解析XtraBackup备份MySQL的原理和过程

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!