Waiting for a read from a relation data file. The parameter track_functions enables tracking of usage of user-defined functions. The pg_stat_archiver view will always have a single row, containing data about the archiver process of the cluster. The parameter track_activities enables monitoring of the current command being executed by any server process. 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. Topics Relevant engine versions Context Causes Actions Relevant engine versions Per-Backend Statistics Functions, pg_stat_get_backend_idset () setof integer. Waiting for changes to a relation data file to reach durable storage. Waiting for a read of a two phase state 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. 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. If, Type of current backend. 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. Also, the collector itself emits a new report at most once per PGSTAT_STAT_INTERVAL milliseconds (500 ms unless altered while building the server). Monitoring systems should choose whether to represent this as missing data, zero or continue to display the last known value. Waiting for a write while initializing a new WAL file. Waits for a buffer pin ( BufferPin ). To minimize skew, stats_fetch_consistency can be set to snapshot, at the price of increased memory usage for caching not-needed statistics data. Waiting for a write to the relation map file. Each buffer header also contains an LWLock, the "buffer content lock", that *does* represent the right to access the data: in the buffer. Waiting for a newly initialized WAL file to reach durable storage. Process ID of the parallel group leader, if this process is a parallel query worker. My application is using Postgres as DBMS, the version of Postgres that i'm using is 10.3 with the extension Postgis installed. IP address of the client connected to this backend. Waiting to access the sub-transaction SLRU cache. The new locking scheme avoids grabbing system-wide exclusive locks in common code paths. Waiting for mapping data to reach durable storage during a logical rewrite. Waiting to read or update dynamic shared memory allocation information. Waiting for a write to a relation data file. Alternatively, you can invoke pg_stat_clear_snapshot(), which will discard the current transaction's statistics snapshot (if any). LWLock:BufferIO - Amazon Relational Database Service AWSDocumentationAmazon RDS and Aurora DocumentationUser Guide Relevant engine versionsContextCausesActions LWLock:BufferIO The easiest way to create a cross-Region replica for Amazon RDS for PostgreSQL is by completing the following steps: On the Amazon RDS console, choose your Amazon RDS for PostgreSQL source instance. Waiting for a write while creating the data directory lock file. The parameter track_functions enables tracking of usage of user-defined functions. Text of this backend's most recent query. There are also several other views, listed in Table28.2, available to show the results of statistics collection. Principal used to authenticate this connection, or NULL if GSSAPI was not used to authenticate this connection. Synchronous state of this standby server. pg_stat_get_backend_client_port ( integer ) integer. 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. 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. Waiting for SSL while attempting connection. Waiting to choose the next subplan during Parallel Append plan execution. Waiting in main loop of startup process for WAL to arrive, during streaming recovery. The lag times reported in the pg_stat_replication view are measurements of the time taken for recent WAL to be written, flushed and replayed and for the sender to know about it. Possible values are: catchup: This WAL sender's connected standby is catching up with the primary. Waiting to read while creating the data directory lock file. This field is truncated if the DN field is longer than NAMEDATALEN (64 characters in a standard build). Write-Ahead Logging (WAL) is a standard method for ensuring data integrity. This facility is independent of the collector process. See Table28.4 for details. The latter will be less if any dead or not-yet-committed rows are fetched using the index, or if any heap fetches are avoided by means of an index-only scan. The pg_stat_database view will contain one row for each database in the cluster, showing database-wide statistics. Waiting for group leader to clear transaction id at transaction end. Amount of transaction data decoded for streaming in-progress transactions to the decoding output plugin while decoding changes from WAL for this slot. 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. Waiting to access the multixact member SLRU cache. The server process is waiting for activity on a socket connected to a user application. checksum_last_failure timestamp with time zone. idle in transaction: The backend is in a transaction, but is not currently executing a query. A transaction can also see its own statistics (not yet flushed out to the shared memory statistics) in the views pg_stat_xact_all_tables, pg_stat_xact_sys_tables, pg_stat_xact_user_tables, and pg_stat_xact_user_functions. Thus, the server expects something to happen that is independent of its internal processes. This view will only contain information on standby servers, since conflicts do not occur on master servers. pg_stat_get_backend_pid ( integer ) integer, pg_stat_get_backend_start ( integer ) timestamp with time zone. workload into more reader nodes. Waiting to read or update vacuum-related information for a B-tree index. For details such as the functions' names, consult the definitions of the standard views. Waiting in main loop of WAL sender process. See, Time when the current transaction was started. to report a documentation issue. Last write-ahead log location sent on this connection, Last write-ahead log location written to disk by this standby server, Last write-ahead log location flushed to disk by this standby server, Last write-ahead log location replayed into the database on this standby server. Waiting for WAL from a stream at recovery. Waiting for a write of a newly created timeline history file. 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. Waiting for a write to a replication slot control file. Waiting for a serialized historical catalog snapshot to reach durable storage. async: This standby server is asynchronous. Current overall state of this backend. might be causing it. When a buffer is read from disk (or written to disk), an IO in progress lock is also acquired, which indicates to other processes that the page is being read (or written) they can queue up if they need to do something with this page. Note that this includes the transactions that are streamed and/or spilled. The pg_statio_all_sequences view will contain one row for each sequence in the current database, showing statistics about I/O on that specific sequence. Waiting to read or truncate multixact information. The LWLock:BufferIO event occurs when Aurora PostgreSQL or RDS for PostgreSQL is waiting for other processes to The pg_stat_bgwriter view will always have a single row, containing global data for the cluster. A backend process is waiting to associate a data block with a buffer in the shared buffer pool. PostgreSQL Entangled in Locks: Attempts to free it - Amit Kapila - Dilip Kumar PGCon 2017 . Waiting in main loop of logical replication launcher process. 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. The pg_stat_gssapi view will contain one row per backend, showing information about GSSAPI usage on this connection. Waiting for a write to a replication slot control file. Here is an example of how wait events can be viewed. These files are stored in the directory named by the stats_temp_directory parameter, pg_stat_tmp by default. (To prevent ordinary users from hiding their activity from the administrator, only superusers are allowed to change these parameters with SET.). However, current-query information collected by track_activities is always up-to-date. The pg_stat_replication_slots view will contain one row per logical replication slot, showing statistics about its usage. Time when the currently active query was started, or if state is not active, when the last query was started. The pg_stat_activity view will have one row per server process, showing information related to the current activity of that process. 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. Number of times WAL files were synced to disk via issue_xlog_fsync request (if fsync is on and wal_sync_method is either fdatasync, fsync or fsync_writethrough, otherwise zero). potential: This standby server is now asynchronous, but can potentially become synchronous if one of current synchronous ones fails. Waiting to read while creating the data directory lock file. The parameter track_io_timing enables monitoring of block read and write times. Waiting to access a parallel query's information about composite types. See, One row only, showing statistics about WAL activity. In addition, background workers registered by extensions may have additional types. 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. (Conflicts occur only on standby servers; see, Number of temporary files created by queries in this database. buffer_io: Waiting for I/O on a data page. Returns the text of this backend's most recent query. (The path case can be distinguished because it will always be an absolute path, beginning with /.). Waiting for a read during recheck of the data directory lock file. Waiting for the version file to be written while creating a database. Using pg_stat_reset() also resets counters that autovacuum uses to determine when to trigger a vacuum or an analyze. Waiting to elect a Parallel Hash participant to allocate a hash table. Table28.17.pg_statio_all_sequences View. The LWLock:BufferIO event occurs when RDS for PostgreSQL or Aurora PostgreSQL is waiting for other processes to finish their I/O operations. Time elapsed between flushing recent WAL locally and receiving notification that this standby server has written and flushed it (but not yet applied it). The track_functions parameter controls exactly which functions are tracked. 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. Waiting for I/O on a multixact offset buffer. Waiting to receive bytes from a shared message queue. Returns the process ID of the server process attached to the current session. Number of transactions in this database that have been committed, Number of transactions in this database that have been rolled back, Number of disk blocks read in this database, Number of times disk blocks were found already in the buffer cache, so that a read was not necessary (this only includes hits in the PostgreSQL buffer cache, not the operating system's file system cache), Number of live rows fetched by sequential scans and index entries returned by index scans in this database, Number of live rows fetched by index scans in this database, Number of rows inserted by queries in this database, Number of rows updated by queries in this database, Number of rows deleted by queries in this database, Number of queries canceled due to conflicts with recovery in this database. This should not be used for data integrity checks. 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. Waiting to write zero bytes to a dynamic shared memory backing file. For client backends, this is the time the client connected to the server. 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. (See Chapter19 for details about setting configuration parameters.). Waiting for confirmation from remote server during synchronous replication.
Postgres 10.3: SELECT queries hang for hours - Stack Overflow Waiting for data to reach durable storage while creating the data directory lock file. Cumulative statistics are collected in shared memory. Type of current backend. PostgreSQL accesses certain on-disk information via SLRU (simple least-recently-used) caches. Waiting for logical rewrite mappings to reach durable storage during a checkpoint. Therefore it is not safe to assume that all files older than last_archived_wal have also been successfully archived. You Waiting for I/O on a serializable transaction conflict SLRU buffer. 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. OID of the database this backend is connected to, Name of the database this backend is connected to.
Locks in PostgreSQL: 4. Locks in memory - Habr 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. In such cases, an older set of per-backend statistics access functions can be used; these are shown in Table28.20. 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. See. 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. When recovery is performed at server start (e.g., after immediate shutdown, server crash, and point-in-time recovery), all statistics counters are reset. Waiting when WAL data is not available from any kind of sources (local, archive or stream) before trying again to retrieve WAL data, at recovery. Superusers and roles with privileges of built-in role pg_read_all_stats (see also Section22.5) can see all the information about all sessions. Number of times in-progress transactions were streamed to the decoding output plugin while decoding changes from WAL for this slot. For an asynchronous standby, the replay_lag column approximates the delay before recent transactions became visible to queries. Waiting for a write while initializing a new WAL file. This lock is used to handle multiple sessions that all require access to the same Waiting for I/O on commit timestamp buffer. Waiting for data to reach durable storage while assigning WAL sync method. Waiting to read or update shared multixact state. Buffer pin waits can be protracted if another process holds an open cursor which last read data from the buffer in question. 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. By default the query text is truncated at 1024 bytes; this value can be changed via the parameter track_activity_query_size. See Table28.4. Waiting for an elected Parallel Hash participant to allocate more batches. Returns the OID of the database this backend is connected to.
The Internals of PostgreSQL : Chapter 8 Buffer Manager - Hironobu SUZUKI Waiting for I/O on a commit timestamp SLRU buffer. Waiting for a read of a serialized historical catalog snapshot. The argument can be bgwriter to reset all the counters shown in the pg_stat_bgwriter view, archiver to reset all the counters shown in the pg_stat_archiver view, wal to reset all the counters shown in the pg_stat_wal view or recovery_prefetch to reset all the counters shown in the pg_stat_recovery_prefetch view. The IO:DataFileRead wait event occurs while data is Waiting to read or update the last value set for a transaction commit timestamp. Waiting for parallel bitmap scan to become initialized. Number of deadlocks detected in this database. Waiting for a logical replication remote server to send data for initial table synchronization. If a backend is in the active state, it may or may not be waiting on some event. Waiting for a barrier event to be processed by all backends. Thanks for letting us know this page needs work. shared_buffers parameter. Table28.12.pg_stat_database_conflicts View. Waiting for parallel workers to finish computing. 28.2.3. Waiting for truncate of mapping data during a logical rewrite. See, One row for each table in the current database, showing statistics about accesses to that specific table. Autovacuum worker or launcher waiting to update or read the current state of autovacuum workers. See, One row for each index in the current database, showing statistics about I/O on that specific index. Streaming only works with top-level transactions (subtransactions can't be streamed independently), so the counter is not incremented for subtransactions. BufferCacheHitRatio and LWLock:BufferIO wait Number of backends currently connected to this database, or NULL for shared objects. Waiting to find or allocate space in shared memory. It can be joined to pg_stat_activity or pg_stat_replication on the pid column to get more details about the connection. PostgreSQL 's statistics collector is a subsystem that supports collection and reporting of information about server activity. Waiting to add a message in shared invalidation queue. See, One row for each tracked function, showing statistics about executions of that function. 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. The columns wal_distance, block_distance and io_depth show current values, and the other columns show cumulative counters that can be reset with the pg_stat_reset_shared function. Sometimes it may be more convenient to obtain just a subset of this information. The idx_tup_read and idx_tup_fetch counts can be different even without any use of bitmap scans, because idx_tup_read counts index entries retrieved from the index while idx_tup_fetch counts live rows fetched from the table. This is used by system processes waiting for activity in their main processing loop. The pg_stat_user_tables and pg_stat_sys_tables views contain the same information, but filtered to only show user and system tables respectively.
postgres/README at master postgres/postgres GitHub Its This event type indicates a process waiting for activity in its main processing loop. Connection string used by this WAL receiver, with security-sensitive fields obfuscated. Waiting for the group leader to update transaction status at end of a parallel operation. The wait_event and state columns are independent. 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. In some cases, the name assigned by an extension will not be available in all server processes; so an LWLock wait event might be reported as just extension rather than the extension-assigned name. potential: This standby server is now asynchronous, but can potentially become synchronous if one of current synchronous ones fails. In rows about other sessions, many columns will be null. purpose is for the same page to be read into the shared buffer. Waiting to write a protocol message to a shared message queue. PostgreSQL utilizes lightweight locks (LWLocks) to synchronize and control access to the buffer content. These access functions use a backend ID number, which ranges from one to the number of currently active backends. - a BufFreeList LWLock was getting acquired to find a free buffer for a page - to change the association of buffer in buffer mapping hash table a LWLock is acquired on a hash partition to which the buffer to be associated belongs and as there were just 16 such partitions, there was huge contention when multiple clients Waiting to read or update replication slot state. Alternatively, one can build custom views using the underlying statistics functions, as discussed in Section28.2.3. 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. Returns the IP address of the client connected to this backend. Waiting for a write of a timeline history file received via streaming replication. It also tracks the total number of rows in each table, and information about vacuum and analyze actions for each table. Waiting to access the multixact offset SLRU cache. Waiting for a read during a file copy operation. Waiting to insert WAL data into a memory buffer. wait_event will contain a name identifying the purpose of the lightweight lock. When using the cumulative statistics views and functions to monitor collected data, it is important to realize that the information does not update instantaneously. The access functions for per-database statistics take a database OID as an argument to identify which database to report on. Re: Improve WALRead() to suck data directly from WAL buffers when possible Send time of last reply message received from standby server. The combination of certificate serial number and certificate issuer uniquely identifies a certificate (unless the issuer erroneously reuses serial numbers). See, One row per database, showing database-wide statistics. The function pg_stat_get_backend_idset provides a convenient way to generate one row for each active backend for invoking these functions. Note that only tables, indexes, and functions in the current database can be seen with these functions.
5mins of Postgres E25: Postgres lock monitoring, LWLocks and the log From pg_stat_activity i noticed that the wait_event_type and wait_event of these queries is as follows: Waiting for data to reach durable storage while creating the data directory lock file. Waiting for parallel workers to finish computing. Additional functions related to statistics collection are listed in Table28.19. Host name of the connected client, as reported by a reverse DNS lookup of client_addr. Presently, accesses to tables and indexes in both disk-block and individual-row terms are counted. In particular, when the standby has caught up completely, pg_stat_replication shows the time taken to write, flush and replay the most recent reported WAL location rather than zero as some users might expect.