[gpfsug-discuss] NDS in Two Site scenario

[Jonathan Buzzard]

On Thu, 2016-07-21 at 12:45 +0000, Mark.Bush at siriuscom.com wrote:
> This is where my confusion sits.  So if I have two sites, and two NDS
> Nodes per site with 1 NSD (to keep it simple), do I just present the
> physical LUN in Site1 to Site1 NDS Nodes and physical LUN in Site2 to
> Site2 NSD Nodes?  

Unless you are going to use a tiebreaker disk you need an odd number of
NSD nodes. If you don't you risk a split brain cluster and well god only
knows what will happen to your file system in such a scenario.

> Or is it that I present physical LUN in Site1 to all 4 NDS Nodes and
> the same at Site2?  (Assuming SAN and not direct attached in this
> case).  I know I’m being persistent but this for some reason confuses
> me.  

That's one way of doing it assuming that you have extended your SAN
across both sites. You present all LUN's to all NSD nodes regardless of
which site they are at. With this method you can use a tiebreaker disk.

Alternatively you present the LUN's at site one to the NSD servers at
site one and all the LUN's at site two to the NSD servers at site two,
and set failure and replication groups up appropriately. However in this
scenario it is critical to have an odd number of NSD servers because you
can only use tiebreaker disks where every NSD node can see the physical
disk aka it's SAN attached (either FC or iSCSI) to all NSD nodes.

That said as others have pointed out, beyond a metropolitan area network
I can't see multi site GPFS working. You could I guess punt iSCSI over
the internet but performance is going to be awful, and iSCSI and GPFS
just don't mix in my experience.

JAB.

-- 
Jonathan A. Buzzard                 Email: jonathan (at) buzzard.me.uk
Fife, United Kingdom.