Statistical reports analyzed from a number of perspectives can be useful not only for grasping the usual database operation but also as a hint for performance improvement. The report output by pgBadger has the following features. By analyzing the log information graphed, it is much more prospective for people to see than the raw log data.
The PostgreSQL buffer manager works very efficiently. In this chapter, the PostgreSQL buffer manager is described.
The first section provides an overview and the subsequent sections describe the following topics: Buffer manager structure How the buffer manager works Ring buffer Fig.
Relations between buffer manager, storage, and backend processes. Overview This section introduces key concepts required to facilitate descriptions in the subsequent sections. Buffer Manager Structure The PostgreSQL buffer manager comprises a buffer table, buffer descriptors, and buffer pool, which are described in the next section.
The buffer pool layer stores data file pages, such as tables and indexes, as well as freespace maps and visibility maps. The buffer pool is an array, i.
The fork numbers of tables, freespace maps and visibility maps are defined in 0, 1 and 2, respectively. The storage manager must be able to cope anyway. How a Backend Process Reads Pages This subsection describes how a backend process reads a page from the buffer manager Fig.
How a backend reads a page from the buffer manager. When a backend process modifies a page in the buffer pool e.
Page Replacement Algorithm When all buffer pool slots are occupied but the requested page is not stored, the buffer manager must select one page in the buffer pool that will be replaced by the requested page.
Typically, in the field of computer science, page selection algorithms are called page replacement algorithms and the selected page is referred to as a victim page. Research on page replacement algorithms has been ongoing since the advent of computer science; thus, many replacement algorithms have been proposed previously.
Flushing Dirty Pages Dirty pages should eventually be flushed to storage; however, the buffer manager requires help to perform this task. In PostgreSQL, two background processes, checkpointer and background writer, are responsible for this task.
If you want to know more details, refer to this discussion on the pgsql-ML and this article. Buffer manager's three-layer structure.
The buffer pool is an array. Each slot stores a data file pages.Since its uses write-ahead logging and distributed configuration, it can provide fault-tolerance and quick recovery from individual server failures. HBase built on top of Hadoop / HDFS and the data stored in HBase can be manipulated using Hadoop’s MapReduce capabilities.
Fixes for visibility and write-ahead-log stability These release contains several fixes to improve the stability of visible data and WAL logging that we wish to highlight here. Prior to this release, data could be prematurely pruned by a vacuum operation when a special snapshot used for catalog scans was presently available.
JGroups is toolkit for reliable group communication.
Processes can join a group, send messages to all members or single members and receive messages from members in the group. At all times, PostgreSQL maintains a write ahead log (WAL) in the pg_wal/ (note that in PostgreSQL and earlier, the name of the directory was pg_xlog/) subdirectory of the cluster’s data directory.
Asynchronous commit provides behavior different from setting fsync = off. fsync is a server-wide setting that will alter the behavior of all transactions. It disables all logic within PostgreSQL that attempts to synchronize writes to different portions of the database, and therefore a system crash (that is, a hardware or operating system crash, not a failure of PostgreSQL itself) could result.
PostgreSQL uses Write-Ahead Logging (WAL), which provides atomicity and durability two of the ACID properties by default. Inserting into an unlogged table would mean PostgreSQL would do inserts without writing into the transaction log, which itself is an I/O operation, making these tables considerably faster than ordinary tables.