addvol commands associate the Data Change Map (DCM) to the data volume by default. This section describes how to associate DCMs to a data volume in an existing VVR configuration. For more information about the DCM object, see Data Change Map.
To associate a Data Change Map to a Data Volume
vxassist command enables you to associate a DCM to a new data volume or an existing data volume.
VVR mirrors the DCM by default. If
loglen is not specified,
vxassist calculates a suitable size for the DCM. For more information, see Determining the region size.
Note If you try to grow a volume that has a DCM, an error message warns you if the DCM is not large enough for the increased size. In this case, dissociate the DCM, grow the volume, and then associate a new DCM to the volume.
VVR calculates the DCM size based on the size of the volume. The default size of the DCM ranges from 4K to 256K depending on the size of the volume. However, you can specify the size of the DCM to a maximum of 2 MB. Internally, the DCM is divided into two maps: the active map and the replay map. Each bit in the DCM represents a contiguous number of blocks in the volume it is associated with, which is referred to as the region.
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The region size is calculated based on the volume size divided by half the DCM size in bits. The minimum region size is K 2 blocks or 32K.
The following table gives examples of the region sizes for volumes of different sizes in a non-CDS (Cross-Platform Data Sharing) disk group and a CDS disk group with the default DCM size and a user-specified DCM size of 2 MB.