When an instance of SQL Server is running in
a Clustered Environment, Microsoft Cluster Service performs two checks to
confirm if the SQL Server resource is still available. One is LooksAlive
and the other one is IsAlive check.
Looks Alive: It’s a basic check in
which the Cluster service queries the Windows Service Control Manager to check
if the SQL Server service is still running. By default this check happens every
5 seconds.
Is Alive: IsAlive check the Cluster
Service connects to the SQL Server instance with the help of
c:\windows\system32\sqsrvres.dll and runs SELECT @@SERVERNAME against the
instance. This check does not check if the user databases are online or not. It
just checks the SQL Server instance availability. This check happens every 60
seconds by default.
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