1 Reply Latest reply: Jun 21, 2011 6:58 PM by Levi Pereira RSS

    ASM LUN design question

    harris baskaran
      Hi,

      We are designing the LUNs for a 7TB databases. In-order to manage it more efficiently we created a 3TB(6 LUNs) Non-Growing Datafile Diskgroup DATA1 and 4TB(17 LUNs) Growing Datafile diskgroup DATA2. For performance reasons we created another +ONLINE diskgroup of two 50GB sized LUNs for redo logs.

      -> In order to multiplex the redo groups should we split the two LUNs to two diskgroups each? That would make diskgroups of one LUN each. Our Storage Admin warned us of performance impacts of single LUN for a disk. Is this a valid concern with ASM?
      -> Or should we create them as single diskgroup two disks of NORMAL redundancy
      -> Or a single diskgroup, two disks with external redundancy

      Can someone also explain to me why Oracle recommend 4 LUNs per diskgroup?

      Thank you.
      Regards,
      Harry
        • 1. Re: ASM LUN design question
          Levi Pereira
          Hi,
          harry2040 wrote:
          Hi,

          We are designing the LUNs for a 7TB databases. In-order to manage it more efficiently we created a 3TB(6 LUNs) Non-Growing Datafile Diskgroup DATA1 and 4TB(17 LUNs) Growing Datafile diskgroup DATA2. For performance reasons we created another +ONLINE diskgroup of two 50GB sized LUNs for redo logs.
          Diskgroup above 2TB I recommend creating LUNs on the size of up to 10% of the total size of the ARRAY. You can explore all available controllers performing balancing the LUNS on the controllers e.g ARRAY 2TB use 10 LUNS of 200GB each. If you have 2 controllers, you can config up 5 LUNS in each Controllers.

          This will help with future maintenance as placing new disks or removing disk to save unused space.

          Oracle ASM data distribution policy is capacity-based. Ensure that Oracle ASM disks in a disk group have the same capacity to maintain balance.
          If in future you need to add new LUNS on Diskgroup you should keep in mind that if you created a 500GB LUN then you must add a new LUN of 500GB.

          I recommend 10% based on my own experience, I do not mean that this is the optimal for your environment, each environment should be analyzed.
          -> In order to multiplex the redo groups should we split the two LUNs to two diskgroups each? That would make diskgroups of one LUN each. Our Storage Admin warned us of performance impacts of single LUN for a disk. Is this a valid concern with ASM?
          If you want multiplex redo I recommend you create 2 DISKGROUPS to ONLINELOG or use DG ONLINELOG+FRA to multiplex redo.
          Redo have a unique characteristic, which is sequential write near zero reading. The database is sensitive to read/write performance of the Redo Logs and should be on a RAID 1 , RAID 0+1 at all since they are accessed sequentially and performance is enhanced in their case by having the disk drive head near the last write location.
          So If you create a single Lun you will can use one controller per Diskgroup, I belive its not so bad because REDO perfom only write and is sequential, but you can increase perfomance you can create 2 Luns and use 2 controllers. The big difference in perfomance is how you create RAID of this LUNS.

          Your Storage Admin is rigth how much more LUNs you can have more performance, this is valid for ASM.
          -> Or should we create them as single diskgroup two disks of NORMAL redundancy
          I do not recommend you use redundancy via ASM (Software) if you can perform redundancy via Storage (Hardware).
          -> Or a single diskgroup, two disks with external redundancy
          I agree with this.
          Can someone also explain to me why Oracle recommend 4 LUNs per diskgroup?
          Because increasing the number of disks in a disk group for maximum data distribution you can get higher I/O bandwidth

          But Oracle Support on the note “Lun Size And Performance Impact With Asm [ID 373242.1]” and on the note “How to Prepare Storage for ASM [ID 452924.1]” say that in different things:

          How to Prepare Storage for ASM
          - Maximize the number of disks in a disk group for maximum data distribution and higher I/O bandwidth.
          - Create large LUNs to reduce LUN management overhead.

          Lun Size And Performance Impact With ASM
          Size alone should not affect the performance of a LUN. The underlying hardware is what counts. There is no magic number for the LUN size. Seek the advice of the storage vendor to recommend the best configuration from a raid 1 or 5 perspective for performance and availability since this may vary between vendors
          For larger LUNs it is recommended using a large ALLOCATION UNIT.

          This should be analyzed and applied the best option for your environment.

          Regards,
          Levi Pereira