一般情况下，在备份完成后，数据尚且不能用于恢复操作，因为备份的数据中可能会包含尚未提交的事务或已经提交但尚未同步至数据文件中的事务。因此，此时数据文件仍处理不一致状态。“准备”的主要作用正是通过回滚未提交的事务及同步已经提交的事务至数据文件也使得数据文件处于一致性状态。读取备份文件夹中的配置文件，然后innobackupex重做已提交事务，回滚未提交事务，之后数据就被写到了备份的数据文件(innodb文件)中，并重建日志文件。这一步隐式调用了2次xtrabackup –prepare。
语法： innobackupex --apply-log --use-memory=G /path/to/BACKUP-DIR/日期文件夹 例子： [root@linuxidc backup]# innobackupex --apply-log /backup/2016-09-04_18-01-55/ 160904 18:04:24 innobackupex: Starting the apply-log operation IMPORTANT: Please check that the apply-log run completes successfully. At the end of a successful apply-log run innobackupex prints "completed OK!". innobackupex version 2.4.4 based on MySQL server 5.7.13 Linux (x86_64) (revision id: df58cf2) xtrabackup: cd to /backup/2016-09-04_18-01-55 xtrabackup: This target seems to be not prepared yet. InnoDB: Number of pools: 1 xtrabackup: xtrabackup_logfile detected: size=8388608, start_lsn=(1626027) xtrabackup: using the following InnoDB configuration for recovery: xtrabackup: innodb_data_home_dir = . xtrabackup: innodb_data_file_path = ibdata1:12M:autoextend xtrabackup: innodb_log_group_home_dir = . xtrabackup: innodb_log_files_in_group = 1 xtrabackup: innodb_log_file_size = 8388608 xtrabackup: using the following InnoDB configuration for recovery: xtrabackup: innodb_data_home_dir = . xtrabackup: innodb_data_file_path = ibdata1:12M:autoextend xtrabackup: innodb_log_group_home_dir = . xtrabackup: innodb_log_files_in_group = 1 xtrabackup: innodb_log_file_size = 8388608 xtrabackup: Starting InnoDB instance for recovery. xtrabackup: Using 104857600 bytes for buffer pool (set by --use-memory parameter) InnoDB: PUNCH HOLE support available InnoDB: Mutexes and rw_locks use GCC atomic builtins InnoDB: Uses event mutexes InnoDB: GCC builtin __sync_synchronize() is used for memory barrier InnoDB: Compressed tables use zlib 1.2.3 InnoDB: Number of pools: 1 InnoDB: Using CPU crc32 instructions InnoDB: Initializing buffer pool, total size = 100M, instances = 1, chunk size = 100M InnoDB: Completed initialization of buffer pool InnoDB: page_cleaner coordinator priority: -20 InnoDB: Highest supported file format is Barracuda. InnoDB: The log sequence number 1626007 in the system tablespace does not match the log sequence number 1626027 in the ib_logfiles! InnoDB: Database was not shutdown normally! InnoDB: Starting crash recovery. InnoDB: Doing recovery: scanned up to log sequence number 1626027 (0%) InnoDB: Creating shared tablespace for temporary tables InnoDB: Setting file './ibtmp1' size to 12 MB. Physically writing the file full; Please wait ... InnoDB: File './ibtmp1' size is now 12 MB. InnoDB: 96 redo rollback segment(s) found. 1 redo rollback segment(s) are active. InnoDB: 32 non-redo rollback segment(s) are active. InnoDB: 5.7.13 started; log sequence number 1626027 xtrabackup: starting shutdown with innodb_fast_shutdown = 1 InnoDB: FTS optimize thread exiting. InnoDB: Starting shutdown... InnoDB: Shutdown completed; log sequence number 1626046 InnoDB: Number of pools: 1 xtrabackup: using the following InnoDB configuration for recovery: xtrabackup: innodb_data_home_dir = . xtrabackup: innodb_data_file_path = ibdata1:12M:autoextend xtrabackup: innodb_log_group_home_dir = . xtrabackup: innodb_log_files_in_group = 2 xtrabackup: innodb_log_file_size = 50331648 InnoDB: PUNCH HOLE support available InnoDB: Mutexes and rw_locks use GCC atomic builtins InnoDB: Uses event mutexes InnoDB: GCC builtin __sync_synchronize() is used for memory barrier InnoDB: Compressed tables use zlib 1.2.3 InnoDB: Number of pools: 1 InnoDB: Using CPU crc32 instructions InnoDB: Initializing buffer pool, total size = 100M, instances = 1, chunk size = 100M InnoDB: Completed initialization of buffer pool InnoDB: page_cleaner coordinator priority: -20 InnoDB: Setting log file ./ib_logfile101 size to 48 MB InnoDB: Setting log file ./ib_logfile1 size to 48 MB InnoDB: Renaming log file ./ib_logfile101 to ./ib_logfile0 InnoDB: New log files created, LSN=1626046 InnoDB: Highest supported file format is Barracuda. InnoDB: Log scan progressed past the checkpoint lsn 1626124 InnoDB: Doing recovery: scanned up to log sequence number 1626133 (0%) InnoDB: Doing recovery: scanned up to log sequence number 1626133 (0%) InnoDB: Database was not shutdown normally! InnoDB: Starting crash recovery. InnoDB: Removed temporary tablespace data file: "ibtmp1" InnoDB: Creating shared tablespace for temporary tables InnoDB: Setting file './ibtmp1' size to 12 MB. Physically writing the file full; Please wait ... InnoDB: File './ibtmp1' size is now 12 MB. InnoDB: 96 redo rollback segment(s) found. 1 redo rollback segment(s) are active. InnoDB: 32 non-redo rollback segment(s) are active. InnoDB: 5.7.13 started; log sequence number 1626133 xtrabackup: starting shutdown with innodb_fast_shutdown = 1 InnoDB: FTS optimize thread exiting. InnoDB: Starting shutdown... InnoDB: Shutdown completed; log sequence number 1626152 160904 18:04:27 completed OK!如果执行正确，其最后输出的几行信息通常如下：completed OK!