2.1.4.1 Log Format and Size

As with SSVM, the CVM dirty region log exists on a log subdisk in a mirrored volume.

A SSVM dirty region log has a recovery map and a single active map. A CVM dirty region log, however, has one recovery map and multiple active maps (one for each node in the cluster). Unlike SSVM, CVM places its recovery map at the beginning of the log.

The CVM dirty region log size is typically larger than a SSVM dirty region log, as it must accommodate active maps for all nodes in the cluster plus a recovery map. The size of each map within the dirty region log is one or more whole blocks. vxassist automatically takes care of allocating a sufficiently large dirty region log.

The log size depends on the volume size and the number of nodes. The log must be large enough to accommodate all maps (one map per node plus a recovery map). Each map should be one block long for each two gigabytes of volume size. For a two-gigabyte volume in a two-node cluster, a log size of five blocks (one block per map) should be sufficient; this is the minimum log size. A four-gigabyte volume in a four-node cluster requires a log size of ten blocks, and so on.