[gpfsug-discuss] maybe a silly question about "old school" gpfs

[Salvatore Di Nardo]

I understand that my test its a bit particular because the client was 
also one of the servers.
Usually clients don't have direct access to the storages, but still it 
made think, hot the things are supposed to work.

For example i did another test with 3 dd's, one each server. All the 
servers was writing to all the luns.
In other words a lun was accessed in parallel by 3 servers.

Its that a problem, or gpfs manage properly the concurrency and avoid 
data corruption?
I'm asking because i was not expecting a server to write to an NSD he 
doesn't own, even if its locally available.
I thought that the general availablity was for failover, not for 
parallel access.


Regards,
Salvatore



On 05/11/14 10:22, Vic Cornell wrote:
> Hi Salvatore,
>
> If you are doing the IO on the NSD server itself and it can see all of 
> the NSDs it will use its "local” access to write to the LUNS.
>
> You need some GPFS clients to see the workload spread across all of 
> the NSD servers.
>
> Vic
>
>
>
>> On 5 Nov 2014, at 10:15, Salvatore Di Nardo <sdinardo at ebi.ac.uk 
>> <mailto:sdinardo at ebi.ac.uk>> wrote:
>>
>> Hello again,
>> to understand better GPFS, recently i build up an test gpfs cluster 
>> using some old hardware that was going to be retired. THe storage was 
>> SAN devices, so instead to use native raids I went for the old school 
>> gpfs. the configuration is basically:
>>
>> 3x servers
>> 3x san storages
>> 2x san switches
>>
>> I did no zoning, so all the servers can see all the LUNs, but on nsd 
>> creation I gave each LUN a primary, secondary and third server. with 
>> the following rule:
>>
>> STORAGE
>> 	primary
>> 	secondary
>> 	tertiary
>> storage1
>> 	server1
>> 	server2 	server3
>> storage2 	server2 	server3 	server1
>> storage3 	server3 	server1 	server2
>>
>>
>>
>> looking at the mmcrnsd, it was my understanding that the primary 
>> server is the one that wrote on the NSD unless it fails, then the 
>> following server take the ownership of the lun.
>>
>> Now come the question:
>> when i did from server 1 a dd surprisingly i discovered that server1 
>> was writing to all the luns. the other 2 server was doing nothing. 
>> this behaviour surprises me because on GSS only the RG owner can 
>> write, so one server "ask" the other server to write to his own 
>> RG's.In fact on GSS can be seen a lot of ETH traffic and io/s on each 
>> server. While i understand that the situation it's different I'm 
>> puzzled about the fact that all the servers seems able to write to 
>> all the luns.
>>
>> SAN deviced usually should be connected to one server only, as 
>> paralled access could create data corruption. In environments where 
>> you connect a SAN to multiple servers ( example VMWARE cloud) its 
>> softeware task to avoid data overwriting between server ( and data 
>> corruption ).
>>
>> Honestly, what  i was expecting is: server1 writing on his own luns, 
>> and data traffic ( ethernet) to the other 2 server , basically asking 
>> *them* to write on the other luns. I dont know if this behaviour its 
>> normal or not. I triied to find a documentation about that, but could 
>> not find any.
>>
>> Could somebody  tell me if this _/"every server write to all the 
>> luns"/_ its intended or not?
>>
>> Thanks in advance,
>> Salvatore
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>
>
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