Understanding and Designing Serviceguard Disaster Recovery Architectures

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 automatic start of packages in the
Recovery data center. In both situations, an operator can check the status of the data in the
volume group in the recovery site and determine whether 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) and Mirror
Consistency Recovery (MCR) must be set to ON for all mirrored logical volumes. This allows
resynchronization of stale extents after failure of a mirror copy or node crash, rather than
requiring a full resynchronization. For SGeRAC clusters, the required settings for MWC and
MCR depend upon whether your Oracle supports resilvering, the LVM volume group version
and the type of Oracle files that will reside on the volume. See Table 6 (page 56) for the
required settings. If both MWC and MCR are set to OFF, Oracle performs the resilvering on
the datafiles, based on the redo logs. If MWC is OFF and MCR is ON it 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. If MWC is ON and MCR is ON, it means that an
incremental resynchronization (of only the changed extents) is required for shared volume
group mirrors after a node crash. Until a resynchronization is complete, 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. Note
that when using the lvdisplay command to view the MWC and MCR settings currently
configured for a logical volume, these values are displayed in the Consistency Recovery field
(a value of MWC indicates MWC=ON and MCR=ON, a value of NOMWC indicates
MWC=OFF and MCR=ON, and a value of NONE indicates MWC=OFF and MCR=OFF).
Table 6 Required Mirror Write Cache and Mirror Consistency Recovery settings for SLVM with RAC
Oracle FilesMirror Consistency
Recovery Setting
Mirror Write
Cache Setting
SLVM Volume
Group Version
Oracle RAC Supports
Resilvering
1
Datafiles, control files, redo
files
ONOFFPrior to 2.1No
Datafiles, control files, redo
files
ONOFF or ON2.1 or laterNo
DatafilesOFFOFFPrior to 2.1Yes
Control files, redo filesONOFF
DatafilesOFFOFF2.1 or laterYes
Control files, redo filesONOFF or ON
1
Currently, no version of Oracle RAC supports resilvering; contact Oracle to determine whether
your version of Oracle supports resilvering.
• Due to the maximum of three images (one 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 Mirrordisk/UX with LVM version 2.0 on HP-UX 11i v3 supports
up to five mirror images, HP recommends that having no more than three mirror copies, as
having three or more mirror copies will adversely affect performance on disk writes.
• If using Oracle Automatic Storage Management (ASM) over SLVM, the SLVM volume groups
and logical volumes used for the ASM devices have the same configuration requirements as
a plain SLVM configuration. ASM over SLVM is supported only with SGeRAC configurations.
56 Extended Distance Cluster Configurations