![]() One parameter that you need to set up when configuring all your PostgreSQL installations is the wal_level. The number of WAL files contained in pg_wal will depend on the value assigned to the parameter checkpoint_segments (or min_wal_size and max_wal_size, depending on the version) in the nf configuration file. They have a unique incremental name in the following format: “00000001 00000000 00000000”. These files have a default size of 16MB (you can change the size by altering the –with-wal-segsize configure option when building the server). ![]() The WALs are stored in the pg_wal directory (or pg_xlog in PostgreSQL versions < 10) under the data directory. The insert position is a Log Sequence Number (LSN), a byte offset into the logs, increasing with each new record. Each WAL record will be appended into a WAL file. Using WAL results in a significantly reduced number of disk writes because only the log file needs to be flushed to disk to guarantee that a transaction is committed, rather than every data file changed by the transaction.Ī WAL record will specify the changes made to the data, bit by bit. Any changes that have not been applied to the data pages can be redone from the REDO logs. REDO logs contain all changes made in the database, and they are used for replication, recovery, online backup, and point-in-time recovery ( PITR). The WALs are the REDO logs in PostgreSQL. ![]() Write-Ahead Log (WAL)Ī Write-Ahead Log is a standard method for ensuring data integrity, and it is automatically enabled by default. So, let’s quickly review a little bit about write-ahead logs. When talking about replication, we will be talking a lot about WALs. In this blog, we will review a little bit of the history of PostgreSQL’s built-in replication features and deep dive into how streaming replication works. It is a topic that you probably have seen over and over but never gets old. Knowledge of replication is a must for anybody managing databases. command_wrappers INFO : datavarehus3 : pg_receivexlog : unexpected termination of replication stream : ERROR : requested WAL segment 00000001000005F700000059 has already been removedĢ017 - 01 - 09 14 : 36 : 02, 097 barman. command_wrappers INFO : datavarehus3 : pg_receivexlog : starting log streaming at 5F7 / 59000000 ( timeline 1 )Ģ017 - 01 - 09 14 : 36 : 02, 096 barman. server INFO : Starting receive - wal for server datavarehus3Ģ017 - 01 - 09 14 : 36 : 02, 095 barman. wal_archiver INFO : No xlog segments found from streaming for datavarehus3. command_wrappers INFO : datavarehus3 : pg_receivexlog : disconnectedĢ017 - 01 - 09 14 : 36 : 02, 049 barman. command_wrappers INFO : datavarehus3 : pg_receivexlog : unexpected termination of replication stream : ERROR : requested WAL segment 00000001000005F700000059 has already been removedĢ017 - 01 - 09 14 : 35 : 01, 829 barman. command_wrappers INFO : datavarehus3 : pg_receivexlog : starting log streaming at 5F7 / 59000000 ( timeline 1 )Ģ017 - 01 - 09 14 : 35 : 01, 828 barman. server INFO : Starting receive - wal for server datavarehus3Ģ017 - 01 - 09 14 : 35 : 01, 796 barman. wal_archiver INFO : No xlog segments found from streaming for datavarehus. command_wrappers INFO : datavarehus : pg_receivexlog : disconnectedĢ017 - 01 - 09 14 : 34 : 02, 594 barman. ![]() command_wrappers INFO : datavarehus : pg_receivexlog : unexpected termination of replication stream : ERROR : requested WAL segment 00000001000005AB00000085 has already been removedĢ017 - 01 - 09 14 : 34 : 01, 592 barman. command_wrappers INFO : datavarehus : pg_receivexlog : starting log streaming at 5AB / 85000000 ( timeline 1 )Ģ017 - 01 - 09 14 : 34 : 01, 591 barman. server INFO : Starting receive - wal for server datavarehusĢ017 - 01 - 09 14 : 34 : 01, 590 barman. wal_archiver INFO : Found 1 xlog segments from file archival for datavarehus. command_wrappers INFO : datavarehus : pg_receivexlog : disconnectedĢ017 - 01 - 09 14 : 33 : 03, 318 barman. ![]()
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