[Linux-cluster] GFS and Simultaneous Access

[berthiaume_wayne at emc.com]

 Hi Bryn.

	I'm curious about the fencing used with iSCSI - (I've used hacks
with iptables
before for Linux based iscsi devices). 

	Also, I've been trying to figure out a way to allow all my nodes
access to the same filesystem/LUN with separate directories for each one
within the same filesystem for simultaneous access. Is this possible or
would this be the place to use the SCSI reservations? This is being done
strictly for testing.

Regards,
Wayne.

-----Original Message-----
From: linux-cluster-bounces at redhat.com
[mailto:linux-cluster-bounces at redhat.com] On Behalf Of Bryn M. Reeves
Sent: Tuesday, April 17, 2007 10:23 AM
To: linux clustering
Subject: Re: [Linux-cluster] GFS over AOE without fencing?

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Kadlecsik Jozsi wrote:
>> If you are using GFS on shared storage then you need fencing. Period.
>>
>> The only way you can guarantee data integrity in this scenario is by
>> completely cutting a failed or misbehaving node off from the storage;
>> either by power cycling it or having the storage reject its access.
>>
>> Otherwise, imagine a situation where a node hangs for some reason and
is
>> ejected from the cluster. At this point none of its locks for the
shared
>> data are valid anymore. Some time later, the node recovers from the
hang
>> and begins flushing writes to the storage -> corruption.
> 
> I see - the sentences above make much more clearer why fencing is
needed.
> Thank you the explanation - and the hint for the possibility to reject

> the access at the storage itself!
>  
> Best regards,
> Jozsef

Hi Jozsef,

No problem! For cutting the access off at the storage there is a new
fence agent in the cluster CVS called fence_scsi - you can use that if
the storage supports SCSI3 reservations. I don't know if that is the
case for AOE though.

Otherwise you could use a regular network power switch, or maybe cook
something up with a custom fencing script (I've used hacks with iptables
before for Linux based iscsi devices).

Kind regards,
Bryn.
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