[Lustre-discuss] Question about failnode

[Kazuki Ohara]

Hi Brain,
Thank you for your answer.

I understood the need of STONISH.

By the way, I doubt the need of the --failnode directive.
I walked into the source code of lustre, but I could not find
the use of the --failnode information except for building log messages.

Are there any more important reasons of the use of the --failnode directive?

Best regards,
Kazuki Ohara

Brian J. Murrell wrote:
> On Thu, 2007-10-25 at 20:58 +0900, Kazuki Ohara wrote:
>> Hi,
>> I have a question about the fainode directive of mkfs.lustre.
>> I hope someone help me.
>>
>> When a shared volume is formatted and mounted as below,
>>   [root at ossnode1 ~]# mkfs.lustre --ost --failnode=ossnode2 \
>>   > --mgsnode=mgsnode at tcp /dev/sda1
>>   [root at ossnode1 ~]# mount -t lustre /dev/sda1 /mnt/ost1
>> ossnode1 knows that sda1 can be accessed by ossnode2.
>>
>> Then, a failover occurs and ossnode2 mounts sda1,
> 
> ossnode2 should in fact, before it does the mount, make as entirely sure
> as it can that ossnode1 does not have it mounted.  The surefire way to
> do that is to kill the power to ossnode1 (assuming there is a power
> controller between the mains and ossnode1 that ossnode2 can operate).
> 
> In the failover game this is called STONITH and is an acronym for "Shoot
> The Other Node In The Head".  All of this is usually coordinated with
> something like Heartbeat.
> 
> The reason for this STONITH action is that in an HA scenario, ossnode2
> only knows that it cannot reach ossnode1.  It does not know why.  It
> could be because it's power failed, it panic'd or any number of reasons.
> Not all of those reasons imply that ossnode1 cannot (and does not) still
> have the disk mounted though.  Only by killing ossnode1 itself, can
> ossnode2 be absolutely sure that ossnode1 does not have the disk
> mounted.
> 
> More than one node mounting an ext{2,3,4} or ldiskfs (which is ext4,
> basically) filesystem is disastrous for that filesystem, so all possible
> measures necessary to prevent that need to be taken.
> 
>> I think there is no way for ossnode2 to know sda1 can be accessed by ossnode1.
>> Does this become a problem?
> 
> It does, hence the steps above.
> 
> b.
> 
> 
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-- 
Kazuki Ohara
Sony Computer Entertainment Inc.
Computer Development Div. Distributed OS Development Dept.
Japan