HP serviceguard t2808-90006 manual Chapter

Page 81

Configuring your Environment for Software RAID

Configuring the Package Control Script and RAID Configuration File

Now consider an XDC configuration such as that shown in Figure 1-3 (DWDM links between data centers). If DC1 fails such that links A and B both fail simultaneously, and DC1's connection to the Quorum Server fails at the same time, Serviceguard ensures that DC2 survives and the package fails over and runs with DC2 local storage.

But if DC1's links A and B fail, and later DC1's link to the Quorum Server fails, then both sets of nodes (DC1 and DC2) will try to obtain the cluster lock from the Quorum Server. If the Quorum server chooses DC1 (which is about to experience complete site failure), then the entire cluster will go down.

But if the Quorum Server chooses DC2 instead, then the application running on DC1 will not be able to write to the remote storage but will continue to write to its local (DC1) storage until site failure occurs (at t3). If the network is set up in such a way that the application cannot communicate with its clients under these circumstances, the clients will not receive any acknowledgement of these writes. HP recommends you configure the network such that when links between the sites fail, the communication links to the application clients also go down.

If the network is configured to prevent the application from communicating with its clients under these circumstances, the clients will not receive any acknowledgement of these writes and after the failover will re-transmit them, and the writes will be committed and acknowledged at DC2. This is the desired outcome; HP recommends you configure the network such that when links between the sites fail, the communication links to the application clients are also shut down.

In the case of an XDC configuration such as that shown in Figure

1-4, there is an additional variable in the possible failure scenarios. Instead of a DWDM link, in this configuration there are two separate LAN and FC links which can experience failure independent of each other. If the network links between the sites fail within a very short period (on the order of 1 second) after t1 (after the storage links had failed), the XDC software on DC1 will not have time to inform the XDC on DC2 of the failure. So DC2 assumes that there were no updates after t1, but there may have been.

When this scenario occurs, disk writes continue on DC1 until t3. In this case, the effective value of the RPO_TARGET parameter is greater than the expected value of 0.

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Contents Page Legal Notices Contents Disaster Scenarios and Their Handling Managing an MD Device Contents Contents Editions and Releases Printing HistoryHP Printing Division Document Organization Intended AudiencePage Related Page Disaster Tolerance Evaluating the Need for Disaster Tolerance Evaluating the Need for Disaster Tolerance High Availability Architecture What is a Disaster Tolerant Architecture?Node 1 fails Pkg B Client ConnectionsDisaster Tolerant Architecture Extended Distance Clusters Understanding Types of Disaster Tolerant ClustersFrom both storage devices Extended Distance Cluster Two Data Center Setup Benefits of Extended Distance Cluster Cluster Extension CLX Cluster CLX for Linux Serviceguard Cluster Shows a CLX for a Linux Serviceguard cluster architectureBenefits of CLX Differences Between Extended Distance Cluster and CLX Continental Cluster New York Cluster Los Angeles ClusterData Cent er a Data Center B Continental ClusterBenefits of Continentalclusters Continental Cluster With Cascading Failover Comparison of Disaster Tolerant SolutionsAttributes Extended Distance Comparison of Disaster Tolerant Cluster SolutionsContinentalclusters Cluster HP-UX onlyUnderstanding Types of Disaster Tolerant Clusters Understanding Types of Disaster Tolerant Clusters Understanding Types of Disaster Tolerant Clusters WAN EVA Protecting Nodes through Geographic Dispersion Disaster Tolerant Architecture GuidelinesOff-line Data Replication Protecting Data through ReplicationPhysical Data Replication On-line Data ReplicationDisadvantages of physical replication in hardware are Advantages of physical replication in hardware areAdvantages of physical replication in software are Logical Data Replication Disadvantages of physical replication in software areDisadvantages of logical replication are Ideal Data Replication Using Alternative Power SourcesPower Circuit 1 node Alternative Power SourcesData Center a Node 3 Power Circuit Creating Highly Available NetworkingDisaster Tolerant Wide Area Networking Disaster Tolerant Local Area NetworkingDisaster Tolerant Cluster Limitations Managing a Disaster Tolerant Environment Manage it in-house, or hire a service?How is the cluster maintained? Additional Disaster Tolerant Solutions Information Building an Extended Distance Dwdm Types of Data Link for Storage NetworkingTwo Data Center and Quorum Service Location Architectures Two Data Center and Quorum Service Location Architectures Server Two Data Centers and Third Location with Dwdm and QuorumTwo Data Center and Quorum Service Location Architectures Rules for Separate Network and Data Links Guidelines on Dwdm Links for Network and Data Guidelines on Dwdm Links for Network and Data Guidelines on Dwdm Links for Network and Data Chapter Configuring your Environment Understanding Software RAID Installing XDC Installing the Extended Distance Cluster SoftwareSupported Operating Systems Prerequisites# rpm -Uvh xdc-A.01.00-0.rhel4.noarch.rpm Verifying the XDC InstallationInstalling the Extended Distance Cluster Software Configuring the Environment Configuring the Environment Configuring the Environment Setting the Value of the Link Down Timeout Parameter Configuring Multiple Paths to StorageCluster Reformation Time and Timeout Values Http//docs.hp.com Using Persistent Device NamesTo Create and Assemble an MD Device Creating a Multiple Disk Device# mdadm -A -R /dev/md0 /dev/hpdev/sde1 /dev/hpdev/sdf1 Chapter Linux #RAIDTAB= # MD RAID Commands Creating and Editing the Package Control Scripts To Create a Package Control ScriptTo Edit the Datarep Variable To Edit the Xdcconfig File parameter To Configure the RAID Monitoring ServiceEditing the raid.conf File Cases to Consider when Setting Rpotarget RPO Target Definitions Chapter Multipledevices and Componentdevices Raidmonitorinterval Configuring your Environment for Software RAID What happens when this disaster occurs Recovery ProcessDisaster Scenario Disaster Scenarios and Their Handling Disaster Scenarios and Their Handling# mdadm --remove /dev/md0 # mdadm -add /dev/md0 Dev/hpdev/mylink-sdf P1 uses a mirror md0 Run the following command to S2 is non-current by less # cmrunpkg packagename Execute the commands that With md0 consisting of only N1, for example Becomes accessible from N2 Center Disaster Scenarios and Their Handling Managing an MD Device Cat /proc/mdstat Viewing the Status of the MD DeviceExample A-1 Stopping the MD Device /dev/md0 Stopping the MD DeviceExample A-2 Starting the MD Device /dev/md0 Starting the MD Device# udevinfo -q symlink -n sdc1 Removing and Adding an MD Mirror Component Disk# mdadm --remove /dev/md0 /dev/hpdev/sde Adding a Mirror Component DeviceIndex 104
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