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jf7640's avatar
jf7640
Level 3
13 years ago

Fencing without fencing?

Hello,

as far as I understand, I/O fencing consists of two separate concepts, which can both be described with the two different meanings of the word "fencing":

  fencing(1): membership arbitration by fighting ("fencing") for the majority of coordination points, via SCSI3 PGR or coordination point servers

  fencing(2): data protection by "building a fence" around data disks using SCSI3 PGR 

 

Would it be possible to have data protection (fencing(2)) but without arbitration by fencing(1)?

 

Reasons for wanting this might be: 

- sufficient heartbeat and low-pri interfaces

- management overhead of CP servers (which have to be documented, patched, monitored etc.)

- asymmetric behaviour with 2 coordinator disks in one storage box and only one in the other

 

As far as I understand, "UseFence = SCSI3" in main.cf enables data protection in the DiskGroup agent but also requires the vxfen module and in order to configure the vxfen module one has to configure a fencing disk group or CP servers. So, as far I can see there is no fencing(2) without fencing(1), but I might have missed a configuration option of vxfen. 

So, is it possible to have only data protection?

 

Kind regards,

Jochen

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  • Gaurav_S's avatar
    Gaurav_S
    13 years ago

    Hi, Option 1 & option 2 together forms 1 solution called IOFencing ... it is not possible to have either of the two ... both will be used on conjuction....

     

    As you correctly said, UseFence=SCSI3 in main.cf enables diskgroup agent to understand that fencing needs to be used which you can say data protection .... this will be achieved by vxfen module in split brain scenario...

    There is a choice of membership arbitration at GAB level but that is not a full proof solution & recommended is to have full fencing setup .. You can check GAB options to find details ..

     

    G

  • Gaurav_S's avatar
    Gaurav_S
    Moderator
    13 years ago

    Hi, Option 1 & option 2 together forms 1 solution called IOFencing ... it is not possible to have either of the two ... both will be used on conjuction....

     

    As you correctly said, UseFence=SCSI3 in main.cf enables diskgroup agent to understand that fencing needs to be used which you can say data protection .... this will be achieved by vxfen module in split brain scenario...

    There is a choice of membership arbitration at GAB level but that is not a full proof solution & recommended is to have full fencing setup .. You can check GAB options to find details ..

     

    G

  • Dagobert's avatar
    Dagobert
    Level 3
    13 years ago

    Hi Gaurav,

    from your comment I get the impression that it is possible to use SCSI3 fencing for data protection and GAB (without SCSI3 fencing) for membership arbitration, however, I struggle to find the correct setting to achieve this. As I see from my tests the VxFEN driver must be loaded and configured, which however does not seem to be possible without a fencing diskgroup. Is it necessary to do customitzed fenmode?

    The rationale for my configuration request goes like this: the configuration is fully symmetric between two datacenters with host mirrored disks via vxvm. As there is only one veritas cluster a cps in a third location imposes too much requirements. The most likely cases to happen are power failures at one site or split brain due to switch power failing. On power failure the surviving node should grab the service groups with disks and continue operation which does not always occur on fencing disks asymetric 2+1 lun distribution across storages in the datacenters. It would be acceptable when one node would panic on split brain due to scsi3 reservation on the dg failing.

    UseFence=SCSI3 in main.cf but SMF vxfen disabled:

     
    2012/07/25 11:59:19 VCS NOTICE V-16-1-52006 UseFence=SCSI3. Fencing is enabled
    2012/07/25 11:59:19 VCS CRITICAL V-16-1-10037 VxFEN driver not configured. Retrying...
    2012/07/25 11:59:34 VCS CRITICAL V-16-1-10037 VxFEN driver not configured. Retrying…
    UseFence=SCSI3 in main.cf and SMF vxfen enabled with vxfenmode=disabled:
     
    2012/07/25 12:00:36 VCS CRITICAL V-16-1-52024 VxFEN driver is configured in mode=4. VCS Stopping. Manually restart VCS after configuring fencing in SCSI3 or SYBASE or customized mode.
    UseFence=None in main.cf and Resource-Property Reservation=SCSI3 at DiskGroup:
     
    2012/07/25 12:03:33 VCS ERROR V-16-10001-1022 (pms10) DiskGroup:PMSProdDg:monitor:Diskgroup cannot be imported with reservations since I/O fencing is not configured.

    Kind regards

      -- Dago