1 maps 更新

1.1 更新规矩

Because cluster map changes may be frequent, as in a very large system where OSDs failures and recoveries are the norm, updates are distributed as incremental maps(增量更新): small messages describing the differences between two successive epochs. In most cases, such updates simply state that one or more OSDs have failed or recovered, although in general they may include status changes for many devices, and multiple updates may be bundled together to describe the difference between distant map epochs. Because cluster map cha