Quorum disk and server I/O

The quorum disk is used to determine the storage system on which server I/O should continue when a path or storage system failure occurs.

The quorum disk is a volume virtualized from an external storage system or from a disk in an iSCSI server, including cloud-based virtual servers (for example, running in Azure), if the server is supported by Universal Volume Manager and supported as a quorum disk. The primary and secondary storage systems check the quorum disk for the physical path statuses.

When the primary and secondary storage systems cannot communicate, the storage systems take the following actions:

1. The primary storage system cannot communicate over the data path and writes this status to the quorum disk.
2. When the secondary storage system detects from the quorum disk that a path failure has occurred, it stops accepting read/write.
3. The secondary storage system communicates to the quorum disk that it cannot accept read/write.
4. When the primary storage system detects that the secondary storage system cannot accept read/write, the primary storage system suspends the pair. Read/write continues to the primary storage system.

If the primary storage system cannot detect from the quorum disk that the secondary storage system cannot accept I/O within five seconds of a communication stoppage, the primary storage system suspends the pair and I/O continues.

If both systems simultaneously write to the quorum disk that communication has stopped, this communication stoppage is considered to be written by the system that receives the first write I/O after the communication stoppage.