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Re: [Linux-cluster] DRBD with GFS applicable for this scenario?



On Fri, 29 Jan 2010 10:33:48 +0500, Zaeem Arshad wrote
> On Thu, Jan 28, 2010 at 3:44 PM, Gordan Bobic <gordan bobich net> wrote:
> 
> You have made interesting observations. Let me see if I can answer these.
> 
> > Hmm... What sort of ping time do you get? I presume you have established
> > that it is on the sensible side.
> 
> It's 7 ms to be precise. I was assuming the distance to be around 35km
> but it seems less.
> 
> >
> > In terms of performance you will need to make sure that machines tend to
> > access only their own sub-paths on the file system (e.g. spool/1 and
> > spool/2, and server 1 doesn't touch spool/2 until server 2 goes down).
> > Otherwise the performance is going to be attrocious since file locks will
> > end up bouncing between the machines. These normally live in cache on a
> > conventional file system so if they have to start getting exchanged at most
> > accesses you are looking at a latency degradation from ~ 50ns down to some
> > milliseconds. If your connectivity is VERY good, if it's 35km I would be
> > surprised if your latencies are better than 10ms, which you'll feel even on
> > the disk latency, let along memory latency - we are talking 200,000x slower
> > in the best case scenario.
> >
> 
> I think this is how DRBD works. It will usually read/write to the
> local device first and in case of local device failure will it switch
> over to the other block over IP. I'd be surprised if it doesn't do
> that.
> 
> > So you propose to have a quorum disk on site 2? OK, that works. The problem
> > is that fencing works by one server fencing another, not itself. So you'll
> > still need a reliable OOB fencing mechanism such as the one I described.
> >
> >
> 
> Yeah. I can probably use the following heuristics to make sure I fall
> back to primary
> 
> Heuristics:
> Check if QD is accessible at SAN2           5
> yahoo check                                           3
> POPXYZ check                                       2
> Volume X visible on SAN1 by Mail1           4
> 
> By using these heuristics and associated scores, I am able to decide
> which server has quorum. I am also looking into the OOB fencing idea
> too. One question that pops in my mind is that while I will have only
> 2 nodes in the DRBD volume, will more than 2 cluster members be able
> to access the GFS volume on top of the DRBD volume?
> 

If you use DRBD on top of the SANs you should access the drbd resource from
the other nodes. I have similar setup: DAS in two servers replicated via DRBD
in active/active mode, then they are exported via iscsi and used from the
nodes (and the storages are nodes too) with multipath multibus.

In your case the fencing is the main problem ... instead of SMS you may use
the mobiles as GPRS modems directly to the remote fence device

> Regards
> 
> --
> Zaeem
> 
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