4 Replies Latest reply: Aug 19, 2013 8:48 AM by Lyxx RSS

    Failure Group Query


      Hello Experts,

      Not trying to beat dead horse but I always had difficulty to understand despite of reading several books about Failure Groups. I am also sure there is something basic which I am missing. I would appreciate your help.


      My doubt is - 


      Fault Tolerant between 2 Failure Groups  Vs  Many Failure Groups


      I understand that

      - Oracle uses failure groups to choose where to place mirrored extents

      - We should have failure groups aligned with failure boundaries/disks sharing same dependency/infrastructure


      I would like to articulate with an example


      suppose I have 4 NAS Servers and each NAS server gives me 4 disks and I want to create normal redundancy diskgroup


      NAS 1 -   Disk1,   Disk2,   Disk3,   Disk4

      NAS 2 -   Disk5,   Disk6,   Disk7,   Disk8

      NAS 3 -   Disk9,   Disk10,  Disk11,  Disk12

      NAS 4 -   Disk13,  Disk14,  Disk15,  Disk16



      Scenario (A) --   this is the ideal scenario I believe


          Diskgroup DG1

          Normal Redundancy

          FG 1 -   Disk 1/2/3/4

           FG 2 -   Disk 5/6/7/8

           FG 3 -   Disk  9/10/11/12

           FG 4 -   Disk 13/14/15/16


      Scenario (B) -  I would like to create only 2 failure groups, each failure group will have disks from 2 NAS Servers


      Diskgroup DG2

        Normal Redundancy

        FG1 -  Disk1/2/3/4/5/6/7/8                  -- disks from 2 nas servers

        FG2 -  Disk 9/10/11/12/13/14/15/16          -- disks from 2 nas servers



      with Scenario A --  we can tolerate 1 Failure group which means failure of 4 disks in one/same failure group

      with Scenario B --  we can tolerate 1 Failure group which means failures of 8 disks in one/same failure group  -- 2 NAS server failures in same failgroup can be tolerated.


      If my above assumption is correct then I am not sure what would be the reason we should go more than 2 failure groups, we get more cover. I have also checked space availability in case we lost

      one disks  ( req mirror mb / usable file mb ) all looks good.


      Could you please tell me what is the drawback of scenario B as compare to A




        • 1. Re: Failure Group Query
          Javier Francisco Ruiz

          In normal redundancy diskgroup 2 failgroups is all you need you will not benefit from have 4 failgroups it will be a waste of storage and more administration work to maintain 4 failgroups.

          • 2. Re: Failure Group Query

            Thanks for your reply, I do have same expression but I get confused when I look at Exadata Implementation. I was trying to understand the concept behind that.


            In full Rack - Exadata has 14 storage cells and they have put each storage cell in one failure group. again, I agree here because of failure boundaries but they can only tolerate one failuregroup = one cell,


            Why did not Oracle go for 2 Failure Groups by putting 6 cells in each failure group, they could have tolerated upto 6 cells ( in same failure group )


            That's the reason for whole confusion.


            Again, thanks for your reply

            • 3. Re: Failure Group Query
              Javier Francisco Ruiz

              Is the diskgroup in exadata a normal redundancy diskgroup? If you look at a external redundancy you will see something even more confusing that Oracle automatically places each disk in it own fail group.

              • 4. Re: Failure Group Query

                That's correct. 14 failure groups with Normal Redundancy.