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When a WAL segment is recycled, its ".ready" and ".done" status files get also automatically removed, however this is not done in a durable manner. Hence, in a subsequent crash, it could be possible that a ".ready" status file is still around with its corresponding segment already gone. If the backend reaches such a state, the archive command would most likely complain about a segment non-existing and would keep retrying, causing WAL segments to bloat pg_wal/, potentially making Postgres crash hard when running out of space. As status files are removed after each individual segment, using durable_unlink() does not completely close the window either, as a crash could happen between the moment the WAL segment is recycled and the moment its status files are removed. This has also some performance impact with the additional fsync() calls needed to make the removal in a durable manner. Doing the cleanup at recovery is not cost-free either as this makes crash recovery potentially take longer than necessary. So, instead, as per an idea of Stephen Frost, make the archiver aware of orphan status files and remove them on-the-fly if the corresponding segment goes missing. Removal failures follow a model close to what happens for WAL segments, where multiple attempts are done before giving up temporarily, and where a successful orphan removal makes the archiver move immediately to the next WAL segment thought as ready to be archived. Author: Michael Paquier Reviewed-by: Nathan Bossart, Andres Freund, Stephen Frost, Kyotaro Horiguchi Discussion: https://postgr.es/m/20180928032827.GF1500@paquier.xyz
PostgreSQL Database Management System ===================================== This directory contains the source code distribution of the PostgreSQL database management system. PostgreSQL is an advanced object-relational database management system that supports an extended subset of the SQL standard, including transactions, foreign keys, subqueries, triggers, user-defined types and functions. This distribution also contains C language bindings. PostgreSQL has many language interfaces, many of which are listed here: https://www.postgresql.org/download See the file INSTALL for instructions on how to build and install PostgreSQL. That file also lists supported operating systems and hardware platforms and contains information regarding any other software packages that are required to build or run the PostgreSQL system. Copyright and license information can be found in the file COPYRIGHT. A comprehensive documentation set is included in this distribution; it can be read as described in the installation instructions. The latest version of this software may be obtained at https://www.postgresql.org/download/. For more information look at our web site located at https://www.postgresql.org/.