![]() ![]() The record of the Heap2 manager relates to vacuuming, here it is in-page vacuum of a table from the system catalog (system objects are easily distinguished with a naked eye by a small number in rel): Postgres$ /usr/lib/postgresql/11/bin/pg_waldump -p /var/lib/postgresql/11/main/pg_wal -s 0/353927BC -e 0/353A7DFCĬertainly, some details can differ from one launch to another, but in this case we get the following. ![]() Let's look at WAL records using the familiar pg_waldump utility. The amount of data retrieved by the SELECT operator does not matter at all in this case, so one row is enough. Now let's perform creation of a table (CREATE TABLE AS SELECT) and remember the WAL location again. Note that the change of the level requires restarting the server. Student$ sudo pg_ctlcluster 11 main restart First we'll set the necessary level (to this end, we will also need to change another parameter - max_wal_senders). And if a failure occurs after completion of the operation, everything needed is already on disk and does not need logging. If a failure occurs while the operation is performed, the data that are already written remain invisible and do not violate the consistency rules. Instead, the data needed are immediately written to disk, and a new object is added to the system catalog and becomes visible at the transaction commit. To save space, the operations related to bulk data processing (such as CREATE TABLE AS SELECT or CREATE INDEX) are not WAL-logged. The minimum possible level is set by the value of wal_level = minimal and ensures only recovery after a failure. The wal_level parameter specifies the level, and each next level includes everything that gets into WAL of the preceding level plus something new. But once we have to maintain the log anyway, we can also adapt it to other tasks by adding some more information to it. The main WAL task is to ensure recovery after a failure. In this article (being the last in this series) we will discuss problems of WAL setup that are unaddressed yet: WAL levels and their purpose, as well as the reliability and performance of write-ahead logging. In the previous articles we already reviewed quite a few important settings that anyway relate to WAL. The size of the necessary WAL files and the recovery time are limited thanks to the checkpoint performed from time to time. Undo your changes and work out what you did wrong.So, we got acquainted with the structure of the buffer cache and in this context concluded that if all the RAM contents got lost due to failure, the write-ahead log (WAL) was required to recover. If PostgreSQL fails to start, you've done something wrong. This can be achieved by moving the pg_xlogĭirectory to another location (while the server is shut down, ofĬourse) and creating a symbolic link from the original location in the It is advantageous if the log is located on a different disk from the Add a symbolic link from the old location to the new location. ![]() If you simply want to move your write-ahead log directory to another location, you should: You should immediately dump yourĭata, run initdb, and reload. As per the documentation:Īfter running this command, it should be possible to start the server,īut bear in mind that the database might contain inconsistent data due Do not just continue using the damaged cluster. If you have to use it, you should REINDEX, dump your database(s), re-initdb, and reload your databases. Pg_resetxlog may corrupt your database, as the documentation warns. If you are unsure, ask on the pgsql-general mailing list or on. Should be used only as a last resort, when the server will not startĭo not run pg_resetxlog unless you know exactly what you are doing and why. Thisįunction is sometimes needed if these files have become corrupted. Pg_resetxlog clears the write-ahead log (WAL). You restored a file system level backup that omitted the pg_xlog directory due to a backup system configuration mistake (this happens more than you'd think, people think "it has log in the name so it must be unimportant I'll leave it out of the backups").įile-system corruption due to a hardware fault or hard drive failure damaged your data directory or potentially evenĪ PostgreSQL bug or operating system bug damaged the write-ahead logs (exceedingly rare). You deleted files you shouldn't have from pg_xlog Pg_resetxlog is a tool of last resort for getting your database running again after: ![]()
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