True if GSSAPI authentication was used for this connection. 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. Waiting for the page number needed to continue a parallel B-tree scan to become available. replication_slot_io: Waiting for I/O on a replication slot. Waiting to setup, drop or use replication origin. This can be a host name, an IP address, or a directory path if the connection is via Unix socket. > However, someone with deeper knowledge of page pinning and buffer manager > internals could certainly devise a better solution. Waiting for a timeline history file received via streaming replication to reach durable storage. The LWLock:BufferIO event occurs when Aurora PostgreSQL or RDS for PostgreSQL is waiting for other processes to Waiting for data to reach durable storage while assigning a new WAL sync method. Waiting to write a protocol message to a shared message queue. This can be used to gauge the delay that. Waiting for a newly created timeline history file to reach durable storage. Waiting to select the starting location of a synchronized table scan. The statistics collector transmits the collected information to other PostgreSQL processes through temporary files. Lag times work automatically for physical replication. Last write-ahead log location already received and written to disk, but not flushed. See. Using pg_stat_reset() also resets counters that autovacuum uses to determine when to trigger a vacuum or an analyze. The reported lag times are not predictions of how long it will take for the standby to catch up with the sending server assuming the current rate of replay. See Section30.5 for more information about the internal WAL function issue_xlog_fsync. Waiting for WAL to reach durable storage during bootstrapping. Time spent reading data file blocks by backends in this database, in milliseconds (if track_io_timing is enabled, otherwise zero), Time spent writing data file blocks by backends in this database, in milliseconds (if track_io_timing is enabled, otherwise zero), Time spent by database sessions in this database, in milliseconds (note that statistics are only updated when the state of a session changes, so if sessions have been idle for a long time, this idle time won't be included), Time spent executing SQL statements in this database, in milliseconds (this corresponds to the states active and fastpath function call in pg_stat_activity), idle_in_transaction_time double precision, Time spent idling while in a transaction in this database, in milliseconds (this corresponds to the states idle in transaction and idle in transaction (aborted) in pg_stat_activity), Total number of sessions established to this database, Number of database sessions to this database that were terminated because connection to the client was lost, Number of database sessions to this database that were terminated by fatal errors, Number of database sessions to this database that were terminated by operator intervention. Waits for lightweight locks ( LWLock ). Waiting to read or update the state of prepared transactions. idle in transaction (aborted): This state is similar to idle in transaction, except one of the statements in the transaction caused an error. Waiting to acquire a speculative insertion lock. Waiting to access the transaction status SLRU cache. Thus, the server expects something to happen that is independent of its internal processes. Port number of the PostgreSQL instance this WAL receiver is connected to. Waiting to read or update vacuum-related information for a B-tree index. Aurora PostgreSQL wait events PDF RSS The following table lists the wait events for Aurora PostgreSQL that most commonly indicate performance problems, and summarizes the most common causes and corrective actions. Monitoring systems should choose whether to represent this as missing data, zero or continue to display the last known value. Waiting to write a protocol message to a shared message queue. wait_event will identify the specific wait point. Waiting to elect a Parallel Hash participant to allocate more batches. See Table28.5 through Table28.13. pg_stat_get_backend_xact_start ( integer ) timestamp with time zone. sync: This standby server is synchronous. A process can wait for the data needed from a client ( Client) or another process ( IPC ). Note that only tables, indexes, and functions in the current database can be seen with these functions. Sometimes it may be more convenient to obtain just a subset of this information. See, One row for each table in the current database, showing statistics about accesses to that specific table. Waiting for a serialized historical catalog snapshot to reach durable storage. Host name of the connected client, as reported by a reverse DNS lookup of, TCP port number that the client is using for communication with this backend, or. Time when this process was started. Waiting for a replication slot to become inactive to be dropped. Several predefined views, listed in Table28.1, are available to show the current state of the system. Waiting to access the multixact offset SLRU cache. Another important point is that when a server process is asked to display any of these statistics, it first fetches the most recent report emitted by the collector process and then continues to use this snapshot for all statistical views and functions until the end of its current transaction. Waiting to read or update sub-transaction information. This documentation is for an unsupported version of PostgreSQL. Returns the process ID of the server process attached to the current session. Waiting for a read of a serialized historical catalog snapshot. In a bitmap scan the output of several indexes can be combined via AND or OR rules, so it is difficult to associate individual heap row fetches with specific indexes when a bitmap scan is used. PostgreSQL's cumulative statistics system supports collection and reporting of information about server activity. Waiting for a replication slot to become inactive so it can be dropped. There have been several occasions when a query is being executed dozens of times simultaneously by one or many users. This facility is independent of the cumulative statistics system. If a backend is in the active state, it may or may not be waiting on some event. Topics Relevant engine versions Context Causes Actions Relevant engine versions The fields returned are a subset of those in the pg_stat_activity view. Waiting to read or update the control file or creation of a new WAL file. The parameter track_io_timing enables monitoring of block read and write times. This is controlled by configuration parameters that are normally set in postgresql.conf. Returns the OID of the database this backend is connected to. It works like this: Detailed Description . In such cases, an older set of per-backend statistics access functions can be used; these are shown in Table28.20. The pg_stat_database view will contain one row for each database in the cluster, showing database-wide statistics. The pg_statio_ views are primarily useful to determine the effectiveness of the buffer cache. Waiting to find or allocate space in shared memory. Thanks for letting us know we're doing a good job! These numbers do not act as stated above; instead they update continuously throughout the transaction. BufferCacheHitRatio and LWLock:BufferIO wait The server process is waiting for a lightweight lock. Waiting for a write of logical rewrite mappings. Waiting for parallel bitmap scan to become initialized. Waiting for other Parallel Hash participants to finish loading a hash table. The server process is waiting for some interaction with another server process. This facility is independent of the collector process. Waiting to update the relation map file used to store catalog to filenode mapping. Waiting to find or allocate space in shared memory. Table28.12.pg_stat_database_conflicts View. Returns the time when the backend's current transaction was started. Waiting in main loop of WAL sender process. Waiting for WAL files required for a backup to be successfully archived. Waiting for a read during reorder buffer management. This field is truncated if the DN field is longer than, Number of WAL files that have been successfully archived, Name of the last WAL file successfully archived, Time of the last successful archive operation, Number of failed attempts for archiving WAL files, Name of the WAL file of the last failed archival operation, Time of the last failed archival operation, Time at which these statistics were last reset, Number of scheduled checkpoints that have been performed, Number of requested checkpoints that have been performed, Total amount of time that has been spent in the portion of checkpoint processing where files are written to disk, in milliseconds, Total amount of time that has been spent in the portion of checkpoint processing where files are synchronized to disk, in milliseconds, Number of buffers written during checkpoints, Number of buffers written by the background writer, Number of times the background writer stopped a cleaning scan because it had written too many buffers, Number of buffers written directly by a backend, Number of times a backend had to execute its own. Such a system would show similar times while new WAL is being generated, but would differ when the sender becomes idle. Waiting for a write while initializing a new WAL file. Total amount of time spent syncing WAL files to disk via issue_xlog_fsync request, in milliseconds (if track_wal_io_timing is enabled, fsync is on, and wal_sync_method is either fdatasync, fsync or fsync_writethrough, otherwise zero). Normally these parameters are set in postgresql.conf so that they apply to all server processes, but it is possible to turn them on or off in individual sessions using the SET command. This wait event information is relevant for all Aurora PostgreSQL 13 versions. Waiting for a replication origin to become inactive so it can be dropped. Autovacuum worker or launcher waiting to update or read the current state of autovacuum workers. See Table28.4 for details. Waiting for data to reach durable storage while creating the data directory lock file. It also tracks the total number of rows in each table, and information about vacuum and analyze actions for each table. Waiting to add a message in shared invalidation queue. Returns the wait event type name if this backend is currently waiting, otherwise NULL. After v9.5, the buffer cache is protected by 128 hash tables each protected by a LWLock. Waiting to update limits on transaction id and multixact consumption. pg_stat_reset_replication_slot ( text ) void. We recommend different actions depending on the causes of your wait event: Observe Amazon CloudWatch metrics for correlation between sharp decreases in the Occasionally i noticed that in random interval of times the dbms become slow and get stuck on a few SELECT queries. Number of times WAL buffers were written out to disk via XLogWrite request. IP address of the client connected to this WAL sender. 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. 106 . Waiting for a WAL file to reach durable storage. Waiting to read or record conflicting serializable transactions. 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. LWLock:BufferIO - Amazon Relational Database Service AWSDocumentationAmazon RDS and Aurora DocumentationUser Guide Relevant engine versionsContextCausesActions LWLock:BufferIO Note that this includes the transactions that are streamed and/or spilled. But processes can also await other events: Waits for input/output ( IO) occur when a process needs to read or write data. Common causes for the LWLock:BufferIO event to appear in top waits include the following: Multiple backends or connections trying to access the same page that's Waiting to add a message to the shared catalog invalidation queue. Waiting in main loop of startup process for WAL to arrive, during streaming recovery. LWTRANCHE_BUFFER_CONTENT @ LWTRANCHE_BUFFER_CONTENT. Waiting to read or update old snapshot control information. Waiting to allocate or assign a transaction id. I've made . Waiting to access the list of finished serializable transactions. Waiting to read or update information about serializable transactions. Provide feedback Waiting to ensure that a table selected for autovacuum still needs vacuuming. Waiting to read or truncate multixact information. Time when this process was started. Waiting for background worker to shut down. See, One row per database, showing database-wide statistics about query cancels due to conflict with recovery on standby servers. Waiting for a read of the relation map file. See Section30.5 for more information about the internal WAL function XLogWrite. Table28.15.pg_stat_replication_slots View, A unique, cluster-wide identifier for the replication slot. Waiting to apply WAL at recovery because it is delayed. The pg_stat_all_tables view will contain one row for each table in the current database (including TOAST tables), showing statistics about accesses to that specific table. The pg_stat_activity view will have one row per server process, showing information related to the current activity of that process. See, Only one row, showing statistics about the WAL receiver from that receiver's connected server. For better performance, stats_temp_directory can be pointed at a RAM-based file system, decreasing physical I/O requirements. Waiting for an asynchronous prefetch from a relation data file. shared_buffers parameter. Waiting for startup process to send initial data for streaming replication. This and other spill counters can be used to gauge the I/O which occurred during logical decoding and allow tuning logical_decoding_work_mem. 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. Users interested in obtaining more detailed information on PostgreSQL I/O behavior are advised to use the PostgreSQL statistics views in combination with operating system utilities that allow insight into the kernel's handling of I/O. See, One row for each backend (including autovacuum worker processes) running, One row for each WAL sender process streaming a base backup, showing current progress. See, One row only, showing statistics about the WAL archiver process's activity. Waiting for a write of mapping data during a logical rewrite. Waiting for a two phase state file to reach durable storage. Waiting during recovery when WAL data is not available from any source (. postgres 26 Heap_Insert Waiting for background worker to shut down. Resets all statistics counters for the current database to zero. Waiting for a read during recheck of the data directory lock file. The full object locks which last (usually) for the duration of a transaction and which you can see in pg_locks have info about them stored in shared memory. See, One row per database, showing database-wide statistics about query cancels due to conflict with recovery on standby servers. 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. This can be used to gauge the delay that synchronous_commit level on incurred while committing if this server was configured as a synchronous standby. Waiting for a write of a newly created timeline history file. Waiting for an elected Parallel Hash participant to decide on future batch growth. See, One row per replication slot, showing statistics about the replication slot's usage. Waiting for I/O on a multixact offset buffer. Waiting for a serialized historical catalog snapshot to reach durable storage. The per-table and per-index functions take a table or index OID. If a backend is in the active state, it may or may not be waiting on some event. However, current-query information collected by track_activities is always up-to-date. Waiting for a buffered file to be truncated. 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 Waiting to read or update replication slot state. Verify whether you have unused indexes, then remove them. This standby's xmin horizon reported by hot_standby_feedback. sync: This standby server is synchronous. Extension: The server process is waiting for activity in an extension module. Waiting for a logical replication remote server to send data for initial table synchronization. Waiting for a write during a file copy operation. Waiting in main loop of logical replication apply process. Waiting for logical rewrite mappings to reach durable storage. The function pg_stat_get_backend_idset provides a convenient way to generate one row for each active backend for invoking these functions. Waiting to read or update notification messages. Lag times work automatically for physical replication. If the argument is NULL, resets statistics for all the replication slots. PostgreSQL's statistics collector is a subsystem that supports collection and reporting of information about server activity. Waiting for recovery conflict resolution for dropping a tablespace. Waiting for any activity when processing replies from WAL receiver in WAL sender process. pg_stat_get_backend_userid ( integer ) oid. Waiting for mapping data to reach durable storage during a logical rewrite. Waiting for a write while adding a line to the data directory lock file. (Some locks have specific names; others are part of a group of locks each with a similar purpose.). Waiting in main loop of autovacuum launcher process. Normally these parameters are set in postgresql.conf so that they apply to all server processes, but it is possible to turn them on or off in individual sessions using the SET command. Buffer pin waits can be protracted if another process holds an open cursor that last read data from the buffer in question. Resetting these counters can cause autovacuum to not perform necessary work, which can cause problems such as table bloat or out-dated table statistics. Statistics Functions. Returns the OID of the user logged into this backend. The combination of certificate serial number and certificate issuer uniquely identifies a certificate (unless the issuer erroneously reuses serial numbers). being read from storage. So the displayed information lags behind actual activity. It is quite possible that user has registered the tranche in one of the backends (by having allocation in dynamic shared memory) in which case other backends won't have that information, so we display extension for such cases. 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. The pg_stat_recovery_prefetch view will contain only one row. Waiting to create or drop the tablespace. Monitoring systems should choose whether to represent this as missing data, zero or continue to display the last known value. Waiting for the relation map file to reach durable storage. Only directly connected standbys are listed; no information is available about downstream standby servers. Java,java,file,io,buffer,Java,File,Io,Buffer,Java10242^n Waiting for a read of a serialized historical catalog snapshot. Waiting for a write of logical rewrite mappings. Wait Events of Type Extension. See Table28.5 through Table28.13. LWLock: The backend is waiting for a lightweight lock. The pg_stat_database view will contain one row for each database in the cluster, plus one for shared objects, showing database-wide statistics. The pg_stat_replication view will contain one row per WAL sender process, showing statistics about replication to that sender's connected standby server. This is a feature, not a bug, because it allows you to perform several queries on the statistics and correlate the results without worrying that the numbers are changing underneath you. The pg_stat_bgwriter view will always have a single row, containing global data for the cluster. Waiting for a relation data file to reach durable storage. If this field is null, it indicates that this is an internal server process. PostgreSQL utilizes lightweight locks (LWLocks) to synchronize and control access to the buffer content. Before PostgreSQL 8.1, all operations of the shared buffer manager itself were protected by a single system-wide lock, the BufMgrLock, which unsurprisingly proved to be a source of contention. Waiting for a write of a two phase state file. 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 a read during reorder buffer management. Buffer pin waits can be protracted if another process holds an open cursor which last read data from the buffer in question. A backend process wants to read a page into shared memory. Definition: lwlock.h:190. Cumulative statistics are collected in shared memory. Waiting for a relation data file to be truncated. The parameter track_wal_io_timing enables monitoring of WAL write times. pg_stat_get_backend_activity_start ( integer ) timestamp with time zone. to keep index reordering low and reduces its impact. Host name of the connected client, as reported by a reverse DNS lookup of client_addr. Table28.19. The access functions for per-database statistics take a database OID as an argument to identify which database to report on. Extensions can register their specific waits ( Extension ). Waiting for an asynchronous prefetch from a relation data file. Waiting to access the commit timestamp SLRU cache. For tranches registered by extensions, the name is specified by extension and this will be displayed as wait_event. Waiting for SLRU data to reach durable storage following a page write. Waiting for logical replication remote server to change state. TCP port number that the client is using for communication with this WAL sender, or, Time when this process was started, i.e., when the client connected to this WAL sender. Number of times in-progress transactions were streamed to the decoding output plugin while decoding changes from WAL for this slot. Waiting for background worker to start up. Additional functions related to the cumulative statistics system are listed in Table28.34. Waiting for a read of a two phase state file. Since collection of statistics adds some overhead to query execution, the system can be configured to collect or not collect information. Waiting to create, drop or use a replication origin. 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. In order to write the disk block into buffer memory, the buffer cache's hash table entry needs updating. Waiting for a write of a WAL page during bootstrapping. Waiting to read or update shared multixact state. Waiting for a read when creating a new WAL segment by copying an existing one. The LWLock that this article will introduce is a lightweight lock (Lightweight Lock) based on SpinLock. Time when this process' current transaction was started, or null if no transaction is active. This and other streaming counters for this slot can be used to tune logical_decoding_work_mem. checksum_last_failure timestamp with time zone. pg_stat_get_backend_wait_event_type ( integer ) text. The LWLock:BufferIO wait event precedes the IO:DataFileRead wait event. Most such locks protect a particular data structure in shared memory. Possible values are: Top-level transaction identifier of this backend, if any. The parameter track_functions enables tracking of usage of user-defined functions. Postgres Source Code Docs: Locking Overview.