Waiting for a write to the relation map file. There are also several other views, listed in Table28.2, available to show the accumulated statistics. Waiting for a logical replication remote server to change state. PostgreSQL Entangled in Locks: Attempts to free it - Amit Kapila - Dilip Kumar PGCon 2017 . Waiting for a two phase state file to reach durable storage. Identifier of this backend's most recent query. 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. This has no effect in a quorum-based synchronous replication. See Table28.4. IP address of the client connected to this backend. The server process is waiting for a heavyweight lock. Waiting for a replication slot control file to reach durable storage. This event type indicates a process waiting for activity in its main processing loop. might be causing it. disabled: This state is reported if track_activities is disabled in this backend. Since collection of statistics adds some overhead to query execution, the system can be configured to collect or not collect information. Other ways of looking at the statistics can be set up by writing queries that use the same underlying statistics access functions used by the standard views shown above. Waiting to read or record conflicting serializable transactions. The server process is waiting for a timeout to expire. If this field is null, it indicates that the client is connected via a Unix socket on the server machine. Send time of last reply message received from standby server. 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. This field is truncated if the principal is longer than NAMEDATALEN (64 characters in a standard build). Waiting for a replication slot control file to reach durable storage. The generated IO patterns are also much worse. Waiting for the group leader to update transaction status at end of a parallel operation. Waiting to read or update shared notification state. Waiting to read or update multixact member mappings. @ LWTRANCHE_REPLICATION_SLOT_IO. As soon as the page is read inside the shared buffer pool, the LWLock:BufferIO lock is released. You can split your Waiting to access the list of predicate locks held by serializable transactions. We're sorry we let you down. Priority of this standby server for being chosen as the synchronous standby in a priority-based synchronous replication. Waiting for a write while creating the data directory lock file. See, One row for each index in the current database, showing statistics about accesses to that specific index. If enabled, calls to user-defined functions and the total time spent in each one are counted as well. Returns the set of currently active backend ID numbers (from 1 to the number of active backends). PostgreSQL 's statistics collector is a subsystem that supports collection and reporting of information about server activity. 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. Priority of this standby server for being chosen as the synchronous standby in a priority-based synchronous replication. 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. The access functions for per-database statistics take a database OID as an argument to identify which database to report on. Waiting to select the starting location of a synchronized table scan. 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. 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. 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. Waiting for a write while creating the data directory lock file. Waiting to elect a Parallel Hash participant to allocate a hash table. quorum: This standby server is considered as a candidate for quorum standbys. Waiting to access the list of predicate locks held by the current serializable transaction during a parallel query. Waiting for a write to a replication slot control file. Since collection of statistics adds some overhead to query execution, the system can be configured to collect or not collect information. A process can wait for the data needed from a client ( Client) or another process ( IPC ). Waiting for the relation map file to reach durable storage. A snapshot is taken the first time cumulative statistics are accessed in a transaction if stats_fetch_consistency is set to snapshot. 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. Resets statistics of the replication slot defined by the argument. 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. OID of this database, or 0 for objects belonging to a shared relation. 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. Waiting for I/O on a clog (transaction status) buffer. When using the cumulative statistics views and functions to monitor collected data, it is important to realize that the information does not update instantaneously. Resets all statistics counters for the current database to zero. Waiting in main loop of startup process for WAL to arrive, during streaming recovery. Waiting for a read while adding a line to the data directory lock file. Waiting for a logical replication remote server to send data for initial table synchronization. Type of current backend. Only directly connected standbys are listed; no information is available about downstream standby servers. Several predefined views, listed in Table28.1, are available to show the current state of the system. 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. The per-index statistics are particularly useful to determine which indexes are being used and how effective they are. 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. (Conflicts occur only on standby servers; see pg_stat_database_conflicts for details.). The server process is waiting for activity on a socket connected to a user application. See, At least one row per subscription, showing information about the subscription workers. PostgreSQL accesses certain on-disk information via SLRU (simple least-recently-used) caches. This counter is incremented each time a transaction is streamed, and the same transaction may be streamed multiple times. The pg_stat_database view will contain one row for each database in the cluster, showing database-wide statistics. Such a system would show similar times while new WAL is being generated, but would differ when the sender becomes idle. See Table28.5 through Table28.13. pg_stat_get_backend_xact_start ( integer ) timestamp with time zone. The overhead of a file is much more than wasting the remainder of a page. Waiting to read or update the last value set for the transaction timestamp. See, One row per connection (regular and replication), showing information about SSL used on this connection. The pg_stat_subscription view will contain one row per subscription for main worker (with null PID if the worker is not running), and additional rows for workers handling the initial data copy of the subscribed tables. See, One row only, showing statistics about the background writer process's activity. See, One row for each table in the current database, showing statistics about I/O on that specific table. If the argument is NULL, reset statistics for all subscriptions. Pointers to free buffers and to the next victim are protected by one buffer strategy lock spinlock. Returns the OID of the user logged into this backend. Time elapsed between flushing recent WAL locally and receiving notification that this standby server has written and flushed it (but not yet applied it). Total amount of time spent writing WAL buffers to disk via XLogWrite request, in milliseconds (if track_wal_io_timing is enabled, otherwise zero). Waiting to acquire a virtual transaction ID lock. It can also count calls to user-defined functions and the total time spent in each one. If this field is null, it indicates that this is an internal server process. 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. Note that this includes the transactions that are streamed and/or spilled. Waiting in main loop of syslogger process. Waiting for background worker to start up. Waiting for logical rewrite mappings to reach durable storage during a checkpoint. See, Time when the current transaction was started. Its This can be used to gauge the delay that, Time elapsed between flushing recent WAL locally and receiving notification that this standby server has written, flushed and applied it. Host name of the connected client, as reported by a reverse DNS lookup of client_addr. Waiting to send bytes to a shared message queue. See. Port number of the PostgreSQL instance this WAL receiver is connected to. The parameter track_wal_io_timing enables monitoring of WAL write times. By default the query text is truncated at 1024 bytes; this value can be changed via the parameter track_activity_query_size. Waiting for WAL buffers to be written to disk. Re: Improve WALRead() to suck data directly from WAL buffers when possible Waiting to read or update the control file or creation of a new WAL file. 39919 LWLock buffer_mapping 5119 Client ClientRead 3116 IO DataFileRead . 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. 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. (The path case can be distinguished because it will always be an absolute path, beginning with /.). Possible values are: Top-level transaction identifier of this backend, if any. Waiting to acquire a lock on page of a relation. 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 fsync call (normally the background writer handles those even when the backend does its own write). Waiting for a read during reorder buffer management. backup: This WAL sender is sending a backup. Waiting for a serialized historical catalog snapshot to reach durable storage. Detailed Description . See, Only one row, showing statistics about the WAL receiver from that receiver's connected server. The server process is waiting for an I/O operation to complete. workload into more reader nodes. 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. Table28.31.pg_statio_all_sequences View, Number of disk blocks read from this sequence. Alternatively, one can build custom views using the underlying cumulative statistics functions, as discussed in Section28.2.24. Waiting for the control file to reach durable storage. Lock: The backend is waiting for a heavyweight lock. The pg_stat_archiver view will always have a single row, containing data about the archiver process of the cluster. Waiting for I/O on a transaction status SLRU buffer. Waiting for another process to be attached to a shared message queue. being read from storage. PostgreSQL is one of the most popular open-source databases in the world and has successful implementations across several mission-critical environments across various domains, using real-time high-end OLTP applications performing millions and billions of transactions per day.
