The aim of WAL is to ensure that the log is written before database records are altered, but this can be subverted by disk drives that falsely report a successful write to the kernel, when in fact they have only cached the data and not yet stored it on the disk. This can be achieved by moving the pg_wal directory to another location (while the server is shut down, of course) and creating a symbolic link from the original location in the main data directory to the new location. It is advantageous if the log is located on a different disk from the main database files.
The numbers do not wrap, but it will take a very, very long time to exhaust the available stock of numbers. Segment files are given ever-increasing numbers as names, starting at 000000010000000000000001. The log record headers are described in access/xlogrecord.h the record content is dependent on the type of event that is being logged. Each segment is divided into pages, normally 8 kB each (this size can be changed via the -with-wal-blocksize configure option). WAL logs are stored in the directory pg_wal under the data directory, as a set of segment files, normally each 16 MB in size (but the size can be changed by altering the -wal-segsize initdb option). Values can be compared to calculate the volume of WAL data that separates them, so they are used to measure the progress of replication and recovery. LSN values are returned as the datatype pg_lsn. The insert position is described by a Log Sequence Number ( LSN) that is a byte offset into the logs, increasing monotonically with each new record. WAL records are appended to the WAL logs as each new record is written. WAL is automatically enabled no action is required from the administrator except ensuring that the disk-space requirements for the WAL logs are met, and that any necessary tuning is done (see Section 30.5). 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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