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What is a Postgres checkpoint?
A checkpoint is a point in the transaction log sequence at which all data files have been updated to reflect the information in the log. All data files will be flushed to disk.
What is checkpoint completion target?
Specifies the target of checkpoint completion as a fraction of total time between checkpoints. This spreads out the checkpoint writes while the system starts working towards the next checkpoint. The default of 0.5 means aim to finish the checkpoint writes when 50% of the next checkpoint is ready.
What is a WAL segment?
A WAL segment is a 16 MB file, by default, and it is internally divided into pages of 8192 bytes (8 KB).
What is Min_wal_size?
min_wal_size is a configuration parameter determining the minimum number of WAL files to retain for reuse at a future checkpoint . min_wal_size was introduced in PostgreSQL 9.5; together with max_wal_size it replaces checkpoint_segments .
What is Effective_cache_size?
effective_cache_size (integer) The effective_cache_size parameter estimates how much memory is available for disk caching by the operating system and within the database itself. The PostgreSQL query planner decides whether it’s fixed in RAM or not.
Where are WAL files?
WAL files are stored in $PGDATA/pg_wal (pg_wal was renamed from pg_xlog in Postgres v10). Typically these are 16 MB files with a 24-character filename made of hex numbers (0-9, A-F).
How does WAL work?
In a system using WAL, all modifications are written to a log before they are applied. Usually both redo and undo information is stored in the log. The purpose of this can be illustrated by an example. Imagine a program that is in the middle of performing some operation when the machine it is running on loses power.
Which is the unique ID of xlog record?
LSN (Log Sequence Number) of XLOG record represents the location where its record is written on the transaction log. LSN of record is used as the unique id of XLOG record. By the way, when we consider how database system recovers, there may be one question; what point does PostgreSQL start to recover from?
When to write the xlog record in PostgreSQL?
The answer is REDO point; that is, the location to write the XLOG record at the moment when the latest checkpoint is started (checkpoint in PostgreSQL is described in Section 9.7 ). In fact, the database recovery processing is strongly linked to the checkpoint processing and both of these processing are inseparable.
How are xlog records written into the Wal buffer?
XLOG records are written into the in-memory WAL buffer by change operations such as insertion, deletion, or commit action. They are immediately written into a WAL segment file on the storage when a transaction commits/aborts. (To be precise, the writing of XLOG records may occur in other cases. The details will be described in Section 9.5 .)