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

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.


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

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.


Denis Cardon

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Denis Cardon
Tranquil IT Systems
44 bvd des pas enchantés
44230 Saint Sébastien sur Loire
tel : +33 (0)

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