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

The Dirty Region Logging (DRL) feature is highly recommended to be used for resynchronization after a node failure. RAID 5 mirrors are not supported, as there is no way to ensure that both data centers have a complete copy of all the data. A FastResync map (also known as FastMirrorResync—FMR) is highly recommended to keep track of changes to a volume when mirror devices become detached (such as after a data center failure). FMR reduces the amount of data needing to be resynchronized after detached mirror devices are reattached. Without FMR, the volume manager can only perform a full resynchronization (that is, it cannot perform an incremental synchronization) when recovering from the failure of a mirror copy or loss of connectivity to a data center. This can have a large impact on performance and availability of the cluster if the disk groups are large. Both DRL and FMR are implemented as bitmaps and are stored together in a version 20 DCO volume that is attached to the “data” volume. Beginning with version 5.0, VxVM and CVM include a new feature, “site-awareness”, which simplifies configuring the disk volumes and mirroring in an Extended Cluster, by allowing the storage in each data center to be tagged with a site ID. A license to enable this feature is included with specific SG SMS A.02.00 product bundles.

If you are using CVM or CFS, the CVM Mirror Detachment Policy must be set to “Global”.

VxVM, CVM and CFS support up to 8 mirror copies, however increasing the number of mirror copies will adversely affect performance on disk writes.

In an Extended Cluster, the VxVM/CVM hot relocation feature can be counter productive. If the inter-site Fiber Channel links fail, or a complete site fails, this feature automatically relocates plexes to the same storage system in the surviving datacenter. This would result in a configuration in which both plexes of a volume are stored on subdisks of the same datacenter, compromising the ability to survive a data center failure at a later time. Therefore it is highly recommended that you disable VxVM’s hot relocation feature off. A convenient way to disable hot relocation for all VxVM/CVM volumes is to prevent the vxrelocd daemon from starting. This can be achieved by commenting out the entry (nohup vxrelocd root &) that invokes the vxrelocd daemon in the startup file (/sbin/rc2.d/S95vxvm-recover) on each node. You should verify these configuration changes after installing VxVM patches or upgrading to a new version of VxVM to ensure that hot relocation stays disabled. Note that preventing vxrelocd from starting will disable the hot relocation feature for all VxVM/CVM volumes on the system. The VxVM Administration Guide provides additional information on how to use the hot relocation feature in a more granular way.

Different CVM/CFS revisions have different limitations and requirements: