[Ocfs2-users] Missing something basic...

[paul fretter (TOC)]

I had similar confusion myself when first looking for a suitable cluster
FS.  I'm not an expert at this, so forgive me if my language appears
simplistic.

There seemed to be 2 basic species:
- There are those which aggregate local storage LUNs from each host into
a single contiguous 'virtual' device, e.g. RedHat GFS etc.  Bear in mind
the4se 'local' LUNS could be a local disk, or a dedicated LUN on a SAN.

- Then there are those which expect all hosts to have direct (shared)
access to the same LUN.  

OCFS2 falls into the latter category.

For disk redundancy you could use a shared disk shelf (e.g. IBM DS4xxx),
create a RAID(1,4 or 5) set in the hardware and present it as a single
shared-access LUN.  All your nodes then have a connection to the LUN
(e.g. by fibrechannel). In effect it is a small SAN!  OCFS2 is then the
glue which manages the file locking and metadata so that nodes don't try
to write to the same blocks at the same time.

By creating a "RAID" using multiple OCFS2 volumes you are bringing the
role of device level redundancy work into the OS, which is effectively
software RAID and is not conducive to high performance or reliability.

Then for high availability of the storage, a good way might be to create
a duplicate shared device and let the hardware perform mirroring for you
(e.g. over fibrechannel, infiniband, iSCSI) etc, and also let the
hardware do the failover for you.

So, by using a SAN with hardware RAID and hardware mirroring to a second
SAN device (with hardware RAID), you can achieve resilience and high
availability, leaving OCFS2 blissfully unaware.

Hope this helps.

Kind regards
Paul Fretter

> -----Original Message-----
> From: ocfs2-users-bounces at oss.oracle.com [mailto:ocfs2-users-
> bounces at oss.oracle.com] On Behalf Of Randy Ramsdell
> Sent: 18 October 2007 14:00
> Cc: ocfs2-users at oss.oracle.com
> Subject: Re: [Ocfs2-users] Missing something basic...
> 
> Benjamin Smith wrote:
> > I'm stumped. I'm doing some research on clustered file systems to be
> deployed
> > over winter break, and am testing on spare machines first.
> >
> > I have two identically configured computers, each with a 10 GB
> > partition, /dev/hda2. I intend to combine these two LAN/RAID1 style
> to
> > represent 10 GB of redundant cluster storage, so that if either
> machine
> > fails, computing can resume with reasonable efficiency.
> >
> > These machines are called "cluster1" and "cluster2", and are
> currently on a
> > local Gb LAN. They are running CentOS 4.4 (recompile of RHEL 4.4)
> I've set up
> > SSH RSA keys so that I can ssh directly from either to the other
> without
> > passwords, though I use a non-standard port, defined in ssh_config
> and
> > sshd_config.
> >
> > I've installed the RPMs without incident. I've set up a cluster
> called "ocfs2"
> > with nodes "cluster1" and "cluster2", with the corresponding LAN IP
> > addresses. I've confirmed that configuration changes populate to
> cluster2
> > when I push the appropriate button in the X11 ocfs2console on
> cluster1. I've
> > checked the firewall(s) to allow inbound TCP to port 7777
connections
> on both
> > machines, and verified this with nmap. I've also tried turning off
> iptables
> > completely. On cluster1, I've formatted and mounted partition
> "oracle"
> > to /meda/cluster using the ocfs2console and I can r/w to this
> partition with
> > other applications. There's about a 5-second delay when
> mounting/unmounting,
> > and the FAQ reflects that this is normal. SELinux is completely off.
> >
> > Questions:
> >
> > 1) How do I get this "oracle" partition to show/mount on host
> cluster2, and
> > subsequent systems added to the cluster? Should I be expecting a
> /dev/* block
> > device to mount, or is there some other program I should be using,
> similar to
> > smbmount?
> >
> As the previous post states, you need a shared storage. A quick and
> easy
> way to do this is to install iscsi-target on another system (target1)
> and then use open-iscsi to log into the target you just created.  So
> have a  third system that create the shared target. Then on cluster1
> log
> into the target to create the ocfs2 cluster FS. At this point, you can
> mount this target on cluster1. On cluster2, log into the target and
> mount as you would normally. Of course you will need the correct
> cluster
> set up. Now you have two systems mounting the shared storage and both
> r/w.
> 
> Note: You may be able to do this with just two systems.  Use cluster1
> as
> the iscsi target system and ocfs2. On cluster1 install iscsi-target
> software and log into the volume share from cluster1 itself. Cluster2
> would just log in to the the target as normal.
> 
> > 2) How do I get this /dev/hda2 (aka "oracle") on cluster1 to combine
> (RAID1
> > style) with /dev/hda2 on cluster2, so that if either host goes down
I
> still
> > have a complete FS to work from? Am I mis-understanding the
abilities
> and
> > intentions of OCFS2? Do I need to do something with NBD, GNBD, ENDB,
> or
> > similar? If so, what's the "recommended" approach?
> >
> >
> Yes you are misunderstanding how ocfs2 works. To use raid for the
> described purposes, you must use it on the target1 system mentioned
> above. On target1, raid two drives or two partitions and then use this
> array as the target volume you export to cluster1 and cluster2. This
> way
> you have a raid array for data protection and ocfs2 for service
> integrity.
> > Thanks,
> >
> > -Ben
> >
> >
> 
> 
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