Network & Disk Heartbeats

Network heartbeat is across the interconnect, every one second, a thread (sending) of CSSD sends a network tcp heartbeat to itself and all other nodes, another thread (receiving) of CSSD receives the heartbeat. If the network packets are dropped or has error, the error correction mechanism on tcp would re-transmit the package, Oracle does not re-transmit in this case. In the CSSD log, you will see a WARNING message about missing of heartbeat if a node does not receive a heartbeat from another node for 15 seconds (50% of misscount). Another warning is reported in CSSD log if the same node is missing for 22 seconds (75% of misscount) and similarly at 90% of misscount and when the heartbeat is missing for a period of 100% of the misscount (i.e. 30 seconds by default), the node is evicted.

Disk heartbeat is between the cluster nodes and the voting disk. CSSD process in each RAC node maintains a heart beat in a block of size 1 OS block in a specific offset by read/write system calls (pread/pwrite), in the voting disk. In addition to maintaining its own disk block, CSSD processes also monitors the disk blocks maintained by the CSSD processes running in other cluster nodes. The written block has a header area with the node name and a counter which is incremented with every next beat (pwrite) from the other nodes. Disk heart beat is maintained in the voting disk by the CSSD processes and If a node has not written a disk heartbeat within the I/O timeout, the node is declared dead. Nodes that are of an unknown state, i.e. cannot be definitively said to be dead, and are not in the group of nodes designated to survive, are evicted, i.e. the node’s kill block is updated to indicate that it has been evicted.

Thus summarizing the heartbeats, N/W Heartbeat is pinged every second, nodes must respond in css_miscount time, failure would lead to node eviction. Similarly Disk Heartbeat, node pings (r/w) voting disk every second, nodes must recieve a response in (long/short) disk timeout time.

Below are the different possibilities of individual heartbeat failures.

Network Ping Disk Ping Reboot
Completes within misscount seconds Completes within Misscount seconds N
Completes within Misscount seconds Takes more than misscount seconds but less than Disktimeout seconds N
Completes within Misscount seconds Takes more than Disktimeout seconds Y
Takes more than Misscount Seconds Completes within Misscount seconds Y

Related Articles:-

GPnP Profile Internals

Network & Disk Heartbeat

OCR Internals

OLR Internals

SCAN Internals

Split Brain

Voting Disk Internals

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