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[Linux-cluster] RE: HA Clustering - Need Help



some quick comments on your post from someone who has tried an active-active cluster on a shared SCSI device.

1. If you want to have the same block partition mounted on two different computers at the same time, then you need some cluster file system like GFS, you can't use ext3. There are other cluster filesystems out there (like lustre) but GFS is most well tied to the RH Cluster Suite and designed for high availability as opposed to paralell computing. 2. If you are going to run GFS in a production environment the recommendation is to not use 2-node. GFS 5 required 3 nodes but GFS 6 offers a 2-node option; However when using two nodes it is harder to know which node is "broken" when something goes wrong, so you'll note a lot of discusson on this list about fencing gone awry and needing some sort of tiebeaker like a quorum disk. If you take care in setting it up a 2-node cluster will work but you'll want to test it extensively before putting it into production. 3. multipathing should work fine and you can build clvm volumes on top of multipath devices. Software RAID is different and not really related.

as for recommendations:
1. don't use SCSI shared storage. I and others have had reliability issues with heavy load in these scenarios.
2.  use more than 2 nodes.
3. go active-passive is possible. as is often pointed out, the entire idea of a high availability cluster is that there is enough processing horsepower to handle the entire remaining load if one node fails. in a 2-node cluster then you'll have to provision each node to be able to run everything. it is far easier to set it up so that one node therefore runs everything and the other node awaits failure than having active-active.

just my $.02
-alan

Date: Tue, 23 Jan 2007 23:03:58 +0530
From: "Net Cerebrum" <netcerebrum gmail com>
Subject: [Linux-cluster] HA Clustering - Need Help
To: linux-cluster redhat com

Hello All,

I am totally new to HA clustering and am trying hard to grasp the
fundamentals in a limited time frame. I have been asked by my company to
create a high availability cluster using Red Hat Cluster Suite on hardware
comprising two servers running RHEL AS 4 and one shared external storage
array. The cluster would be running in Active-Active state. Oracle Database
version 9 (not RAC) would run on one of the servers while the Oracle
Applications version 11 would run on the other. In case of failure of either
of the servers, the service would be started on the other server. Both the
servers (nodes) would be connected to the storage array through two
redundant SCSI controllers.

Since the storage has redundant controllers, both the servers would be
connected to the storage array using two channels each and the requirement
is to make it an Active-Active Load Balanced configuration using a multipath
software. The storage vendor has suggested using the multipath option with
the mdadm software for creating multipath devices on the storage array.

I have gone through the manuals and since this is my first attempt at high
availabilty clustering I have many doubts and questions. What file system
should be used on the external storage ? Is it better to use ext3 or Red Hat
GFS ? At certain places it is mentioned that GFS should be used only if the
number of nodes is 3 or more and GULM is being used. Since we have only two
nodes, we plan to use DLM.  It is also mentioned that GFS and CLVM may not
work on a software RAID device. Would the multipath devices created
(/dev/md0, /dev/md1, etc) be considered to be software RAID devices, though
in the real sense they are not ? Further the development team is not too
sure about the compatibility between GFS and Oracle Database and
Applications. What could be the pros and cons of using  ext3 file system in
this scenario ?

The development team just wants one filesystem to be used on the storage
which would be mounted as /oracle on both the servers / nodes and all the
binaries and data would reside on this. Since this filesystem is going to be
mounted at boot time, my understanding is that no mounting or unmounting of
any filesystem will take place during the failover so the cluster
configuration should reflect that. The documentation repeatedly refers to
mounting of the file systems when failover takes place so that's giving rise
to a little confusion. Further there are references to a quorum partition in
documentation but I have not been able to find any provision to make use of
the same in the cluster configuration tool.

Please help me in clarifying these issues and suggest me how to go about
setting this cluster. I would be really grateful for any suggestions and
references.

Thanks,


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