Re: [Xen-users] HA Xen on 2 servers!! No NFS, special hardware, DRBD or iSCSI...

[Chris de Vidal <chris@xxxxxxxxxx>]

--- Javier Guerra <javier@xxxxxxxxxxx> wrote:
> On Wednesday 07 June 2006 7:33 am, Eric Windisch wrote:
> > I have been thinking of testing NBD for a similar setup, instead of
> > AoE.  My understanding is that NBD natively supports raid-1 mirroring
> > with logic particularly useful for RAID-over-TCP.  That is, during a
> 
> i think you're mixing NBD and DRBD.

No he got it right.  Similar technologies.  One advantage of NBD is it also 
offers RAID 5.


> as i see it, DRBD lets you have two block devices, each one on it's own box, 
> and maintain them as mirrors.  any modification on one gets propagated to the 
> other one.  now, if you want to make that "RAID1" available to a third box 
> (and a fourth, fifth, etc) it's usually shared with GNBD (but it could be 
> AoE, iSCSI, ieee 1394, anything).
> 
> to Chris: yes, you could get what you want, just make a big data disk (or 
> partition, or LVM PV) 'mirrored' with DRBD, and use that for the DomU's 
> storage.  you'll get live migration if the backing storage is name-equivalent 
> on both boxes (easy to do with LVM).  if one hangs up, just reboot it on the 
> other box.

Well that's what I thought, but what happens if you want to do a live migration 
and then take the
first node down for maintenance?

For example:
DRBD running as primary on Node1
DRBD partition exported to both boxes using GNBD (or iSCSI or AoE...)
Live migrate DomU1 from Node1 to Node2, which sets you up like this:
                [ DomU1 ]
                    |
  [ LVM  ]       [ LVM  ]
  [ GNBD ]------>[ GNBD ]
[ DRBD pri ]   [ DRBD sec ]
[  Node1   ]   [  Node2   ]

OK so far so good.

But now I want to take Node1 down for maintenance.  I cannot do that without 
making the secondary
DRBD partition the new primary, which I cannot do without umounting the current 
primary DRBD
partition, which I cannot do unless I suspend DomU1.

So as far as I can see, DRBD does not enable live migration + power-off first 
node without some
sort of suspend/resume interruption.  My use of AoE would allow that but Eric 
is right -- slight
network interruptions will cause a resync.

I wonder how often that would happen though, with a crossover cable?  Perhaps 
it's tolerable...
and if the guest OS is Linux, install the fast sync patch into the DomU...

I guess the regular migration suspend/resume interruption isn't too bad, I'm 
just reaching for the
holy grail ;-)

CD

TenThousandDollarOffer.com

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