pg_resetxlog、pg_resetwal工具的三个用法

pg_resetxlog、pg_resetwal工具的两个用法

 

pg_resetwal — reset the write-ahead log and other control information of a PostgreSQL database cluster

pg_resetwal [ --force | -f ] [ --dry-run | -n ] [option...] [ --pgdata | -D ] datadir

 

一、PostgreSQL恢复pg_control文件：

1.需要下面四个参数：
-x XID set next transaction ID
在pg_clog下面，找到最大的文件编号，+1 后面跟上5个0
如：0000

-x = 0x000100000

-m MXID set next and oldest multitransaction ID
在pg_multixact/offsets下面，找到最大的文件编号，+1 后面跟上4个0
如：0000
-m = 0x00010000

-O OFFSET set next multitransaction offset
在pg_multixact/members下面，找到最大的文件编号，+1 后面跟上4个0
如：0000
-m = 0x00010000

-l XLOGFILE force minimum WAL starting location for new transaction log
找到pg_wal下面最新的日志文件，编号+1，然后分别去时间线、高32位、低32位：
如：000000010000000000000002
那么最新的日志文件就是000000010000000000000003
那么参数为：
-l 000000010000000000000003

2.执行恢复：
1）touch pg_control
2）pg_resetxlog -x 0x000100000 -m 0x00010000 -O 0x00010000 -l 000000010000000000000003 -f $PGDATA

当然，-m参数如果报错，也可以不要：
pg_resetxlog -x 0x000100000 -O 0x00010000 -l 000000010000000000000003 -f ￥PGDATA

 

二、安全清理不必要的日志文件：
1）cd $PGDATA/pg_xlog/
2）pg_ctl stop -D $PGDATA -m fast
3）pg_controldata记录清理前的信息，并记录：NextXID NextOID给下面使用
4）pg_resetxlog -o 24584 -x 1745 -f $PGDATA
5）查看清理后大小
du -sh

 

三、使用pg_resetxlog来重置事务ID来访问被修改的数据

例如删除数据的xid为100，那么我们回退到99，那么删除到操作还不可见，因此就能看到被删除的数据，但是删除是已经发生的，当我们提升xid到100时，删除就生效，你将无法访问到删除的数据。

被重置的xid之后的操作还是存在，无法抹除。当在xid为99时，我们再插入一条数据，那么这个时候访问表，我们将得到原来删除了表，在插入一条记录的情况。删除和插入将在一个xid下。

 

因此，使用重置xid的方式，我们也必须在重置之后，将现在的表备份出来，简单方法是create test_old  as select * from test;的方式来做。因为随着xid的增长，误操作也会被重现。

参考我之前的：https://www.cnblogs.com/kuang17/p/10615164.html

 

四、参数详解：

-x：
A safe value for the next transaction ID (-x) can be determined by looking for the numerically
largest file name in the directory pg_clog under the data directory, adding one, and then multiplying by 1048576. Note that the file names are in hexadecimal. It is usually easiest to specify the option value in hexadecimal too. For example, if 0011 is the largest entry in pg_clog, -x 0x1200000 will work (five trailing zeroes provide the proper multiplier).

-m：
A safe value for the next multitransaction ID (-m) can be determined by looking for the numerically largest file name in the directory pg_multixact/offsets under the data directory, adding one,and then multiplying by 65536. As above, the file names are in hexadecimal, so the easiest way to do this is to specify the option value in hexadecimal and add four zeroes

-O：
A safe value for the next multitransaction offset (-O) can be determined by looking for the numerically largest file name in the directory pg_multixact/members under the data directory, adding one, and then multiplying by 65536. As above, the file names are in hexadecimal, so the easiest way to do this is to specify the option value in hexadecimal and add four zeroes

-l：
The WAL starting address (-l) should be larger than any WAL segment file name currently existing
in the directory pg_xlog under the data directory. These names are also in hexadecimal and have
three parts. The first part is the “timeline ID” and should usually be kept the same. Do not choose a value larger than 255 (0xFF) for the third part; instead increment the second part and reset the third part to 0. For example, if 00000001000000320000004A is the largest entry in pg_xlog, -l 0x1,0x32,0x4B will work; but if the largest entry is 000000010000003A000000FF, choose -l 0x1,0x3B,0x0 or more.

https://www.postgresql.org/docs/12/app-pgresetwal.html