RAC_OCR and VD files in asm.

Question

Hi All,
    From 11g r2 onwards we can keep OCR and voting disk files in ASM +DG in that case,
1. if ocr and VD files are in ASM DG then which process needs to start first ? how can we start asm dg first before cluster?
2. why we need to keet VD files in odd numbers ?
Please give me some link or way to understand this.
Thanks,

Answer 1

1. if ocr and VD files are in ASM DG then which process needs to start first ? how can we start asm dg first before cluster?
Oracle Clusterware does not rely on ASM to access the Voting Files because the voting disks are placed directly on ASMDISK (not in a Diskgroup).
Voting files not depend of Diskgroup (with state mounted) to be accessed, does not mean that we don’t need the diskgroup, diskgroup and voting file are linked by their settings.
CSSD identifies the vote disks based on the discovery string in the gpnp profile and at this point after the discovery of the vote disks has direct access to it.
Within the disk header of ASM disks with Voting files there is markers that define the starting AU (vfstart) and ending AU (vfend) for the voting disk.
On startup, CSSD will scan all ASM disks (using the ASM discovery string from the GPnP profile) for these markers once found, CSSD use them without any direct relationship to an ASM instance
2. why we need to keet VD files in odd numbers ?
You should configure odd number of voting disks because far as voting files are concerned, a node must be able to access more than the half of the voting files at any time (simple majority).
In order to be able to tolerate a failure of n voting files, one must have at least 2n+1 configured. (n= number of voting files) for the cluster.
If a node in cluster lost majority of disk heartbeat they must be kicked from cluster.
To ensure that will lost the majority (i.e greater than 50% of disks) you must have odd number, because you can't lost a half disk.
If you have a even number of disk you can't apply quorum concept and it is not acceptable in a cluster.