[Linux-cluster] High Availability options

[Steven Dake]

On Thu, 2006-07-13 at 14:45 -0500, Stephen Kirkpatrick wrote:
> Lon Hohberger wrote:
> > On Thu, 2006-07-13 at 12:14 -0500, Stephen Kirkpatrick wrote:
> >
> >   
> >> Thanks for the reply.  I wondered if it was possible to have HA for 
> >> stateful applications.  At least
> >> I know my options better now.  I can tolerate brief downtime, like the 
> >> time it would take a failover
> >> node to take over, although no downtime would have been preferred.
> >>
> >> Any suggestions on HA solutions for a non-SAN deployment?
> >>     

Stephen,

Coming in on this thread late (so I may be missing some details),
stateful HA application failover can be achieved by using something like
rgmanager + checkpointing, or availability management framework +
checkpointing.

The rgmanager code is in the current cluster tree.  AMF and
Checkpointing APIs are available via the openais project
(http://developer.osdl.org/dev/openais).  At the moment rgmanager is in
production state and AMF is more alpha-ish.  checkpointing is
production.

No downtime is not possible - this would mean availability=100%.  There
is always some time associated with detection and recovery from a fault.
The above openais project can detect application or node faults in the
15-30 msec range on 2.6 kernels.  Node faults can be tuned to 40-60msec
depending on number of nodes.  This still provides high levels of
availability and if an application is properly designed and tested, with
standard models should provide 6 to 7 nines of availability.

Regards
-steve

> >
> > You don't need a SAN if you don't intend to share data.  
> >
> > If you intend to share data but do not want a SAN, you'll need to set up
> > DRBD or CLVM+Cluster Mirroring+GNBD (Note: cluster mirroring is still in
> > a bit of development).  The main advantage to the latter approach is
> > simultaneous read/write access from both machines in the cluster.  The
> > advantage to DRBD is that it is easier to set up.
> >
> > Alternatively, you could use NFS mounts from a good NAS (a NetApp filer,
> > as an example) as your source for shared dat.
> >
> > You can use linux-cluster or Linux-HA to do failover with or without a
> > SAN; the choice is up to you.  Linux-HA supports DRBD out of the box,
> > but CLVM will still need parts of linux-cluster running in order to
> > operate.
> >
> > If you want support, you can use Red Hat Cluster Suite (which is
> > effectively linux-cluster w/ support).
> >
> > Good luck!
> >
> > -- Lon
> >
> > --
> > Linux-cluster mailing list
> > Linux-cluster at redhat.com
> > https://www.redhat.com/mailman/listinfo/linux-cluster
> >
> >
> >   
> Thanks for the info.  If I understood correctly what I was reading
> about DRDB, it sounds like it runs entirely over an IP network.
> This sounds like what I had originally envisioned for this project,
> as it is a small deployment.
> 
> I was envisioning SCSI storage in two nodes with individual
> internal RAID arrays as depicted in the diagram below,
> 
> 
> +----------------+          +------------------+
> | Primary server |            | Secondary server |
> |                |            |                  |
> |  -----------   |/          \|   -----------    |
> |     RAID-1     |------------|      RAID-1      |
> |  -----------   |\    |     /|   -----------    |
> |                |     |      |                  |
> +----------------+     |      +------------------+
>                        |
>                        +-- Data sync over IP network?
> 
> 
> I was thinking fibre channel storage when I said no SAN.  The
> cost of deploying fibre storage is a bit much for this small
> project.  However, if there is a good way to utilize shared
> SCSI storage and a good HA solution to go along, I would be open
> to researching it.
> 
> In your response, you indicated that Linux-HA supports DRDB out
> of the box.  Does the Red Hat Cluster Suite support this too?
> The Cluster Project FAQ just mentions the CLVM/GNBD solution.
> 
> Thanks,
> Stephen Kirkpatrick
> 
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