Instance Recovery Phases
The first phase of instance recovery is called cache recovery or rolling forward, and involves reapplying all of the changes recorded in the online redo log to the data files. Because rollback data is recorded in the online redo log, rolling forward also regenerates the corresponding undo segments.
instance recovery的第一个阶段是 cache recovery ,或者叫 rolling forward,包括重新将所有来自online redo log 的变更记录写入数据文件中.因为rollback数据是记录在redo log里面, rolling forward的数据也可以从undo里面重新生成.
Rolling forward proceeds through as many online redo log files as necessary to bring the database forward in time. After rolling forward, the data blocks contain all committed changes recorded in the online redo log files. These files could also contain uncommitted changes that were either saved to the data files before the failure, or were recorded in the online redo log and introduced during cache recovery.
Rolling Forward通过读取online redo log的数据将数据库forward到指定时间.在rolling forward以后所有已经commit的数据块都记录在online redo log里面.redo log也会保留所有uncommited的变更包含已经保存在数据文件里面的,或者保存在online redo里面而且在cache recovery中恢复的.
After the roll forward, any changes that were not committed must be undone. Oracle Database uses the checkpoint position, which guarantees that every committed change with an SCN lower than the checkpoint SCN is saved on disk. Oracle Database applies undo blocks to roll back uncommitted changes in data blocks that were written before the failure or introduced during cache recovery. This phase is called rolling back or transaction recovery.
roll forward以后,所有没有commit的数据必须回退.Oracle使用checkpoint的检查点信息. 即将所有小于SCN的变更写入数据文件在红.同时Oracle也使用undo中的数据来rollback所有没有commit的数据,这个阶段叫做rolling back 或者 transaction recovery
Figure 13-6 illustrates rolling forward and rolling back, the two steps necessary to recover from database instance failure.
Oracle Database can roll back multiple transactions simultaneously as needed. All transactions that were active at the time of failure are marked as terminated. Instead of waiting for the SMON process to roll back terminated transactions, new transactions can roll back individual blocks themselves to obtain the required data.
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