Waiting for an immediate synchronization of a relation data file to durable storage. In all other states, it shows the last query that was executed. Waiting for a write during a file copy operation. Waiting to apply WAL at recovery because it is delayed. catchup: This WAL sender's connected standby is catching up with the primary. Returns a record of information about the backend with the specified process ID, or one record for each active backend in the system if NULL is specified. Time when this process was started. TCP port number that the client is using for communication with this WAL sender, or -1 if a Unix socket is used, Time when this process was started, i.e., when the client connected to this WAL sender. Table28.26.pg_stat_database_conflicts View, Number of queries in this database that have been canceled due to dropped tablespaces, Number of queries in this database that have been canceled due to lock timeouts, Number of queries in this database that have been canceled due to old snapshots, Number of queries in this database that have been canceled due to pinned buffers, Number of queries in this database that have been canceled due to deadlocks. Waiting for a write while initializing a new WAL file. Waiting to read or update the state of prepared transactions. Waiting to read or update the replication progress. The pg_stat_all_indexes view will contain one row for each index in the current database, showing statistics about accesses to that specific index. Connection string used by this WAL receiver, with security-sensitive fields obfuscated. Waiting to acquire a speculative insertion lock. If this field is null, it indicates either that the client is connected via a Unix socket on the server machine or that this is an internal process such as autovacuum. Waiting for background worker to start up. Waiting during base backup when throttling activity. You Waiting for truncate of mapping data during a logical rewrite. The parameter track_counts controls whether statistics are collected about table and index accesses. (Conflicts occur only on standby servers; see pg_stat_database_conflicts for details.). Waiting to access the list of predicate locks held by the current serializable transaction during a parallel query. Waiting for a read during reorder buffer management. Waiting for a read when creating a new WAL segment by copying an existing one. The functions for per-function statistics take a function OID. Only directly connected standbys are listed; no information is available about downstream standby servers. The per-table and per-index functions take a table or index OID. number of buffers needed by the current workload, The size of the shared buffer pool not being well balanced with the number of pages being evicted by other In addition, background workers registered by extensions may have additional types. Write-Ahead Logging (WAL) is a standard method for ensuring data integrity. Waiting for a write of mapping data during a logical rewrite. Thanks for letting us know this page needs work. The server process is waiting for some interaction with another server process. If a backend is in the active state, it may or may not be waiting on some event. If you've got a moment, please tell us how we can make the documentation better. DN of the issuer of the client certificate, or NULL if no client certificate was supplied or if SSL is not in use on this connection. Waiting for a read of the relation map file. Re: Improve WALRead() to suck data directly from WAL buffers when possible When a server, including a physical replica, shuts down cleanly, a permanent copy of the statistics data is stored in the pg_stat subdirectory, so that statistics can be retained across server restarts. Cumulative statistics are collected in shared memory. All temporary files are counted, regardless of why the temporary file was created, and regardless of the, Number of deadlocks detected in this database, Time spent reading data file blocks by backends in this database, in milliseconds, Time spent writing data file blocks by backends in this database, in milliseconds, Number of queries in this database that have been canceled due to dropped tablespaces, Number of queries in this database that have been canceled due to lock timeouts, Number of queries in this database that have been canceled due to old snapshots, Number of queries in this database that have been canceled due to pinned buffers, Number of queries in this database that have been canceled due to deadlocks, Number of sequential scans initiated on this table, Number of live rows fetched by sequential scans, Number of index scans initiated on this table, Number of live rows fetched by index scans, Number of rows updated (includes HOT updated rows), Number of rows HOT updated (i.e., with no separate index update required), Estimated number of rows modified since this table was last analyzed, Last time at which this table was manually vacuumed (not counting, Last time at which this table was vacuumed by the autovacuum daemon, Last time at which this table was manually analyzed, Last time at which this table was analyzed by the autovacuum daemon, Number of times this table has been manually vacuumed (not counting, Number of times this table has been vacuumed by the autovacuum daemon, Number of times this table has been manually analyzed, Number of times this table has been analyzed by the autovacuum daemon, Number of index scans initiated on this index, Number of index entries returned by scans on this index, Number of live table rows fetched by simple index scans using this index, Number of disk blocks read from this table, Number of disk blocks read from all indexes on this table, Number of buffer hits in all indexes on this table, Number of disk blocks read from this table's TOAST table (if any), Number of buffer hits in this table's TOAST table (if any), Number of disk blocks read from this table's TOAST table indexes (if any), Number of buffer hits in this table's TOAST table indexes (if any), Number of disk blocks read from this index, Number of disk blocks read from this sequence, Number of times this function has been called, Total time spent in this function and all other functions called by it, in milliseconds, Total time spent in this function itself, not including other functions called by it, in milliseconds, Process ID of the server process handling the current session, Returns a record of information about the backend with the specified PID, or one record for each active backend in the system if, Returns the timestamp of the current statistics snapshot, Reset all statistics counters for the current database to zero (requires superuser privileges by default, but EXECUTE for this function can be granted to others. (See Chapter20 for details about setting configuration parameters.). For more information, see LWLock:buffer_content (BufferContent). Waiting for a read when creating a new WAL segment by copying an existing one. It can also count calls to user-defined functions and the total time spent in each one. This can be used to gauge the delay that synchronous_commit level remote_write incurred while committing if this server was configured as a synchronous standby. When the buffer manager receives a request, PostgreSQL uses the buffer_tag of the desired page. Number of disk blocks read from this table, Number of disk blocks read from all indexes on this table, Number of buffer hits in all indexes on this table, Number of disk blocks read from this table's TOAST table (if any), Number of buffer hits in this table's TOAST table (if any), Number of disk blocks read from this table's TOAST table indexes (if any), Number of buffer hits in this table's TOAST table indexes (if any). wait_event will identify the type of lock awaited. Listen The most possible reason for why you see LWLockTranche/buffer_mapping wait event in PostgreSQL Well, if you are here you probably came across an issue where your database had CPU spikes. From pg_stat_activity i noticed that the wait_event_type and wait_event of these queries is as follows: active: The backend is executing a query. The pg_stat_archiver view will always have a single row, containing data about the archiver process of the cluster. All temporary files are counted, regardless of why the temporary file was created (e.g., sorting or hashing), and regardless of the log_temp_files setting. Resets statistics to zero for a single SLRU cache, or for all SLRUs in the cluster. See, One row per database, showing database-wide statistics about query cancels due to conflict with recovery on standby servers. Resets statistics for a single subscription shown in the pg_stat_subscription_stats view to zero. Waiting to send bytes to a shared message queue. Waiting for parallel query dynamic shared memory allocation lock. Waiting to read or update transaction status. There are also several other views, listed in Table28.2, available to show the results of statistics collection. Waiting to get a snapshot or clearing a transaction id at transaction end. Number of sequential scans initiated on this table, Number of live rows fetched by sequential scans, Number of index scans initiated on this table, Number of live rows fetched by index scans, Number of rows updated (includes HOT updated rows), Number of rows HOT updated (i.e., with no separate index update required), Estimated number of rows modified since this table was last analyzed, Estimated number of rows inserted since this table was last vacuumed, Last time at which this table was manually vacuumed (not counting VACUUM FULL), Last time at which this table was vacuumed by the autovacuum daemon, Last time at which this table was manually analyzed, last_autoanalyze timestamp with time zone, Last time at which this table was analyzed by the autovacuum daemon, Number of times this table has been manually vacuumed (not counting VACUUM FULL), Number of times this table has been vacuumed by the autovacuum daemon, Number of times this table has been manually analyzed, Number of times this table has been analyzed by the autovacuum daemon. The parameter track_activities enables monitoring of the current command being executed by any server process. Waiting in a cost-based vacuum delay point. gorthx on Twitter This is the only column in this view that returns a value reflecting current state; all other columns return the accumulated values since the last reset. Waiting for WAL files required for a backup to be successfully archived. Number of times transactions were spilled to disk while decoding changes from WAL for this slot. See, One row only, showing statistics about the WAL archiver process's activity. The management of the buffers in PostgreSQL consists of a buffer descriptor that contains metadata about the buffer and the buffer content that is read from the disk. This view will only contain information on standby servers, since conflicts do not occur on master servers. Timeout: The server process is waiting for a timeout to expire. Process ID of the subscription worker process, OID of the relation that the worker is synchronizing; null for the main apply worker, Last write-ahead log location received, the initial value of this field being 0, Process ID of a backend or WAL sender process, Version of SSL in use, or NULL if SSL is not in use on this connection, Name of SSL cipher in use, or NULL if SSL is not in use on this connection, Number of bits in the encryption algorithm used, or NULL if SSL is not used on this connection, True if SSL compression is in use, false if not, or NULL if SSL is not in use on this connection, Distinguished Name (DN) field from the client certificate used, or NULL if no client certificate was supplied or if SSL is not in use on this connection. See, One row per WAL sender process, showing statistics about replication to that sender's connected standby server. Waiting in main loop of logical replication apply process. See, One row per database, showing database-wide statistics. Serial number of the client certificate, or NULL if no client certificate was supplied or if SSL is not in use on this connection. pg_stat_get_activity, the underlying function of the pg_stat_activity view, returns a set of records containing all the available information about each backend process. Heavyweight locks, also known as lock manager locks or simply locks, primarily protect SQL-visible objects such as tables. Waiting for background worker to shut down. Returns the time when the backend's most recent query was started. Statistics Functions. block. Waiting to acquire an advisory user lock. Waiting in main loop of logical launcher process. So the statistics will show static information as long as you continue the current transaction. OID of the user logged into this WAL sender process, Name of the user logged into this WAL sender process, Name of the application that is connected to this WAL sender. The LWLock:BufferIO wait event precedes the IO:DataFileRead wait event. Waiting to read or update shared multixact state. Such a system would show similar times while new WAL is being generated, but would differ when the sender becomes idle. pg_stat_get_activity ( integer ) setof record. BufferCacheHitRatio metric dips. The parameter track_activities enables monitoring of the current command being executed by any server process. Thus, the server expects something to happen that is independent of its internal processes. See, One row per connection (regular and replication), showing information about SSL used on this connection. The pg_stat_ssl view will contain one row per backend or WAL sender process, showing statistics about SSL usage on this connection. Waiting for an update to the control file to reach durable storage. See, One row for each index in the current database, showing statistics about accesses to that specific index. Waiting for a write when creating a new WAL segment by copying an existing one. This is used by system processes waiting for activity in their main processing loop. Waiting for data to reach durable storage while assigning WAL sync method. Time at which the last data page checksum failure was detected in this database (or on a shared object), or NULL if data checksums are not enabled. idle in transaction: The backend is in a transaction, but is not currently executing a query. Waiting to read or update information about. For example, to show the PIDs and current queries of all backends: Table28.35. Sometimes it may be more convenient to obtain just a subset of this information. The generated IO patterns are also much worse. Waiting for a read of a timeline history file. Process ID of the parallel group leader, if this process is a parallel query worker. Waiting for logical replication remote server to change state. Number of data page checksum failures detected in this database (or on a shared object), or NULL if data checksums are not enabled. (To prevent ordinary users from hiding their activity from the administrator, only superusers are allowed to change these parameters with SET.). Presently, the collector can count accesses to tables and indexes in both disk-block and individual-row terms. Waiting for action on logical replication worker to finish. The pg_statio_all_tables view will contain one row for each table in the current database (including TOAST tables), showing statistics about I/O on that specific table. Waiting for logical rewrite mappings to reach durable storage during a checkpoint. to keep index reordering low and reduces its impact. being read from storage. When recovery is performed at server start (e.g., after immediate shutdown, server crash, and point-in-time recovery), all statistics counters are reset. Table28.19. Waiting for a read from a replication slot control file. The IO:DataFileRead wait event occurs while data is The pg_statio_user_indexes and pg_statio_sys_indexes views contain the same information, but filtered to only show user and system indexes respectively. Topics Relevant engine versions Context Causes Actions Relevant engine versions Waiting in main loop of checkpointer process. Resets some cluster-wide statistics counters to zero, depending on the argument. Waiting in main loop of syslogger process. Waiting in main loop of autovacuum launcher process. Waiting to perform an operation on a list of locks held by serializable transactions. Waiting in WAL receiver to receive data from remote server. Number of WAL files that have been successfully archived, Name of the WAL file most recently successfully archived, last_archived_time timestamp with time zone, Time of the most recent successful archive operation, Number of failed attempts for archiving WAL files, Name of the WAL file of the most recent failed archival operation, last_failed_time timestamp with time zone, Time of the most recent failed archival operation. IP address of the client connected to this WAL sender. Waiting for data to reach durable storage while adding a line to the data directory lock file. These times represent the commit delay that was (or would have been) introduced by each synchronous commit level, if the remote server was configured as a synchronous standby. Therefore, a bitmap scan increments the pg_stat_all_indexes.idx_tup_read count(s) for the index(es) it uses, and it increments the pg_stat_all_tables.idx_tup_fetch count for the table, but it does not affect pg_stat_all_indexes.idx_tup_fetch. Waiting to read or update dynamic shared memory allocation information. Waiting for a read during a file copy operation. When the number of actual disk reads is much smaller than the number of buffer hits, then the cache is satisfying most read requests without invoking a kernel call. Waiting for the version file to be written while creating a database. Waiting to select the starting location of a synchronized table scan. Waiting in WAL receiver to establish connection to remote server. This counts top-level transactions only, and is not incremented for subtransactions. Calling, Reset statistics for a single table or index in the current database to zero (requires superuser privileges by default, but EXECUTE for this function can be granted to others), Reset statistics for a single function in the current database to zero (requires superuser privileges by default, but EXECUTE for this function can be granted to others), Set of currently active backend ID numbers (from 1 to the number of active backends), Time when the most recent query was started, IP address of the client connected to this backend, TCP port number that the client is using for communication, Wait event type name if backend is currently waiting, otherwise NULL. lock_manager Waiting for a read from a timeline history file during walsender timeline command. This field is truncated if the DN field is longer than NAMEDATALEN (64 characters in a standard build). Waiting to read data from the client while establishing a GSSAPI session. Waiting for a write of a timeline history file received via streaming replication. [prev in list] [next in list] [prev in thread] [next in thread] List: postgresql-general Subject: Re: [HACKERS] [PATCH] Refactoring of LWLock tranches From: Ildus Kurbangaliev <i.kurbangaliev postgrespro ! I am not the DBA, but receive reports occasionally when an application is causing load on the system. Per-Backend Statistics Functions, pg_stat_get_backend_idset () setof integer. A process acquires an LWLock in a shared mode to read from the buffer and . Waiting for a write to a relation data file. The pg_stat_user_indexes and pg_stat_sys_indexes views contain the same information, but filtered to only show user and system indexes respectively. Buffer pin waits can be protracted if another process holds an open cursor which last read data from the buffer in question. Waiting for startup process to send initial data for streaming replication. Resets statistics for a single function in the current database to zero. Number of decoded transactions sent to the decoding output plugin for this slot. Waiting for SLRU data to reach durable storage following a page write. Waiting in main loop of the statistics collector process. The pg_stat_bgwriter view will always have a single row, containing global data for the cluster. Time when the currently active query was started, or if state is not active, when the last query was started. Each such lock protects a particular data structure in shared memory. If the standby server has entirely caught up with the sending server and there is no more WAL activity, the most recently measured lag times will continue to be displayed for a short time and then show NULL. However, these statistics do not give the entire story: due to the way in which PostgreSQL handles disk I/O, data that is not in the PostgreSQL buffer cache might still reside in the kernel's I/O cache, and might therefore still be fetched without requiring a physical read. Possible types are. The pg_stat_slru view will contain one row for each tracked SLRU cache, showing statistics about access to cached pages. Postgres Source Code Docs: Locking Overview. I'd like to know more about what these locks could imply if anything. Waiting for WAL from a stream at recovery. to report a documentation issue. A transaction can also see its own statistics (as yet untransmitted to the collector) in the views pg_stat_xact_all_tables, pg_stat_xact_sys_tables, pg_stat_xact_user_tables, and pg_stat_xact_user_functions. Waiting to read or update multixact member mappings. The pg_stat_ssl view will contain one row per backend or WAL sender process, showing statistics about SSL usage on this connection. Waiting to allocate or free a replication slot. Waiting for the group leader to update transaction status at end of a parallel operation. The type of event for which the backend is waiting, if any; otherwise NULL. A backend process is waiting to associate a data block with a buffer in the shared buffer pool. Using pg_stat_reset() also resets counters that autovacuum uses to determine when to trigger a vacuum or an analyze. This is consistent with the goal of measuring synchronous commit and transaction visibility delays for recent write transactions. events. Waiting for parallel query dynamic shared memory allocation. There are also several other views, listed in Table28.2, available to show the accumulated statistics. See, One row per connection (regular and replication), showing information about SSL used on this connection. Waiting to acquire a lock on a non-relation database object. The pg_statio_all_indexes view will contain one row for each index in the current database, showing statistics about I/O on that specific index. Possible types are autovacuum launcher, autovacuum worker, logical replication launcher, logical replication worker, parallel worker, background writer, client backend, checkpointer, archiver, startup, walreceiver, walsender and walwriter. All temporary files are counted, regardless of why the temporary file was created, and regardless of the log_temp_files setting. A database-wide ANALYZE is recommended after the statistics have been reset. Indexes can be used by simple index scans, bitmap index scans, and the optimizer. (See Chapter19 for details about setting configuration parameters.). disabled: This state is reported if track_activities is disabled in this backend. Resets all statistics counters for the current database to zero. Waiting for a write during a file copy operation. Each buffer header also contains an LWLock, the "buffer content lock", that *does* represent the right to access the data: in the buffer. Waiting to read or update a process' fast-path lock information. Total amount of data written to temporary files by queries in this database. The optimizer also accesses indexes to check for supplied constants whose values are outside the recorded range of the optimizer statistics because the optimizer statistics might be stale. Name of this database, or NULL for shared objects. Waiting to send bytes to a shared message queue. The parameter track_wal_io_timing enables monitoring of WAL write times. Time when this process' current transaction was started, or null if no transaction is active. To reduce confusion for users expecting a different model of lag, the lag columns revert to NULL after a short time on a fully replayed idle system. Waiting for a serialized historical catalog snapshot to reach durable storage. wait_event will identify the specific wait point. wait_event will identify the specific wait point. (For example, in psql you could issue \d+ pg_stat_activity.) If the argument is NULL, resets statistics for all the replication slots. Priority of this standby server for being chosen as the synchronous standby in a priority-based synchronous replication. Wait Events of Type Extension. All temporary files are counted, regardless of why the temporary file was created (e.g., sorting or hashing), and regardless of the, Total amount of data written to temporary files by queries in this database. The pg_stat_gssapi view will contain one row per backend, showing information about GSSAPI usage on this connection. A process acquires an LWLock in a shared mode to read from the buffer and an exclusive mode to write to the buffer. Additional functions related to the cumulative statistics system are listed in Table28.34. Waiting to read or update information about the state of synchronous replication. Waiting to insert WAL into a memory buffer. See, One row for each table in the current database, showing statistics about I/O on that specific table. The pg_stat_user_functions view will contain one row for each tracked function, showing statistics about executions of that function. Waiting for the control file to reach durable storage. Connection string used by this WAL receiver, with security-sensitive fields obfuscated.