ubifs- Master Node

原创译文：

主节点（master node）存储在逻辑擦除块 LEB1 和 LEB2。
LEBs 逻辑擦除块是UBI 创建抽象出来的概念。UBI 建立物理擦除块（PEBs）和逻辑擦除块（LEBs）的映射关系，因此，LEB1 和 LEB2 可能映射到flash（确切的说是UBI device）的任何位置，但是，UBI会一直记录他们的位置。
使用两个擦除块是为了保存两份master node，用于恢复使用。

The master node stores the position of all on-flash structures that are not at fixed logical positions. The master node itself is written repeatedly to logical eraseblocks (LEBs) one and two.

LEBs are an abstraction created by UBI. UBI maps physical eraseblocks (PEBs) to LEBs, so LEB one and two can be anywhere on the flash media (strictly speaking, the UBI device), however UBI always records where they are.

Two eraseblocks are used in order to keep two copies of the master node. This is done for the purpose of recovery, because there are two situations that can cause a corruptor missing master node.

The first is that there could be a loss of power at the same instant that the master node is being written. The second is that there could be degradation or corruption of the flash media itself.

In the first case, recovery is possible because the previous version of the master node can be used. In the second case, recovery is not possible because it cannot be determined reliably what is a valid master node version.

In that latter case, a userspace utility program would be needed to analyze all the nodes on the media and attempt to fix or recreate corrupt or missingnodes.

Having two copies of the master node makes it possible to determine which situation has arisen, and respond accordingly.

Master Node 保存在Master Area， Master node 指向Wandering Tree的 root 节点。