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 Understanding and Designing Serviceguard Disaster Tolerant Architectures Fifth Edition: > Chapter 2 Building an Extended Distance Cluster Using Serviceguard

Special requirements and recommendations for using LVM and SLVM in Extended Clusters
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  • For LVM volume groups using Mirrordisk/UX mirroring, it is necessary to activate the volume groups in the package without LVM quorum (using the vgchange –q n option), after failures involving the loss of one of the disk arrays. This is because LVM won’t allow a volume group to be activated unless more than 50 percent of the disks in the volume group are present.

NOTE: Activating LVM volume groups without quorum may cause data loss in some rare scenarios involving multiple failures, so this option should be used carefully. In some cases, it may be better to require LVM quorum in the package configuration, which will result in the package failing to startup if quorum is not present. Or, you could disable LVM quorum for the packages in the Recovery data center, and disable the packages from automatically starting in the Recovery data center. Then for either case, an operator can check the status of the data in the volume group in the recovery site and determine whether or not it is safe to proceed and start the package without LVM quorum.

  • If MirrorDisk/UX is used with LVM volume groups, Mirror Write Cache (MWC) must be set to ON for all mirrored logical volumes. This will allow for resynchronization of stale extents after failure of a mirror copy or node crash, rather than requiring a full resynchronization. For SLVM (concurrently activated) volume groups, Mirror Write Cache must be set to OFF for mirrored logical volumes (e.g., NOMWC must be used), and Mirror Consistency Recovery must be set to ON. This means that a full resynchronization is required for shared volume group mirrors after a node crash, which can have a significant impact on recovery time. Until a resynchronization has completed, data integrity is vulnerable to any additional failures. You must ensure that the mirror copies reside in different data centers, so it is recommended to configure physical volume groups for the disk devices in each data center, and to use Group Allocation Policy for all mirrored logical volumes.

  • Due to the maximum of 3 images (1 original image plus two mirror copies) allowed in MirrorDisk/UX, if JBODs are used for application data, only one data center can contain JBODs while the other data center must contain disk arrays with hardware mirroring. The data center containing the JBODs must mirror the data between the local disks by using MirrorDisk/UX mirroring. While LVM2 on HP-UX 11i v3 supports up to 7 mirror images, we recommend against having more than 3 mirror copies, as having 3 or more mirror copies will adversely affect performance on disk writes. VxVM, CVM and CFS support up to 32 mirror copies, so it is possible to have JBODS in both data centers, however increasing the number of mirror copies will adversely affect performance on disk writes.



Configuration requirements for all Extended Distance Clusters
  		 


Special requirements and recommendations for using VxVM, CVM and CFS in Extended Clusters  
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