DNS MP – guilty once again?

When a DNS Zone is Active Directory integrated and you have several DCs, the discovery for the DNS Domain generates useless traffic and high CPU usage. One of the discovered properties is the “Primary Server”, alas every DC for DNS integrated zones is the primary server for the zone, since the DNS Domain is discovered on every DC we have a race condition in which every DC may override the previous discovery. This in turn generates, as detailed by Fabrizio in a previous post, a configuration reload on every DC. The configuration reload, at the state, is a CPU intensive operation…

This is the definition of the DNS Domain class, as you can see the PrimaryServer is not a key, so every instance is identified by the DomainName property.

<ClassType ID=”Microsoft.Windows.DNSServer.Library.DNSDomain” Accessibility=”Public” Abstract=”false” Base=”Microsoft.Windows.DNSServer.Library.Component” Hosted=”false” Singleton=”false”>
  <Property ID=”DomainName” Type=”string” Key=”true” CaseSensitive=”false” Length=”256″ MinLength=”0″ />
  <Property ID=”PrimaryServer” Type=”string” Key=”false” CaseSensitive=”false” Length=”256″ MinLength=”0″ />
  <Property ID=”DynamicUpdates” Type=”string” Key=”false” CaseSensitive=”false” Length=”256″ MinLength=”0″ />
  <Property ID=”NameServers” Type=”string” Key=”false” CaseSensitive=”false” Length=”256″ MinLength=”0″ />
</ClassType>

By default the discovery runs every 6 hours with no sync time, if, for example, the zone is distributed on 6 DCs we can have one config reload per DC per hour, more DCs mean more configuration reload. This can quickly become an issue. Once again a configuration reload is a CPU intensive operation for the Health Service process.

The DNS MP potentially has another issue, for each DNS Zone it discovers the zone SerialNumber. The SerialNumber is incremented by the DNS for every change in the zone, so we can assume it will change on every discovery cycle. From my first research this property is never used for monitoring purposes. Since the discovery runs every 6 hours it means a reload every 6 hours, this is not dramatic, but if bad MP coding habits sum up, then it can add to the health service total cpu usage, bringing the net impact of monitoring to an unacceptable level. We have DCs and ISA servers well above a 5% average cpu utilization for the HealthService process. With the previous DNS MP we had HS CPU usage over 10% on average.

<ClassType ID=”Microsoft.Windows.DNSServer.Library.Zone” Accessibility=”Public” Abstract=”false” Base=”Microsoft.Windows.DNSServer.Library.Component” Hosted=”true” Singleton=”false”>
  <Property ID=”ZoneName” Type=”string” Key=”true” CaseSensitive=”false” Length=”256″ MinLength=”0″ />
  <Property ID=”ZoneType” Type=”string” Key=”false” CaseSensitive=”false” Length=”256″ MinLength=”0″ />
  <Property ID=”IsReverseZone” Type=”bool” Key=”false” CaseSensitive=”false” Length=”256″ MinLength=”0″ />
  <Property ID=”IsADIntegrated” Type=”bool” Key=”false” CaseSensitive=”false” Length=”256″ MinLength=”0″ />
  <Property ID=”AllowDynamicUpdates” Type=”string” Key=”false” CaseSensitive=”false” Length=”256″ MinLength=”0″ />
  <Property ID=”AllowZoneTransfers” Type=”int” Key=”false” CaseSensitive=”false” Length=”256″ MinLength=”0″ />
  <Property ID=”PrimaryServerName” Type=”string” Key=”false” CaseSensitive=”false” Length=”256″ MinLength=”0″ />
  <Property ID=”ZoneFileName” Type=”string” Key=”false” CaseSensitive=”false” Length=”256″ MinLength=”0″ />
  <Property ID=”UseWINS” Type=”bool” Key=”false” CaseSensitive=”false” Length=”256″ MinLength=”0″ />
  <Property ID=”SerialNumber” Type=”int” Key=”false” CaseSensitive=”false” Length=”256″ MinLength=”0″ />
  <Property ID=”HostName” Type=”string” Key=”true” CaseSensitive=”false” Length=”256″ MinLength=”0″ />
  <Property ID=”NameServers” Type=”string” Key=”false” CaseSensitive=”false” Length=”256″ MinLength=”0″ />
  <Property ID=”MasterServers” Type=”string” Key=”false” CaseSensitive=”false” Length=”256″ MinLength=”0″ />
  <Property ID=”WINSServers” Type=”string” Key=”false” CaseSensitive=”false” Length=”256″ MinLength=”0″ />
  <Property ID=”Interval” Type=”int” Key=”false” CaseSensitive=”false” Length=”256″ MinLength=”0″ />
</ClassType>

My modest advice:

1. closely monitor instance space change every time you import a new MP (it can be done with a SQL Query). Frequent changes in a static environment means bad discovery processes.
2. define a custom rule to collect HS CPU Usage and report on it for average usage above 5%
3. override bad discovery scripts with more polite ones (as we did for ISA and DNS)
4. raise your voice and ask Microsoft for more testing efforts in MPs, patching and so on. We should concentrate on bringing value building monitoring blocks and reports for LOB apps not on fixing bugs in Microsoft code, shouldn’t we?