PostgreSQL wal 日志与规档 & backup 备份的有效性检测 - pg_waldump pg_verifybackup

作者

digoal

日期

2022-03-18

标签

PostgreSQL , pg_verifybackup , pg_waldump , 检测 , 有效性

使用 pg_waldump 检查wal文件、归档文件是否有损坏.
使用 pg_verifybackup 检查pg_basebackup备份的数据文件是否有损坏.

pg_waldump 检查wal文件是否有损坏的原理是检查checksum, 因为wal的checksum是强制开启的.

pg_verifybackup will use that information to invoke pg_waldump to parse those write-ahead log records. The --quiet flag will be used, so that pg_waldump will only report errors, without producing any other output.
While this level of verification is sufficient to detect obvious problems such as a missing file or one whose internal checksums do not match, they aren't extensive enough to detect every possible problem that might occur when attempting to recover.
For instance, a server bug that produces write-ahead log records that have the correct checksums but specify nonsensical actions can't be detected by this method.

https://www.postgresql.org/docs/13/app-pgverifybackup.html

src/backend/access/transam/xlogreader.c

/*  
 * CRC-check an XLOG record.  We do not believe the contents of an XLOG  
 * record (other than to the minimal extent of computing the amount of  
 * data to read in) until we've checked the CRCs.  
 *  
 * We assume all of the record (that is, xl_tot_len bytes) has been read  
 * into memory at *record.  Also, ValidXLogRecordHeader() has accepted the  
 * record's header, which means in particular that xl_tot_len is at least  
 * SizeOfXLogRecord.  
 */  
static bool  
ValidXLogRecord(XLogReaderState *state, XLogRecord *record, XLogRecPtr recptr)  
{  
        pg_crc32c       crc;  
        /* Calculate the CRC */  
        INIT_CRC32C(crc);  
        COMP_CRC32C(crc, ((char *) record) + SizeOfXLogRecord, record->xl_tot_len - SizeOfXLogRecord);  
        /* include the record header last */  
        COMP_CRC32C(crc, (char *) record, offsetof(XLogRecord, xl_crc));  
        FIN_CRC32C(crc);  
        if (!EQ_CRC32C(record->xl_crc, crc))  
        {  
                report_invalid_record(state,  
                                                          "incorrect resource manager data checksum in record at %X/%X",  
                                                          LSN_FORMAT_ARGS(recptr));  
                return false;  
        }  
        return true;  
}  

例子

cd $PGDATA/pg_wal  
pg_waldump -q -p ./ 000000010000000100000045 000000010000000100000076  
pg_waldump: fatal: error in WAL record at 1/47000028: invalid record length at 1/47000060: wanted 24, got 0   

wal文件会被重命名循环使用, 文件内容中可能还有残留信息, 没有初始化. 因此在使用pg_waldump读取时会报错, 属于正常现象.

postgres=# show wal_init_zero;  
 wal_init_zero   
---------------  
 off  
(1 row)  

使用pg_waldump检查归档文件不会有如上问题.

期望 PostgreSQL 增加什么功能?

PolarDB for PostgreSQL云原生分布式开源数据库

PostgreSQL 解决方案集合

德哥 / digoal's github - 公益是一辈子的事.