[Linux-cluster] clvm mirroring target status

[Benjamin Marzinski]

On Fri, Feb 04, 2005 at 05:52:31PM +0100, Filip Sergeys wrote:
> Hi,
> 
> We are going to install a linux cluster with 2 gnbd servers (no SPOF)
> and gfs + clvm on the cluster nodes (4 nodes). I have two options, if I
> read the docs well, for duplicating data on the gnbd servers:
> 1) using clvm target mirroring on the cluster nodes
> 2) use drbd underneath to mirror discs. Basically two disks per machine:
> 1 live disk which is mirrored with drbd to the second disk in the second
> machine and the other way around in the second machine 
> (so the second disk in the first machine is thus the mirror from the
> first (="live") disk in the second machine(sounds complicated, but it is
> just hard to write down)). 
> Both live disks from each machine will be combined as one logical disk
> (If I understood well, this is possible).
> 
> Question: what is the status of clvm mirroring? Is it stable? 
> Suppose it is stable, so I have a choice: which one of the options would
> any of you choose? Reason? (Stability, performance, ...)

I'm still not sure if cluster mirroring is available for testing (I don't
think that it is). It's defintely not considered stable.

I'm also sort of unsure about your drbd solution.
As far as I know, drbd only allows write access on one node at a time. So,
if the first machine uses drbd to write to a local device and one on the second
machine, the second machine cannot write to that device. drbd is only useful
for active passive setups.  If you are using pool multipathing to multipath
between the two gnbd servers, you could set it to failover mode, and modify
the fencing agent that you are using to fence the gnbd_server, to make it tell
drbd to fail over when you fence the server.

I have never tried this, but it seems reasonable. One issue would be how to
bring the failed server back up, since the devices are going to be out of sync.

http://www.drbd.org/start.html says that drbd still only allows write access
to one node at a time.

sorry :(

-Ben

> I found two hits on google concerning clvm mirroring, but both say it is
> not finished yet. However the most recent one is from june 2004. 
> I cannot test either because we have no spare machine. I'm going to buy
> two machine so I need to know which disk configuration I will be using.
> 
> Thanks in advance,
> 
> Regards,
> 
> Filip Sergeys
> 
> 
> 
> http://64.233.183.104/search?q=cache:r1Icx--aI2YJ:www.spinics.net/lists/gfs/msg03439.html+clvm+mirroring+gfs&hl=nl&start=12
> https://www.redhat.com/archives/linux-cluster/2004-June/msg00028.html
> 
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