mysql 启动失败，数据恢复
Forcing InnoDB Recovery提供了6个等级的修复模式，需要注意的是值大于3的时候，会对数据文件造成永久的破坏，不可恢复。六个等级的介绍摘抄如下：
Forcing InnoDB Recovery|
1 (SRV_FORCE_IGNORE_CORRUPT)
Lets the server run even if it detects a corrupt page. Tries to make SELECT * FROM tbl_name jump over corrupt index records and pages, which helps in dumping tables.
2 (SRV_FORCE_NO_BACKGROUND)
Prevents the master thread and any purge threads from running. If a crash would occur during the purge operation, this recovery value prevents it.
3 (SRV_FORCE_NO_TRX_UNDO)
Does not run transaction rollbacks after crash recovery.
4 (SRV_FORCE_NO_IBUF_MERGE)
Prevents insert buffer merge operations. If they would cause a crash, does not do them. Does not calculate table statistics. This value can permanently corrupt data files. After using this value, be prepared to drop and recreate all secondary indexes.
5 (SRV_FORCE_NO_UNDO_LOG_SCAN)
Does not look at undo logs when starting the database: InnoDB treats even incomplete transactions as committed. This value can permanently corrupt data files.
6 (SRV_FORCE_NO_LOG_REDO)
Does not do the redo log roll-forward in connection with recovery. This value can permanently corrupt data files. Leaves database pages in an obsolete state, which in turn may introduce more corruption into B-trees and other database structures.
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使用方法如下，在mysql配置文件中，添加或修改以下配置的值
my.cnf|
[mysqld]
innodb_force_recovery = 1
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根据查到的博文提到的方法，我的修复步骤如下：
- 因为我无法启动mysql，所以首先要想办法启动mysql，然后dump数据。从innodb_force_recovery的值1开始尝试，看mysql能否在该修复模式下启动，不到万不得已，不要尝试值为4及以上。
- 在我这里，mysql在值为2时可以启动，这是stop掉数据库，然后备份数据
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sudo service mysql stop
mysqldump -u root -p --all-databases > all-databases.sql
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- 删除掉出错的数据文件
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mv ib_logfile0 ib_logfile0.bak
mv ib_logfile1 ib_logfile1.bak
mv ibdata1 ibdata1.bak
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- 启动mysql，然后从备份文件恢复数据
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sudo service mysql start
mysql -u root -p < all-databases.sql
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- 因为在修复模式下，在插入数据时报错，也就是说此时是不能写入数据的。所以就关闭掉了修复模式
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[mysqld]
innodb_force_recovery = 0
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restart mysql后，再次恢复数据
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sudo service mysql restart
mysql -u root -p --default-character-set=utf8 < all-databases.sql
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- 再次重启下mysql，现在mysql可以正常启动了，并且数据也回复成功。
