xenpool                  XCP2 in xenpool
==================              ====================
drbd-sr1 (primary)    <----->   drbd-sr1 (secondary)
 exported to xenpool
 through iscsi target

drbd-sr2 (secondary)  <----->   drbd-sr2 (primary)
                                 exported to xenpool
                                 through iscsi target

I have a few year of practice of drbd with primary/secondary setup on 
iscsi san, and I am pretty confident the data should be safe with this 
kind of setup, but I'm wondering of the behavior of a dom0 xen in this 
situation.
I'll give it a shot in the next few days.

Thanks for your input.

Denis





> On 06.03.2012 13:47, Denis Cardon wrote:
> > Hello everyone,
> >
> > I wanted to know if any of you had some experience with building a
> > redundant xcp/xenserver cluster based on drbd. In this setup I would
> > like to avoid a separate iscsi SAN, and have replicated local storage.
> >
> > There are docs on the internet showing how to set up a dual primary
> > drbd SR on xenserver/xcp. It works fine with live migration and all,
> > but when trying disaster scenario, like severing the drbd replication
> > link, one gets a split brain scenario that cannot be unsolved. It
> > should be doable if xapi had a smart understanding of drbd, but
> > currently it is just a SR like others. So it is a no-go for the time
> > being.
> >
> > I also tried a drbd primary-secondary scenario, with re-export with
> > iet iscsi target, but for the moment I have unresolved performance
> > issues (will try also with scst target).
> >
> > So I wanted to know if any of you had any experience or hints with
> > this kind of setup : a dual active/active XCP servers with local SR
> > replicated with primary-secondary DRBD, re-exported with iscsi or aoe
> > or whatever.
> >
> > Cheers,
> >
> > Denis Cardon
>
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--
Denis Cardon
Tranquil IT Systems
44 bvd des pas enchantés
44230 Saint Sébastien sur Loire
tel : +33 (0) 2.40.97.57.57
http://www.tranquil-it-systems.fr


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