[Linux-cluster] clvm mirroring target status

Sat Feb 5 09:55:13 UTC 2005

I'll try to explain it a bit more structured

Host A
--------
Disk A 
Disk Bm (mirrored disk of disk B in host B, unmounted)

Host B
--------
Disk B
Disk Am (mirrored disk of disk A in host A, unmounted)

Normal working situation:
---------------------------------
Disk A and Disk B are exported with GNBD. If I understood well, I can combine 
them into one logical disk for the clusternodes with clvm (striped maybe, 
don't know, need to read more about it).
Disk Am and Bm are basically only used as mirroring for A and B. THis is  done 
with drbd. So they are not taking part in rw actions in any way.

Host B goes down:
------------------------
Heartbeat says it is down, I cut the power.
This is what I think needs to be done:
-Heartbeat moves the virtual IP address of host B to Host A. This is the IP 
address by which disk B was exported
-Mount disk Bm read/write. 
-Export Bm with GNBD. The clusters should now be able to continue working, I 
think transparently (need to test that to know).

Concequences:
-------------------
Bringing host B back in the game needs a manual intervention. 
-Basically al services on the cluster nodes need to stop writing. 
-Sync the disk from Bm to B
-Give host B back its virtual ip address
-mount B read/write
-umount Bm in host A
-start all services again on the nodes.
=> I know this is not perfect. But we can live with that. This will need to 
happen after office hours. The thing is that we don't have the budget for 
shared storage and certainly not for a redundant shared storage solution 
because most entry level shared storages are SPOFs. 

I need to find out more about that multipathing. I am not sure how to use it 
in this configuration. 
If you have idea's for improvement, they are welcome. 

Regards,

Filip

PS. Thanx for your answer on the clvm mirroring state.

On Friday 04 February 2005 21:00, Benjamin Marzinski wrote:
> 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/g
> >fs/msg03439.html+clvm+mirroring+gfs&hl=nl&start=12
> > https://www.redhat.com/archives/linux-cluster/2004-June/msg00028.html
> >
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