Why drives with voting rights are always an odd number

I have a query that why voting disk are always recommended in odd numbers.

The voting disk must reside on a shared disk that is accessible by all nodes in the cluster. For high availability, Oracle recommends that you have several disks with voting rights. Oracle Clusterware can be configured to manage records with multiple voting rights (multiplexing), but you must have an odd number of voting records, such as three, five and so on. Oracle Clusterware supports a maximum of 32 disks with voting rights. If you set a single voting disk, then you must use external mirror to provide redundancy.

A node must be able to access more than half the disks with right to vote at any time. For example, if you have five voting disks configured, then a node must be able to access at least three drives with right to vote at any time. If a node cannot access the required minimum number of discs with right to vote, it is expelled or removed from the cluster.

Here I have a table:
Total number of vdisk (n) Minimum vdisk required by nodes (> n/2) no vdisk can be failed at the same time
1               1                         0
2               1                         1
3               2                         1
4               2                         2
5               3                         2
6               3                         3
7               4                         3
Here, it is clear that it does not matter as the vdisk 4 or 5, the number of vdisk can be failed at the same time would be 2. So, not sure why an odd number of voting records are recommended.

Please suggest.

Kind regards
Dheeraj van Voorde

(1) why don't we not create an odd number of voting record?
With regard to the vote of the disks are concerned, a node must be able to access exclusively more than half of the discs with the right to vote at any time. So if you want to be able to tolerate a failure of n disks to vote, you must have at least 2n + 1 configured. (n = 1 means 3 discs with voting rights). You can configure up to 32 drives with right to vote, providing protection against the 15 simultaneous disk failures.
Oracle recommends that customers use drives 3 or more with right to vote in Oracle RAC 10g Release 2. Note: For best availability, 3 files of voting should be separate physical disks. It is recommended to use an odd number as 4 drives will be more highly available than 3 disks, 3 1/2 is 1.5... rounded up to 2, 4 1/2 is 2, once we lose 2 discs, our cluster fails with 4 disks with voting rights or 3 discs with right to vote.

(2) only the cluster checks actually for the counting of the votes before node eviction? If so, could you expain this process briefly?
Yes. If you lose half or more than all your discs with right to vote, then nodes get evicted from the cluster, or nodes to expel from the cluster.

(3) what is the logic behind the documentaion note which says "each node must be able to see more than half the disks with right to vote at any time"?
The answer to the first question is will also answer this question.

answered by: Silva
Reference: Question on voting drive
Question on voting drive

Tags: Database

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