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. Waiting for a write to the relation map file. Number of blocks zeroed during initializations, Number of times disk blocks were found already in the SLRU, so that a read was not necessary (this only includes hits in the SLRU, not the operating system's file system cache), Number of disk blocks written for this SLRU, Number of blocks checked for existence for this SLRU, Number of flushes of dirty data for this SLRU. Waiting to find or allocate space in shared memory. This facility is independent of the cumulative statistics system. Waiting for a newly created timeline history file to reach durable storage. Waiting for WAL to be flushed in WAL sender process. 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 BM_IO_IN_PROGRESS flag acts as a kind of lock, used to wait for I/O on a: buffer to complete (and in releases before 14, it was accompanied by a: per-buffer LWLock). Note that this includes data that is streamed and/or spilled. Waiting to access a parallel query's information about type modifiers that identify anonymous record types. 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 during recovery when WAL data is not available from any source (. Waiting for a replication slot to become inactive to be dropped. Returns the text of this backend's most recent query. For more information on lightweight locks, see Locking Overview. Waiting to elect a Parallel Hash participant to allocate more batches. Port number of the PostgreSQL instance this WAL receiver is connected to. Table28.17.pg_statio_all_sequences View. Conversely, if it's known that statistics are only accessed once, caching accessed statistics is unnecessary and can be avoided by setting stats_fetch_consistency to none. Alternatively, one can build custom views using the underlying statistics functions, as discussed in Section28.2.3. Waiting for a read during a file copy operation. The server process is waiting for a heavyweight lock. See Table28.4. It is used per the rules above. This lock is used to handle multiple sessions that all require access to the same Waiting to allocate or free a replication slot. Waiting for data to reach durable storage while creating the data directory lock file. This can be a host name, an IP address, or a directory path if the connection is via Unix socket. Waiting for background worker to start up. 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 read from a relation data file. Waiting for a read from a replication slot control file. 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. checksum_last_failure timestamp with time zone. might be causing it. IPC: The server process is waiting for some activity from another process in the server. Waiting to update limits on transaction id and multixact consumption. See, One row for each index in the current database, showing statistics about I/O on that specific index. If this field is null, it indicates that this is an internal server process. Most such locks protect a particular data structure in shared memory. Waiting for SSL while attempting connection. You can split your Its purpose is for the same page to be read into the shared buffer. 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. Lock: The backend is waiting for a heavyweight lock. potential: This standby server is now asynchronous, but can potentially become synchronous if one of current synchronous ones fails. Waiting for the relation map file to reach durable storage. 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. For more information, see LWLock:buffer_content (BufferContent). In such cases, an older set of per-backend statistics access functions can be used; these are shown in Table28.20. Waiting to retrieve messages from the shared catalog invalidation queue. 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. For more information, see LWLock:buffer_mapping. 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. Waiting for a replication slot control file to reach durable storage. [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 ! Heavyweight locks, also known as lock manager locks or simply locks, primarily protect SQL-visible objects such as tables. Waiting for a write of a timeline history file received via streaming replication. Waiting for data to reach durable storage while creating the data directory lock file. pg_stat_reset_replication_slot ( text ) void. Definition: lwlock.h:190. 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. The pg_stat_ssl view will contain one row per backend or WAL sender process, showing statistics about SSL usage on this connection. buffer_mapping: Waiting to associate a data block with a buffer in the buffer pool. David Christensen on Twitter. For tranches registered by extensions, the name is specified by extension and this will be displayed as wait_event. PostgreSQL utilizes lightweight locks (LWLocks) to synchronize and control access to the buffer content. Waiting for recovery conflict resolution for dropping a tablespace. Waiting to apply WAL during recovery because of a delay setting. Here is an example of how wait events can be viewed. Waiting to read while creating the data directory lock file. Only directly connected standbys are listed; no information is available about downstream standby servers. A process acquires an LWLock in a shared mode to read from the buffer and . Waiting for a read of a two phase state file. See, One row per connection (regular and replication), showing information about SSL used on this connection. Time when this process was started. IP address of the client connected to this WAL sender. Waiting to read or update multixact member mappings. 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. - 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 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. Total number of WAL full page images generated, Number of times WAL data was written to disk because WAL buffers became full. Waiting for any activity when processing replies from WAL receiver in WAL sender process. Waiting to read or update information about. See Table28.5 through Table28.13. pg_stat_get_backend_xact_start ( integer ) timestamp with time zone. Waiting for I/O on a multixact offset SLRU buffer. Waiting to add a message in shared invalidation queue. A process can wait for the data needed from a client ( Client) or another process ( IPC ). The pg_statio_user_tables and pg_statio_sys_tables views contain the same information, but filtered to only show user and system tables respectively. Waiting for other process to be attached in shared message queue. Waiting to access a parallel query's information about composite types. You Wait event name if backend is currently waiting, otherwise NULL. The LWLock:BufferIO event occurs when Aurora PostgreSQL or RDS for PostgreSQL is waiting for other processes to Buffer pin waits can be protracted if another process holds an open cursor which last read data from the buffer in question. Waiting for an elected Parallel Hash participant to allocate a hash table. Waiting for a read from a timeline history file during a walsender timeline command. Waiting to read or update the state of prepared transactions. The parameter track_counts controls whether statistics are collected about table and index accesses. The pg_stat_user_functions view will contain one row for each tracked function, showing statistics about executions of that function. Waiting to replace a page in WAL buffers. BufferCacheHitRatio metric dips. wait_event will identify the specific wait point. For details such as the functions' names, consult the definitions of the standard views. Waiting in main loop of checkpointer process. Waiting for I/O on an async (notify) buffer. This can be used to gauge the delay that. Identifier of this backend's most recent query. Returns the wait event name if this backend is currently waiting, otherwise NULL. Several predefined views, listed in Table28.1, are available to show the current state of the system. The LWLock:BufferIO event occurs when RDS for PostgreSQL or Aurora PostgreSQL is waiting for other processes to finish their I/O operations. streaming: This WAL sender is streaming changes after its connected standby server has caught up with the primary. 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. See, One row for each table in the current database, showing statistics about accesses to that specific table. Synchronous state of this standby server. 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. Since collection of statistics adds some overhead to query execution, the system can be configured to collect or not collect information. If you've got a moment, please tell us what we did right so we can do more of it. Waiting for a write of a two phase state file. Monitoring systems should choose whether to represent this as missing data, zero or continue to display the last known value. The WALWriteLock wait occurs while PostgreSQL flushes WAL records to disk or during a WAL segment switch.. How to reduce this wait . The per-index statistics are particularly useful to determine which indexes are being used and how effective they are. The pg_stat_wal_receiver view will contain only one row, showing statistics about the WAL receiver from that receiver's connected server. idle in transaction (aborted): This state is similar to idle in transaction, except one of the statements in the transaction caused an error. We recommend different actions depending on the causes of your wait event: Observe Amazon CloudWatch metrics for correlation between sharp decreases in the Returns the OID of the user logged into this backend. For better performance, stats_temp_directory can be pointed at a RAM-based file system, decreasing physical I/O requirements. Last write-ahead log location already received and flushed to disk, the initial value of this field being the first log location used when WAL receiver is started, Timeline number of last write-ahead log location received and flushed to disk, the initial value of this field being the timeline number of the first log location used when WAL receiver is started, last_msg_send_time timestamp with time zone, Send time of last message received from origin WAL sender, last_msg_receipt_time timestamp with time zone, Receipt time of last message received from origin WAL sender, Last write-ahead log location reported to origin WAL sender, Time of last write-ahead log location reported to origin WAL sender, Replication slot name used by this WAL receiver, Host of the PostgreSQL instance this WAL receiver is connected to.