Waiting for logical rewrite mappings to reach durable storage during a checkpoint. might be causing it. We're sorry we let you down. Waiting between writes while performing a checkpoint. If, Type of current backend. This includes the sync time when wal_sync_method is either open_datasync or open_sync. 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. In addition, background workers registered by extensions may have additional types. Number of times transactions were spilled to disk while decoding changes from WAL for this slot. Waiting for a read during reorder buffer management. Table28.31.pg_statio_all_sequences View, Number of disk blocks read from this sequence. (See Chapter20 for details about setting configuration parameters.). Using pg_stat_reset() also resets counters that autovacuum uses to determine when to trigger a vacuum or an analyze. Note that only tables, indexes, and functions in the current database can be seen with these functions. Last write-ahead log location already received and written to disk, but not flushed. Such a system would show similar times while new WAL is being generated, but would differ when the sender becomes idle. backup: This WAL sender is sending a backup. Several predefined views, listed in Table28.1, are available to show the current state of the system. disabled: This state is reported if track_activities is disabled in this backend. postgres7 Slru--1. The following wait events are a subset of the list in Amazon Aurora PostgreSQL wait events. Waiting for background worker to shut down. The pg_stat_wal_receiver view will contain only one row, showing statistics about the WAL receiver from that receiver's connected server. Waiting while sending synchronization requests to the checkpointer, because the request queue is full. 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. If the argument is NULL, reset statistics for all subscriptions. Waiting to read or update the state of prepared transactions. The pg_stat_user_functions view will contain one row for each tracked function, showing statistics about executions of that function. See Section30.5 for more information about the internal WAL function issue_xlog_fsync. Waiting for a write when creating a new WAL segment by copying an existing one. 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. For more information, see LWLock:buffer_mapping. Table28.12.pg_stat_database_conflicts View. Waiting for SLRU data to reach durable storage during a checkpoint or database shutdown. The new locking scheme avoids grabbing system-wide exclusive locks in common code paths. Waiting to access the multixact offset SLRU cache. Waiting to manage an extension's space allocation in shared memory. PostgreSQL Entangled in Locks: Attempts to free it - Amit Kapila - Dilip Kumar PGCon 2017 . Number of deadlocks detected in this database. The pg_stat_database_conflicts view will contain one row per database, showing database-wide statistics about query cancels occurring due to conflicts with recovery on standby servers. 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. If enabled, calls to user-defined functions and the total time spent in each one are counted as well. 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. Waiting to acquire a virtual transaction ID lock. Each individual server process flushes out accumulated statistics to shared memory just before going idle, but not more frequently than once per PGSTAT_MIN_INTERVAL milliseconds (1 second unless altered while building the server); so a query or transaction still in progress does not affect the displayed totals and the displayed information lags behind actual activity. 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 to read or update the last value set for the transaction timestamp. 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. Thus, the server expects something to happen that is independent of its internal processes. Restrict the maximum number of connections to the database as a best practice. 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. Waiting for mapping data to reach durable storage during a logical rewrite. The access functions for per-database statistics take a database OID as an argument to identify which database to report on. The pg_stat_slru view will contain one row for each tracked SLRU cache, showing statistics about access to cached pages. The server process is waiting for an I/O operation to complete. sync: This standby server is synchronous. Waiting to read or update multixact offset mappings. 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 If the argument is NULL, resets statistics for all the replication slots. BK_1935: "IObuffer_locks,ControlLock()"IOControlLockControlLockIOSlruSharedData Waiting for I/O on a clog (transaction status) buffer. Streaming only works with top-level transactions (subtransactions can't be streamed independently), so the counter is not incremented for subtransactions. Waiting in background writer process, hibernating. Waiting for an elected Parallel Hash participant to finish allocating more buckets. It can be joined to pg_stat_activity or pg_stat_replication on the pid column to get more details about the connection. 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. Lag times work automatically for physical replication. fastpath function call: The backend is executing a fast-path function. Waiting for WAL to reach durable storage during bootstrapping. PostgreSQL also supports reporting dynamic information about exactly what is going on in the system right now, such as the exact command currently being executed by other server processes, and which other connections exist in the system. I'd like to know more about what these locks could imply if anything. OID of the database this backend is connected to, Name of the database this backend is connected to. Alternatively, one can build custom views using the underlying statistics functions, as discussed in Section28.2.3. Each individual server process transmits new statistical counts to the collector just before going idle; so a query or transaction still in progress does not affect the displayed totals. buffer_mapping: Waiting to associate a data block with a buffer in the buffer pool. 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. See, At least one row per subscription, showing information about the subscription workers. your experience with the particular feature or requires further clarification, Logical decoding plugins may optionally emit tracking messages; if they do not, the tracking mechanism will simply display NULL lag. 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. Sometimes it may be more convenient to obtain just a subset of this information. This should not be used for data integrity checks. PostgreSQL accesses certain on-disk information via SLRU (simple least-recently-used) caches. Waiting to read or write relation cache initialization file. Waiting to choose the next subplan during Parallel Append plan execution. This is consistent with the goal of measuring synchronous commit and transaction visibility delays for recent write transactions. From the Actions drop-down menu, choose Create Read Replica. The pg_statio_ views are primarily useful to determine the effectiveness of the buffer cache. LWLock:buffer_mapping. Waiting to read or update information about. Waiting for a replication slot control file to reach durable storage. The parameter track_io_timing enables monitoring of block read and write times. Waiting to read or update shared multixact state. Waiting in main loop of WAL writer process. Waiting for parallel query dynamic shared memory allocation. Waiting to receive bytes from a shared message queue. This standby's xmin horizon reported by hot_standby_feedback. Please refer to your browser's Help pages for instructions. Waiting for a read when creating a new WAL segment by copying an existing one. Waiting to read or write a data page in memory. active: The backend is executing a query. Waiting to acquire a lock on page of a relation. In such cases, an older set of per-backend statistics access functions can be used; these are shown in Table28.35. The server process is idle. Priority of this standby server for being chosen as the synchronous standby in a priority-based synchronous replication. replication_origin: Waiting to read or update the replication progress. Number of times WAL buffers were written out to disk via XLogWrite request. Doing this helps Waiting for a write of a timeline history file received via streaming replication. Waiting to access a parallel query's information about composite types. Waiting for a read from a replication slot control file. Client: The server process is waiting for some activity on a socket from user applications, and that the server expects something to happen that is independent from its internal processes. Waiting to manage space allocation in shared memory. Waiting for a read of a two phase state file. For example, to show the PIDs and current queries of all backends: Table28.20. Waiting in background writer process, hibernating. Waiting to write a protocol message to a shared message queue. Verify whether you have unused indexes, then remove them. Returns the time when this process was started. Waiting for WAL to reach durable storage during bootstrapping. Each shared buffer has an I/O lock that is associated with the LWLock:BufferIO wait event, each time a block (or Waiting to send bytes to a shared message queue. 106 . Table28.6.