4 Replies Latest reply: Jul 17, 2014 11:00 AM by Dude! RSS

    How many threads do rebalance on ASM volume files?

    1856219

      I'd made some test, which show me that ASM volume files (ACFS) has only one I/O thread per volume file. This mean that, if you have 1Tb volume file and you want to rebalance it you can do it with only one I/O thread.

       

      I execute this (alter diskgroup data rebalance power 10) which contains only database files and has 24 disks. Through iostat I can see that, all disks was loaded by many I/O operations!

      But if I execute (alter diskgroup acfs rebalance power 10) which contains only 2 volume files, 1Tb each and has 24 disks. Through iostat I can see that, in one time loaded only one disk! And after some time later another disk was loaded, but always in one time loaded only ONE disk!


      It is shows me that asm rebalance for asm vol file do by single I/O operation compared rebalance operation for rdbms files which going with multiply I/O operations.

        • 1. Re: How many threads do rebalance on ASM volume files?
          Aritra-Oracle

          Hi,

           

           

          ASM has exactly same behaviour as with database files - ASM performs the rebalance on a per file basis till 11GR2.

          The ASM metadata files (1-9) get rebalanced first. The ASM then rebalances the volume file 256, DRL file 257, and so on.

           

           

          From this we see that the ASM rebalances volume files (and other ASM files), not the OS files in the associated file system(s).

          So,probably ,it was moving extent related to that file from each disk ,so you would have seen this .

           

           

           

          Regards,

          Aritra    

          • 2. Re: How many threads do rebalance on ASM volume files?
            Dude!

            ACFS volumes are a separate feature of ASM to store non database files. ASM is not RAID and rebalancing is not on a disk but on a file basis. You can increase the rebalance power, but it can only do one I/O tread per file. As far as I understand, the ACFS volume is a single file from the ASM technical view and the rebalance operaton of the ACFS volume is on the basis of the ACFS volume file and not on the files stored inside it.

            • 3. Re: How many threads do rebalance on ASM volume files?
              1856219

              Yes, you right. But my question is, why ASM can't rebalance extents within volume files with multiply threads?

              • 4. Re: How many threads do rebalance on ASM volume files?
                Dude!

                I'm not the developer of ASM and can only guess, but generally I would imagine it depends on the number of extends that need to be rebalanced. Similarly, a single process or single thread can only run on one CPU regardless of how many CPU's are in the system. So as far as I understand, multiple rebalance processes will not fight over a single file extend.