XtraBackup是percona公司提供的开源工具，以热备Innodb表著称而被广泛采用。
XtraBackup对 Innodb的备份之所以是热备，无需锁表，是基于Innodb自身的崩溃恢复机制，它首先复制所有的Innodb数据文件，这样复制出来的文件肯定是不 一致的，然后对每个文件进行崩溃恢复处理，最终达到一致。就和MySQL在启动Innodb的时候一样，会通过比较数据文件头和redo log文件头信息来检查数据是否是一致的，如果不一致就尝试通过前滚(把redo log中所有提交的事务写入数据文件)和回滚(从数据文件中撤销所有redo log中未提交的事务引起的修改)来使数据达到最终一致。
XtraBackup在启动的时候会记录一个LSN(log sequence number)，然后就把所有的Innodb数据文件复制出来，这样复制出来的数据文件是不一致的，但是XtraBackup会在后台运行一个进程把所有 对redo log file的修改记录下来，只要有了这个数据，就能进行崩溃恢复。只所以要额外记录下来，是因为MySQL自身的redo log file是可重用的。
以上的操作是由xtrabackup二进制程序(比如xtrabackup_55)完成的，如果使用 innobackupex 脚本，刚才的步骤完成以后，innobackupex就会去备份MyISAM表和.frm文件，这时要保证数据的一致性就会先锁表了，通过FLUSH TABLES WITH READ LOCK命令锁表然后把文件复制出来，再释放掉这个锁。
在恢复数据的时候，要经过 prepare(recovery)和restore两个步骤。在prepare结束以后，Innodb的表恢复到了复制Innodb文件结束的时间点， 这个时间点也就是锁表复制MyISAM表的起点，所以最终数据是一致的。一般我们在恢复的时候执行两次prepare，是因为第二次prepare会帮助 我们生成redo log文件，从而加快MySQL数据库启动的速度。
我们再来看一下实际备份的日志来理解这个过程：
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110701 03 : 29 : 13 innobackupex: Starting ibbackup with command: xtrabackup_55 -- defaults-file="/home/mysql/3306/my.cnf" --backup --suspend-at-end --log-stream --target-dir=./
innobackupex: Waiting for ibbackup (pid = 22334 ) to suspend
innobackupex: Suspend file ' /home/mysql/3306/data/xtrabackup_suspended '
xtrabackup: suspend - at - end is enabled.
xtrabackup: uses posix_fadvise().
xtrabackup: cd to / home / mysql / 3306 / data
xtrabackup: Target instance is assumed as followings.
xtrabackup: innodb_data_home_dir = / home / mysql / 3306 / data
xtrabackup: innodb_data_file_path = ibdata1:512M:autoextend
xtrabackup: innodb_log_group_home_dir = / home / mysql / 3306 / redolog
xtrabackup: innodb_log_files_in_group = 3
xtrabackup: innodb_log_file_size = 134217728
110701 3 : 29 : 13 InnoDB: Using Linux native AIO
110701 3 : 29 : 13 InnoDB: Warning: allocated tablespace 268 , old maximum was 0
xtrabackup: Stream mode.
>> log scanned up to ( 2371741708 )
110701 03 : 29 : 15 innobackupex: Continuing after ibbackup has suspended
innobackupex: Starting to backup InnoDB tables and indexes
innobackupex: from original InnoDB data directory ' /home/mysql/3306/data '
innobackupex: Backing up as tar stream ' ibdata1 '
>> log scanned up to ( 2371741708 )
>> log scanned up to ( 2371742105 )
>> log scanned up to ( 2371742105 )
innobackupex: Backing up file ' /home/mysql/3306/data/test/t.ibd '
>> log scanned up to ( 2371742115 )
innobackupex: Backing up files ' /home/mysql/3306/data/banping/*.ibd ' ( 16 files)
......
110701 03 : 29 : 35 innobackupex: Connected to database with mysql child process (pid = 22630 )
>> log scanned up to ( 2371742526 )
110701 03 : 29 : 39 innobackupex: Starting to lock all tables...
>> log scanned up to ( 2371742526 )
>> log scanned up to ( 2371742526 )
110701 03 : 29 : 51 innobackupex: All tables locked and flushed to disk
110701 03 : 29 : 51 innobackupex: Starting to backup .frm, .MRG, .MYD, .MYI,
innobackupex: .TRG, .TRN, .ARM, .ARZ, .CSM, .CSV and .opt files in
innobackupex: subdirectories of ' /home/mysql/3306/data '
innobackupex: Backing up files ' /home/mysql/3306/data/banping/*.{frm,MYD,MYI,MRG,TRG,TRN,ARM,ARZ,CSM,CSV,opt,par} ' ( 17 files)
innobackupex: Backing up file ' /home/mysql/3306/data/test/t.frm '
......
110701 03 : 29 : 53 innobackupex: Finished backing up .frm, .MRG, .MYD, .MYI, .TRG, .TRN, .ARM, .ARZ, .CSV, .CSM and .opt files
innobackupex: Resuming ibbackup
xtrabackup: The latest check point ( for incremental): ' 2371742526 '
>> log scanned up to ( 2371742526 )
xtrabackup: Transaction log of lsn ( 2371741708 ) to ( 2371742526 ) was copied.
110701 03 : 29 : 55 innobackupex: All tables unlocked
110701 03 : 29 : 55 innobackupex: Connection to database server closed
innobackupex: Backup created in directory ' /home/mysql/backup/data/3306 '
innobackupex: MySQL binlog position: filename ' bin.000014 ' , position 309836330 mysql,information_schema,performance_schema
innobackupex: MySQL slave binlog position: master host '' , filename '' , position
innobackupex: You must use - i ( -- ignore-zeros) option for extraction of the tar stream.
110701 03 : 29 : 55 innobackupex: completed OK!
