1 Reply Latest reply on May 24, 2013 10:34 AM by rarain

    confusion about  asm disk group

      I`m reading the Grid installation guide on linux guide(Oracle® Grid Infrastructure Installation Guide 11g Release 2 (11.2) for Linux E22489-08). Below text from the Document makes me confused.
      Normal redundancy
      In a normal redundancy disk group, to increase performance and reliability,
      Oracle ASM by default uses two-way mirroring. A normal redundancy disk
      group requires a minimum of two disk devices (or two failure groups). The
      effective disk space in a normal redundancy disk group is half the sum of the
      disk space in all of its devices.
      For Oracle Clusterware files, Normal redundancy disk groups provide 3
      voting disk files, 1 OCR and 2 copies (one primary and one secondary mirror).
      With normal redundancy, the cluster can survive the loss of one failure group.
      For most installations, Oracle recommends that you select normal redundancy.
      It says in a normal redundancy setting for ASM disk group. We need at least two disk devices. 3 voting disk files 1 ocr and 2 copies(So it should be 3 ocr files).

      Then it gives a table (Table 3–5 Total Oracle Clusterware Storage Space Required by Redundancy Type)
      In the tables it shows
      Normal redundancy leve need
      3 disks not 2. (question 1: The text i posted above says it should be 2)
      600 MB space for OCR ( question 2 : we already know one OCR need 300, if normal level need 3 ocr then here should be 900. Why it`s 600)
      900 MB space for voding disk
      1.5 GB for total

      So here is my questions
      1. Why the form says it need 3 disk instead 2?
      2. Why 3 ocr need 600m?
        • 1. Re: confusion about  asm disk group

          Why the form says it need 3 disk instead 2?

          In normal case minimum disk requirement for Normal Redundancy diskgroup is 2 but If you configure diskgroup for Voting disk and OCR disk with Normal redudancy then minimum disk requirment is three because Oracle Clusterware configure multiplex odd number of voting disk(i.e. 3) across the three different disk.

          2. Why 3 ocr need 600m?

          As far as OCR is concerned, it is maintained as like ASM file and since diskgroup redundancy is normal so ASM will maintained one mirror copy for OCR file. One OCR disk requirment is 300m so in this way for Normal redundancy diskgroup it become 600mb (one OCR file + one Mirror OCR file).