62.1. Database File Layout

This section describes the storage format at the level of files and directories.

Traditionally, the configuration and data files used by a database cluster are stored together within the cluster's data directory, commonly referred to as LTDATA (after the name of the environment variable that can be used to define it). A common location for LTDATA is /var/lib/lightdb-x/data. Multiple clusters, managed by different server instances, can exist on the same machine.

The LTDATA directory contains several subdirectories and control files, as shown in Table 62.1. In addition to these required items, the cluster configuration files lightdb.conf, lt_hba.conf, and lt_ident.conf are traditionally stored in LTDATA, although it is possible to place them elsewhere.

Table 62.1. Contents of LTDATA

Item Description
LT_VERSIONA file containing the major version number of LightDB
baseSubdirectory containing per-database subdirectories
current_logfilesFile recording the log file(s) currently written to by the logging collector
globalSubdirectory containing cluster-wide tables, such as pg_database
lt_commit_tsSubdirectory containing transaction commit timestamp data
lt_dynshmemSubdirectory containing files used by the dynamic shared memory subsystem
lt_logicalSubdirectory containing status data for logical decoding
lt_multixactSubdirectory containing multitransaction status data (used for shared row locks)
lt_notifySubdirectory containing LISTEN/NOTIFY status data
lt_replslotSubdirectory containing replication slot data
lt_serialSubdirectory containing information about committed serializable transactions
lt_snapshotsSubdirectory containing exported snapshots
lt_statSubdirectory containing permanent files for the statistics subsystem
lt_stat_tmpSubdirectory containing temporary files for the statistics subsystem
lt_subtransSubdirectory containing subtransaction status data
lt_tblspcSubdirectory containing symbolic links to tablespaces
lt_twophaseSubdirectory containing state files for prepared transactions
lt_walSubdirectory containing WAL (Write Ahead Log) files
lt_xactSubdirectory containing transaction commit status data
lightdb.auto.confA file used for storing configuration parameters that are set by ALTER SYSTEM
lightdb.optsA file recording the command-line options the server was last started with
lightdb.pidA lock file recording the current postmaster process ID (PID), cluster data directory path, postmaster start timestamp, port number, Unix-domain socket directory path (could be empty), first valid listen_address (IP address or *, or empty if not listening on TCP), and shared memory segment ID (this file is not present after server shutdown)

For each database in the cluster there is a subdirectory within LTDATA/base, named after the database's OID in pg_database. This subdirectory is the default location for the database's files; in particular, its system catalogs are stored there.

Note that the following sections describe the behavior of the builtin heap table access method, and the builtin index access methods. Due to the extensible nature of LightDB, other access methods might work differently.

Each table and index is stored in a separate file. For ordinary relations, these files are named after the table or index's filenode number, which can be found in pg_class.relfilenode. But for temporary relations, the file name is of the form tBBB_FFF, where BBB is the backend ID of the backend which created the file, and FFF is the filenode number. In either case, in addition to the main file (a/k/a main fork), each table and index has a free space map (see Section 62.3), which stores information about free space available in the relation. The free space map is stored in a file named with the filenode number plus the suffix _fsm. Tables also have a visibility map, stored in a fork with the suffix _vm, to track which pages are known to have no dead tuples. The visibility map is described further in Section 62.4. Unlogged tables and indexes have a third fork, known as the initialization fork, which is stored in a fork with the suffix _init (see Section 62.5).

Caution

Note that while a table's filenode often matches its OID, this is not necessarily the case; some operations, like TRUNCATE, REINDEX, CLUSTER and some forms of ALTER TABLE, can change the filenode while preserving the OID. Avoid assuming that filenode and table OID are the same. Also, for certain system catalogs including pg_class itself, pg_class.relfilenode contains zero. The actual filenode number of these catalogs is stored in a lower-level data structure, and can be obtained using the pg_relation_filenode() function.

When a table or index exceeds 1 GB, it is divided into gigabyte-sized segments. The first segment's file name is the same as the filenode; subsequent segments are named filenode.1, filenode.2, etc. This arrangement avoids problems on platforms that have file size limitations. (Actually, 1 GB is just the default segment size. The segment size can be adjusted using the configuration option --with-segsize when building LightDB.) In principle, free space map and visibility map forks could require multiple segments as well, though this is unlikely to happen in practice.

A table that has columns with potentially large entries will have an associated TOAST table, which is used for out-of-line storage of field values that are too large to keep in the table rows proper. pg_class.reltoastrelid links from a table to its TOAST table, if any. See Section 62.2 for more information.

The contents of tables and indexes are discussed further in Section 62.6.

Tablespaces make the scenario more complicated. Each user-defined tablespace has a symbolic link inside the LTDATA/lt_tblspc directory, which points to the physical tablespace directory (i.e., the location specified in the tablespace's CREATE TABLESPACE command). This symbolic link is named after the tablespace's OID. Inside the physical tablespace directory there is a subdirectory with a name that depends on the LightDB server version, such as PG_9.0_201008051. (The reason for using this subdirectory is so that successive versions of the database can use the same CREATE TABLESPACE location value without conflicts.) Within the version-specific subdirectory, there is a subdirectory for each database that has elements in the tablespace, named after the database's OID. Tables and indexes are stored within that directory, using the filenode naming scheme. The pg_default tablespace is not accessed through lt_tblspc, but corresponds to LTDATA/base. Similarly, the pg_global tablespace is not accessed through lt_tblspc, but corresponds to LTDATA/global.

The pg_relation_filepath() function shows the entire path (relative to LTDATA) of any relation. It is often useful as a substitute for remembering many of the above rules. But keep in mind that this function just gives the name of the first segment of the main fork of the relation — you may need to append a segment number and/or _fsm, _vm, or _init to find all the files associated with the relation.

Temporary files (for operations such as sorting more data than can fit in memory) are created within LTDATA/base/ltsql_tmp, or within a ltsql_tmp subdirectory of a tablespace directory if a tablespace other than pg_default is specified for them. The name of a temporary file has the form ltsql_tmpPPP.NNN, where PPP is the PID of the owning backend and NNN distinguishes different temporary files of that backend.