WAL 日志我们可以做什么？ ## Part 01 WAL 日志简介 www.highgo.com ## Write Ahead Log Files - WAL 日志一般存储在$PGDATA/pg_xlog内，他们一般以类似于0000000100000002000000D4的文件存储。 ![Image](/uploads/documents/d/5/8/a/d58a5f37b8956629f5ab2ca83436c8eb/p4_1

pg_ident.conf | 10 global | 11 pg_clog | 12 pg_snapshots | 13 pg_multixact | 14 PG_VERSION | 15 pg_xlog | 16 pg_hba.conf | 17 pg_stat_tmp | 18 #### 9.25. System Information Functions Table 9-57 shows ## wal_keep_segments(integer) Specifies the minimum number of past log file segments kept in the pg_xlog directory, in case a standby server needs to fetch them for streaming replication. Each segment is archive, if WAL archiving is in use.) This sets only the minimum number of segments retained in pg_xlog; the system might need to retain more segments for WAL archival or to recover from a checkpoint.

pg_ident.conf | 10 global | 11 pg_clog | 12 pg_snapshots | 13 pg_multixact | 14 PG_VERSION | 15 pg_xlog | 16 pg_hba.conf | 17 pg_stat_tmp | 18 pg_subtrans | 19 (19 rows) #### 9.25. System Information ## wal_keep_segments(integer) Specifies the minimum number of past log file segments kept in the pg_xlog directory, in case a standby server needs to fetch them for streaming replication. Each segment is archive, if WAL archiving is in use.) This sets only the minimum number of segments retained in pg_xlog; the system might need to retain more segments for WAL archival or to recover from a checkpoint.

pg_ident.conf | 10 global | 11 pg_clog | 12 pg_snapshots | 13 pg_multixact | 14 PG_VERSION | 15 pg_xlog | 16 pg_hba.conf | 17 pg_stat_tmp | 18 pg_subtrans | 19 (19 rows) #### 9.25. System Information ## wal_keep_segments(integer) Specifies the minimum number of past log file segments kept in the pg_xlog directory, in case a standby server needs to fetch them for streaming replication. Each segment is archive, if WAL archiving is in use.) This sets only the minimum number of segments retained in pg_xlog; the system might need to retain more segments for WAL archival or to recover from a checkpoint.

pg_ident.conf | 10 global | 11 pg_clog | 12 pg_snapshots | 13 pg_multixact | 14 PG_VERSION | 15 pg_xlog | 16 pg_hba.conf | 17 pg_stat_tmp | 18 pg_subtrans | 19 (19 rows) #### 9.25. System Information wal_keep_segments(integer) Specifies the minimum number of past log file segments kept in the pg_xlog directory, in case a standby server needs to fetch them for streaming replication. Each segment is archive, if WAL archiving is in use.) This sets only the minimum number of segments retained in pg_xlog; the system might need to retain more segments for WAL archival or to recover from a checkpoint.

## wal_keep_segments(integer) Specifies the minimum number of past log file segments kept in the pg_xlog directory, in case a standby server needs to fetch them for streaming replication. Each segment is archive, if WAL archiving is in use.) This sets only the minimum number of segments retained in pg_xlog; the system might need to retain more segments for WAL archival or to recover from a checkpoint. Point-in-Time Recovery (PITR) At all times, PostgreSQL maintains a write ahead log (WAL) in the pg_xlog/ subdirectory of the cluster's data directory. The log records every change made to the database's

## wal_keep_segments(integer) Specifies the minimum number of past log file segments kept in the pg_xlog directory, in case a standby server needs to fetch them for streaming replication. Each segment is archive, if WAL archiving is in use.) This sets only the minimum number of segments retained in pg_xlog; the system might need to retain more segments for WAL archival or to recover from a checkpoint. Point-in-Time Recovery (PITR) At all times, PostgreSQL maintains a write ahead log (WAL) in the pg_xlog/ subdirectory of the cluster's data directory. The log records every change made to the database's

wal_keep_segments(integer) Specifies the minimum number of past log file segments kept in the pg_xlog directory, in case a standby server needs to fetch them for streaming replication. Each segment is archive, if WAL archiving is in use.) This sets only the minimum number of segments retained in pg_xlog; the system might need to retain more segments for WAL archival or to recover from a checkpoint. Point-In-Time Recovery (PITR) At all times, PostgreSQL maintains a write ahead log (WAL) in the pg_xlog/ subdirectory of the cluster's data directory. The log records every change made to the database's

Point-In-Time Recovery (PITR) At all times, PostgreSQL maintains a write ahead log (WAL) in the pg_xlog/ subdirectory of the cluster's data directory. The log describes every change made to the database's executed might look like this: test ! -f /mnt/server/archivedir/00000001000000A900000065 && cp pg_xlog/00000001000000A9000 A similar command will be generated for each new file to be archived. The archive resolved reasonably quickly. The pg_xlog/ directory will continue to fill with WAL segment files until the situation is resolved. (If the filesystem containing pg_xlog/ fills up, PostgreSQL will do a PANIC

Point-In-Time Recovery (PITR) At all times, PostgreSQL maintains a write ahead log (WAL) in the pg_xlog/ subdirectory of the cluster's data directory. The log describes every change made to the database's executed might look like this: test ! -f /mnt/server/archivedir/00000001000000A900000065 && cp pg_xlog/00000001000000A9 A similar command will be generated for each new file to be archived. The archive resolved reasonably quickly. The pg_xlog/ directory will continue to fill with WAL segment files until the situation is resolved. (If the filesystem containing pg_xlog/ fills up, PostgreSQL will do a PANIC