[Linux-cluster] Howto define two-node cluster in enterpriseenvironment

[Thomas Sjolshagen]

 On Mon, 10 Jan 2011 21:21:45 +0100, "Kit Gerrits" 
 <kitgerrits at gmail.com> wrote:
> Hello fellow administrator,
>
> If you have a SAN...
> Why can't you have the SAN publish the same LUN to the two cluster 
> nodes
> simultaneously?

 You can, but you minimally need to guarantee (not believe or think, but 
 guarantee!) that both nodes do not

 a) write to the same sectors, file systems or LVM volumes at the same 
 time (this is actually a whole lot more difficult to do than most people 
 think) - including boot sectors, partition tables, LVM metadata, etc, 
 etc,

 b) think they're exclusively accessing the LUN I.e. there must be 
 something on the nodes - an application, OS tool or something else - 
 that understands that there is more than one reader & writer to a LUN 
 and thus synchronizes this.

> It is only used as a raw device, so there should be no ugly 
> filesystem
> side-effects.

 File systems only serve to make this a lot more obvious to the end user 
 or administrator since it's integrity tends to get shot fairly quickly 
 and there are integrity checks in place. On raw devices, you get the 
 "benefit" of ignorance about the fact that your data is corrupt, unless 
 b) above is true.

 Hth,

 // Thomas

>
>
> Regards,
>
> Kit
>
> -----Original Message-----
> From: linux-cluster-bounces at redhat.com
> [mailto:linux-cluster-bounces at redhat.com] On Behalf Of Andreas 
> Bleischwitz
> Sent: maandag 10 januari 2011 11:25
> To: linux-cluster at redhat.com
> Subject: [Linux-cluster] Howto define two-node cluster in
> enterpriseenvironment
>
> Hello list,
>
> I recently ran into some questions regarding a two-node cluster in an
> enterprise environment, where single-point-of-failures were tried to 
> be
> eliminated whenever possible.
>
> The situation is the following:
> Two-node cluster, SAN-based shared storage - multipathed; host-based
> mirrored, bonded NICS, Quorum device as tie-breaker.
>
> Problem:
> The quorum device is the single-point-of-failure as the SAN-device 
> could
> fail and hence the quorum-disc wouldn't be accessible.
> The quorum-disc can't be host-based mirrored, as this would require 
> cmirror
> - which depends on a quorate cluster.
> One solution: use storage-based mirroring - with extra costs, limited 
> to no
> support with mixed storage vendors.
> Another solution: Use a third - no service - node which has to have 
> the same
> SAN-connections as the other two nodes out of cluster reasons. This 
> node
> will idle most of the time and therefore be very uneconomic.
>
> How are such situations usually solved using RHCS? There must be a 
> way of
> configuring a two-nodecluster without having a SPOF defined.
>
> HP had a quorum-host with their no longer maintained Service Guard, 
> which
> could do quorum for more than on cluster at once.
>
> Any suggestions appreciated.
>
> Best regrads,
>
> Andreas Bleischwitz
>
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