7 Replies Latest reply on Aug 8, 2018 7:07 AM by Martin PRESSLABER

    DAX fallbacks

    Martin PRESSLABER

      Hi,

      could you explain what DAX fallbacks mean in detail and how to avoid them? Or is it ok to have fallbacks?

      Unfortunately googling does not help...

       

      MANpage says: fallbacks == Number of commands completed by the software, which DAX could not complete

       

      OK, but why and what does it mean for the DB?

       

      Running RAC on M7 with InMemory:

      1st node=DAX commands 35.608.411 with 18 fallbacks

      2nd node=DAX commands 63.563.607 with 2.656.188 fallbacks

       

      Is it just a bad day for the second node? Any ideas how I could dig deeper at OS or DB level?

       

      Thanks in advance and regards

      - Martin

        • 1. Re: DAX fallbacks
          Mike LaRosa -Oracle

          Hi Martin,

           

          This is the Sparc T-Series Server area, Looks like your asking about a DataBase function ?  can you add more details so we can try and move this to a better suited area for questions ?

           

          What database application ?  what version ?

           

          Thanks

           

          Mike

          • 2. Re: DAX fallbacks
            Martin PRESSLABER

            Hi Mike,

            it is database related, but I am talking about a SPARC hardware feature:

            What Is the SPARC M7 Data Analytics Accelerator?

             

            Perhaps I could ask that question in an In-Memory database sections; but it is a hardware feature :-)

             

            Thanks for your support and kind regards

            - Martin

            • 3. Re: DAX fallbacks
              Martin PRESSLABER

              That's also a good explanation:

               

              https://swisdev.oracle.com/DAX/DAXwhatis.php

               

              Regards M

              • 4. Re: DAX fallbacks
                Mike LaRosa -Oracle

                Hi Martin,

                 

                Did some research, You are correct the DAX functionality is done in hw aka at the chip level....  back to your questions....

                 

                 

                === Caveat, These answers coming from a hardware guy, you may get more detail from the DB guys who code for this.

                 

                 

                could you explain what DAX fallbacks mean in detail and how to avoid them? Or is it ok to have fallbacks?

                 

                 

                MANpage says: fallbacks == Number of commands completed by the software, which DAX could not complete

                 

                 

                 

                === As you found in the manpage,  simply telling us how many transactions DAX completed and how

                === many it did not aka fallbacks.

                 

                 

                 

                OK, but why and what does it mean for the DB?

                 

                 

                === It would seem node 2 may have hit the wall so to speak in DAX's capability and the fallbacks snowballed,

                === might be evidence that DAX does retry transactions....

                 

                Running RAC on M7 with InMemory:

                 

                1st node=DAX commands 35.608.411 with 18 fallbacks

                 

                2nd node=DAX commands 63.563.607 with 2.656.188 fallbacks

                 

                 

                 

                Is it just a bad day for the second node? Any ideas how I could dig deeper at OS or DB level?

                 

                === suggest running this by the RAC/DB guys for more details on what DAX is doing and how to resolve this issue.

                • 5. Re: DAX fallbacks
                  Martin PRESSLABER

                  Hi,

                   

                  did an update to QFSDP Apr18 and now both nodes are using DAX with less fallbacks now:

                  n1 53.235.926 commands with 10 fallbacks

                  n2 33.866.192 commands with 18 fallbacks

                   

                  And that within a week... seems that patching increased the usage, but it is very difficult to say, what queries took advantage from DAX...

                  But still I think that these 2 million fallbacks where an issue before... (without a negative effect, perhaps for the run time)

                  I will need to compare some AWRs ...

                   

                  Any how... still very strange that it seems no one is knowing or caring about these DAX engines... but the idea behind is very very cool so I am wondering about that... ;-)

                   

                  Thank you for your interest,

                  regards

                  - Martin

                  • 6. Re: DAX fallbacks
                    Mike LaRosa -Oracle

                    Morning,

                     

                    Hoping someone here can answer Martin's questions more fully than I am able too

                     

                    Thaanks

                     

                    Mike

                    • 7. Re: DAX fallbacks
                      Martin PRESSLABER

                      Hi,

                      thanks for your support...

                       

                      a good documentation would be enough... "fallback" does not sound good :-)

                       

                      thx and regards

                      - martin