Storing Voting Files on Oracle ASM

Oracle ASM manages voting files differently from other files that it stores. If you choose to store your voting files in Oracle ASM, then Oracle ASM stores all the voting files for the cluster in the disk group you choose. You cannot use voting files stored in Oracle ASM and voting files not stored in Oracle ASM in the same cluster.

Once you configure voting files on Oracle ASM, you can only make changes to the voting files' configuration using the crsctl replace votedisk command. This is true even in cases where there are no working voting files. Despite the fact that crsctl query css votedisk reports zero vote disks in use, Oracle Clusterware remembers the fact that Oracle ASM was in use and the replace verb is required. Only after you use the replace verb to move voting files back to non-Oracle ASM storage are the verbs add css votedisk and delete css votedisk again usable.

The number of voting files you can store in a particular Oracle ASM disk group depends upon the redundancy of the disk group.

By default, Oracle ASM puts each voting file in its own failure group within the disk group. A failure group is a subset of the disks in a disk group. Failure groups define disks that share components, such that if one fails then other disks sharing the component might also fail. An example of what you might define as a failure group would be a set of SCSI disks sharing the same SCSI controller. Failure groups are used to determine which Oracle ASM disks to use for storing redundant data. For example, if two-way mirroring is specified for a file, then redundant copies of file extents must be stored in separate failure groups.

The redundancy level that you choose for the Oracle ASM disk group determines how Oracle ASM mirrors files in the disk group, and determines the number of disks and amount of disk space that you require. If the voting files are in a disk group, then the disk groups that contain Oracle Clusterware files (OCR and voting files) have a higher minimum number of failure groups than other disk groups because the voting files are stored in quorum failure groups.

A quorum failure group is a special type of failure group that is used to store the Oracle Clusterware voting files. The quorum failure group is used to ensure that a quorum of the specified failure groups are available. When Oracle ASM mounts a disk group that contains Oracle Clusterware files, the quorum failure group is used to determine if the disk group can be mounted if there is a loss of one or more failure groups. Disks in the quorum failure group do not contain user data, therefore a quorum failure group is not considered when determining redundancy requirements in respect to storing user data.

Redundancy levels include:

• External redundancy: An external redundancy disk group requires a minimum of one disk device. The effective disk space in an external redundancy disk group is the sum of the disk space in all of its devices.

  Because Oracle ASM does not mirror data in an external redundancy disk group, Oracle recommends that you use external redundancy with storage devices such as RAID, or other similar devices that provide their own data protection mechanisms.

• Normal redundancy: 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, a normal redundancy disk group requires a minimum of three disk devices (two of the three disks are used by failure groups and all three disks are used by the quorum failure group) and provides three voting files and one OCR and mirror of the OCR. When using a normal redundancy disk group, the cluster can survive the loss of one failure group.

• High redundancy: In a high redundancy disk group, Oracle ASM uses three-way mirroring to increase performance and provide the highest level of reliability. A high redundancy disk group requires a minimum of three disk devices (or three failure groups). The effective disk space in a high redundancy disk group is one-third the sum of the disk space in all of its devices.

  For Oracle Clusterware files, a high redundancy disk group requires a minimum of five disk devices (three of the five disks are used by failure groups and all five disks are used by the quorum failure group) and provides five voting files and one OCR and two mirrors of the OCR. With high redundancy, the cluster can survive the loss of two failure groups.

Using the crsctl replace votedisk command, you can move a given set of voting files from one Oracle ASM disk group into another, or onto a certified file system. If you move voting files from one Oracle ASM disk group to another, then you can change the number of voting files by placing them in a disk group of a different redundancy level as the former disk group.