From 1bee1a37250bb0cd9c062094644887ec336b055b Mon Sep 17 00:00:00 2001 From: Bruce Momjian Date: Thu, 18 Feb 2010 03:16:09 +0000 Subject: [PATCH] Wordsmithing of HS and SR documentation, with some wording improvements from Erik Rijkers too. --- doc/src/sgml/high-availability.sgml | 487 ++++++++++++++-------------- 1 file changed, 252 insertions(+), 235 deletions(-) diff --git a/doc/src/sgml/high-availability.sgml b/doc/src/sgml/high-availability.sgml index e868e232d6..48f6e98f10 100644 --- a/doc/src/sgml/high-availability.sgml +++ b/doc/src/sgml/high-availability.sgml @@ -1,4 +1,4 @@ - + High Availability, Load Balancing, and Replication @@ -497,7 +497,7 @@ protocol to make nodes agree on a serializable transactional order. continuous recovery mode, reading the WAL files from the primary. No changes to the database tables are required to enable this capability, so it offers low administration overhead compared to some other - replication approaches. This configuration also has relatively low + replication solutions. This configuration also has relatively low performance impact on the primary server. @@ -516,14 +516,14 @@ protocol to make nodes agree on a serializable transactional order. It should be noted that the log shipping is asynchronous, i.e., the WAL - records are shipped after transaction commit. As a result there is a + records are shipped after transaction commit. As a result, there is a window for data loss should the primary server suffer a catastrophic - failure: transactions not yet shipped will be lost. The length of the - window of data loss can be limited by use of the + failure; transactions not yet shipped will be lost. The size of the + data loss window can be limited by use of the archive_timeout parameter, which can be set as low - as a few seconds if required. However such a low setting will + as a few seconds. However such a low setting will substantially increase the bandwidth required for file shipping. - If you need a window of less than a minute or so, it's probably better + If you need a window of less than a minute or so, it is probably better to consider record-based log shipping. @@ -531,8 +531,8 @@ protocol to make nodes agree on a serializable transactional order. The standby server is not available for access, since it is continually performing recovery processing. Recovery performance is sufficiently good that the standby will typically be only moments away from full - availability once it has been activated. As a result, we refer to this - capability as a warm standby configuration that offers high + availability once it has been activated. As a result, this is called + a warm standby configuration which offers high availability. Restoring a server from an archived base backup and rollforward will take considerably longer, so that technique only offers a solution for disaster recovery, not high availability. @@ -592,7 +592,7 @@ protocol to make nodes agree on a serializable transactional order. recovery.conf file on the standby server. Normal recovery processing would request a file from the WAL archive, reporting failure if the file was unavailable. For standby processing it is normal for - the next WAL file to be unavailable, so we must be patient and wait for + the next WAL file to be unavailable, so the standby must wait for it to appear. For files ending in .backup or .history there is no need to wait, and a non-zero return code must be returned. A waiting restore_command can be @@ -639,8 +639,8 @@ if (!triggered) and design. One potential option is the restore_command command. It is executed once for each WAL file, but the process running the restore_command is created and dies for - each file, so there is no daemon or server process, and we cannot - use signals or a signal handler. Therefore, the + each file, so there is no daemon or server process, and + signals or a signal handler cannot be used. Therefore, the restore_command is not suitable to trigger failover. It is possible to use a simple timeout facility, especially if used in conjunction with a known archive_timeout @@ -749,7 +749,7 @@ if (!triggered) Starting with PostgreSQL version 9.0, you can use streaming replication (see ) to - achieve the same with less effort. + achieve the same benefits with less effort. @@ -763,17 +763,18 @@ if (!triggered) PostgreSQL includes a simple streaming replication - mechanism, which lets the standby server to stay more up-to-date than - file-based log shipping allows. The standby connects to the primary + mechanism, which allows the standby server to stay more up-to-date than + file-based log shipping. The standby connects to the primary and the primary starts streaming WAL records from where the standby left off, and continues streaming them as they are generated, without waiting for the WAL file to be filled. So with streaming replication, - archive_timeout does not need to be configured. + archive_timeout does not need to be changed to reduce + possible data loss. Streaming replication relies on file-based continuous archiving for - making the base backup and for allowing a standby to catch up if it's + making the base backup and for allowing a standby to catch up if it is disconnected from the primary for long enough for the primary to delete old WAL files still required by the standby. @@ -783,6 +784,7 @@ if (!triggered) The short procedure for configuring streaming replication is as follows. For full details of each step, refer to other sections as noted. + @@ -806,16 +808,16 @@ if (!triggered) Set and authentication options - (see pg_hba.conf) so that the standby server can connect to - the pseudo replication database of the primary server (see + (see pg_hba.conf) so the standby server can connect to + the replication pseudo-database on the primary server (see ). On systems that support the keepalive socket option, setting , and - helps the master to notice - a broken connection promptly. + helps the master promptly + notice a broken connection. @@ -843,7 +845,7 @@ if (!triggered) If you're setting up the standby server for high availability purposes, set up WAL archiving, connections and authentication like the primary server, because the standby server will work as a primary server after - failover. If you're setting up the standby server for e.g reporting + failover. If you're setting up the standby server for reporting purposes, with no plans to fail over to it, configure the standby accordingly. @@ -863,7 +865,7 @@ if (!triggered) a standby. If this parameter is on, the server will not end recovery when the end of archived WAL is reached, but will keep trying to continue recovery using restore_command - and by connecting to the primary server as specified by + and by connecting to the primary server as specified by the primary_conninfo setting. @@ -873,9 +875,10 @@ if (!triggered) restore_end_command (string) - In standby-mode, restore_command (and restore_end_command) is set to a - simple command or script like in PITR. pg_standby or similar tools - that wait for the next WAL file to arrive, cannot be used with + With standby_mode enabled, restore_command + (and restore_end_command) should be set to a + simple command or script like in PITR. pg_standby or similar tools + that wait for the next WAL file to arrive cannot be used with streaming replication, as the server handles retries and waiting itself. Set restore_command as you would if you were recovering using a Continuous archiving backup (see ). @@ -886,18 +889,18 @@ if (!triggered) primary_conninfo (string) - Specifies a connection string which is used for the standby server + Specifies a connection string to be used for the standby server to connect with the primary. This string is in the same format as described in . If any option is unspecified in this string, then the corresponding environment variable (see ) is checked. If the - environment variable is not set either, then the indicated built-in + environment variable is not set either, then defaults are used. The built-in replication requires that a host name (or host address) - or port number which the primary server listens on should be - specified in this string, respectively. Also ensure that a role with + or port number which the primary server listens on be + specified in this string. Also ensure that a role with the SUPERUSER and LOGIN privileges on the primary is set (see ). Note that @@ -916,8 +919,6 @@ if (!triggered) Specifies a trigger file whose presence ends recovery in the standby. If no trigger file is specified, the standby never exits recovery. - - This setting has no effect if standby_mode is off. @@ -927,22 +928,23 @@ if (!triggered) Start the PostgreSQL server on the standby. The standby - server will go into recovery mode and proceeds to receive WAL records + server will go into recovery mode and proceed to receive WAL records from the primary and apply them continuously. + Authentication - It's very important that the access privilege for replication are set - properly so that only trusted users can read the WAL stream, because it's - easy to extract serious information from it. + It is very important that the access privilege for replication be setup + properly so that only trusted users can read the WAL stream, because it is + easy to extract privileged information from it. - Only superuser is allowed to connect to the primary as the replication + Only the superuser is allowed to connect to the primary as the replication standby. So a role with the SUPERUSER and LOGIN privileges needs to be created in the primary. @@ -963,7 +965,7 @@ host replication foo 192.168.1.100/32 md5 - The host name and port number of the primary, user name to connect as, + The host name and port number of the primary, connection user name, and password are specified in the recovery.conf file or the corresponding environment variable on the standby. For example, if the primary is running on host IP 192.168.1.50, @@ -992,7 +994,7 @@ primary_conninfo = 'host=192.168.1.50 port=5432 user=foo password=foopass' If the standby server fails then no failover need take place. If the standby server can be restarted, even some time later, then the recovery - process can also be immediately restarted, taking advantage of + process can also be restarted immediately, taking advantage of restartable recovery. If the standby server cannot be restarted, then a full new standby server instance should be created. @@ -1000,8 +1002,8 @@ primary_conninfo = 'host=192.168.1.50 port=5432 user=foo password=foopass' If the primary server fails and the standby server becomes the new primary, and then the old primary restarts, you must have - a mechanism for informing old primary that it is no longer the primary. This is - sometimes known as STONITH (Shoot The Other Node In The Head), which is + a mechanism for informing the old primary that it is no longer the primary. This is + sometimes known as STONITH (Shoot The Other Node In The Head), which is necessary to avoid situations where both systems think they are the primary, which will lead to confusion and ultimately data loss. @@ -1017,11 +1019,11 @@ primary_conninfo = 'host=192.168.1.50 port=5432 user=foo password=foopass' - Once failover to the standby occurs, we have only a + Once failover to the standby occurs, there is only a single server in operation. This is known as a degenerate state. The former standby is now the primary, but the former primary is down - and might stay down. To return to normal operation we must - fully recreate a standby server, + and might stay down. To return to normal operation, a standby server + must be recreated, either on the former primary system when it comes up, or on a third, possibly new, system. Once complete the primary and standby can be considered to have switched roles. Some people choose to use a third @@ -1059,31 +1061,31 @@ primary_conninfo = 'host=192.168.1.50 port=5432 user=foo password=foopass' - Running queries in recovery is in many ways the same as normal running - though there are a large number of usage and administrative points - to note. + Running queries in recovery mode is similar to normal query operation, + though there are a several usage and administrative differences + noted below. User's Overview - Users can connect to the database while the server is in recovery - and perform read-only queries. Read-only access to catalogs and views - will also occur as normal. + Users can connect to the database server while it is in recovery + mode and perform read-only queries. Read-only access to system + catalogs and views will also occur as normal. The data on the standby takes some time to arrive from the primary server so there will be a measurable delay between primary and standby. Running the same query nearly simultaneously on both primary and standby might therefore - return differing results. We say that data on the standby is eventually + return differing results. Eventually, the standby will be consistent with the primary. Queries executed on the standby will be correct with regard to the transactions that had been recovered at the start of the query, or start of first statement, in the case of serializable transactions. In comparison with the primary, the standby returns query results that could have been obtained on the primary - at some exact moment in the past. + at some moment in the past. @@ -1091,38 +1093,39 @@ primary_conninfo = 'host=192.168.1.50 port=5432 user=foo password=foopass' transaction_read_only will be forced to be true, regardless of the default_transaction_read_only setting in postgresql.conf. It can't be manually set to false either. As a result, all transactions - started during recovery will be limited to read-only actions only. In all + started during recovery will be limited to read-only actions. In all other ways, connected sessions will appear identical to sessions initiated during normal processing mode. There are no special commands - required to initiate a connection at this time, so all interfaces - work normally without change. After recovery finishes, the session + required to initiate a connection so all interfaces + work unchanged. After recovery finishes, the session will allow normal read-write transactions at the start of the next transaction, if these are requested. - Read-only here means "no writes to the permanent database tables". - There are no problems with queries that make use of transient sort and + "Read-only" above means no writes to the permanent database tables. + There are no problems with queries that use transient sort and work files. - The following actions are allowed + The following actions are allowed: - Query access - SELECT, COPY TO including views and SELECT RULEs + Query access - SELECT, COPY TO including views and + SELECT rules - Cursor commands - DECLARE, FETCH, CLOSE, + Cursor commands - DECLARE, FETCH, CLOSE - Parameters - SHOW, SET, RESET + Parameters - SHOW, SET, RESET @@ -1131,17 +1134,17 @@ primary_conninfo = 'host=192.168.1.50 port=5432 user=foo password=foopass' - BEGIN, END, ABORT, START TRANSACTION + BEGIN, END, ABORT, START TRANSACTION - SAVEPOINT, RELEASE, ROLLBACK TO SAVEPOINT + SAVEPOINT, RELEASE, ROLLBACK TO SAVEPOINT - EXCEPTION blocks and other internal subtransactions + EXCEPTION blocks and other internal subtransactions @@ -1149,76 +1152,80 @@ primary_conninfo = 'host=192.168.1.50 port=5432 user=foo password=foopass' - LOCK TABLE, though only when explicitly in one of these modes: - ACCESS SHARE, ROW SHARE or ROW EXCLUSIVE. + LOCK TABLE, though only when explicitly in one of these modes: + ACCESS SHARE, ROW SHARE or ROW EXCLUSIVE. - Plans and resources - PREPARE, EXECUTE, DEALLOCATE, DISCARD + Plans and resources - PREPARE, EXECUTE, + DEALLOCATE, DISCARD - Plugins and extensions - LOAD + Plugins and extensions - LOAD - These actions produce error messages + These actions produce error messages: - Data Manipulation Language (DML) - INSERT, UPDATE, DELETE, COPY FROM, TRUNCATE. + Data Manipulation Language (DML) - INSERT, + UPDATE, DELETE, COPY FROM, + TRUNCATE. Note that there are no allowed actions that result in a trigger being executed during recovery. - Data Definition Language (DDL) - CREATE, DROP, ALTER, COMMENT. - This also applies to temporary tables currently because currently their + Data Definition Language (DDL) - CREATE, + DROP, ALTER, COMMENT. + This also applies to temporary tables also because currently their definition causes writes to catalog tables. - SELECT ... FOR SHARE | UPDATE which cause row locks to be written + SELECT ... FOR SHARE | UPDATE which cause row locks to be written - RULEs on SELECT statements that generate DML commands. + Rules on SELECT statements that generate DML commands. - LOCK TABLE, in short default form, since it requests ACCESS EXCLUSIVE MODE. - LOCK TABLE that explicitly requests a mode higher than ROW EXCLUSIVE MODE. + LOCK TABLE, in short default form, since it requests ACCESS EXCLUSIVE MODE. + LOCK TABLE that explicitly requests a mode higher than ROW EXCLUSIVE MODE. - Transaction management commands that explicitly set non-read only state + Transaction management commands that explicitly set non-read-only state: - BEGIN READ WRITE, - START TRANSACTION READ WRITE + BEGIN READ WRITE, + START TRANSACTION READ WRITE - SET TRANSACTION READ WRITE, - SET SESSION CHARACTERISTICS AS TRANSACTION READ WRITE + SET TRANSACTION READ WRITE, + SET SESSION CHARACTERISTICS AS TRANSACTION READ WRITE - SET transaction_read_only = off + SET transaction_read_only = off @@ -1226,9 +1233,10 @@ primary_conninfo = 'host=192.168.1.50 port=5432 user=foo password=foopass' - Two-phase commit commands - PREPARE TRANSACTION, COMMIT PREPARED, - ROLLBACK PREPARED because even read-only transactions need to write - WAL in the prepare phase (the first phase of two phase commit). + Two-phase commit commands - PREPARE TRANSACTION, + COMMIT PREPARED, ROLLBACK PREPARED + because even read-only transactions need to write WAL in the + prepare phase (the first phase of two phase commit). @@ -1238,20 +1246,20 @@ primary_conninfo = 'host=192.168.1.50 port=5432 user=foo password=foopass' - LISTEN, UNLISTEN, NOTIFY since they currently write to system tables + LISTEN, UNLISTEN, NOTIFY - Note that current behaviour of read only transactions when not in + Note that the current behaviour of read only transactions when not in recovery is to allow the last two actions, so there are small and subtle differences in behaviour between read-only transactions - run on standby and during normal running. - It is possible that the restrictions on LISTEN, UNLISTEN, NOTIFY and - temporary tables may be lifted in a future release, if their internal - implementation is altered to make this possible. + run on a standby and run during normal operation. + It is possible that LISTEN, UNLISTEN, + NOTIFY, and temporary tables might be allowed in a + future release. @@ -1264,9 +1272,9 @@ primary_conninfo = 'host=192.168.1.50 port=5432 user=foo password=foopass' Users will be able to tell whether their session is read-only by - issuing SHOW transaction_read_only. In addition a set of - functions allow users to - access information about Hot Standby. These allow you to write + issuing SHOW transaction_read_only. In addition, a set of + functions () allow users to + access information about the standby server. These allow you to write functions that are aware of the current state of the database. These can be used to monitor the progress of recovery, or to allow you to write complex programs that restore the database to particular states. @@ -1274,15 +1282,15 @@ primary_conninfo = 'host=192.168.1.50 port=5432 user=foo password=foopass' In recovery, transactions will not be permitted to take any table lock - higher than RowExclusiveLock. In addition, transactions may never assign + higher than RowExclusiveLock. In addition, transactions may never assign a TransactionId and may never write WAL. Any LOCK TABLE command that runs on the standby and requests - a specific lock mode higher than ROW EXCLUSIVE MODE will be rejected. + a specific lock mode higher than ROW EXCLUSIVE MODE will be rejected. - In general queries will not experience lock conflicts with the database - changes made by recovery. This is becase recovery follows normal + In general queries will not experience lock conflicts from the database + changes made by recovery. This is because recovery follows normal concurrency control mechanisms, known as MVCC. There are some types of change that will cause conflicts, covered in the following section. @@ -1303,22 +1311,23 @@ primary_conninfo = 'host=192.168.1.50 port=5432 user=foo password=foopass' There are also additional types of conflict that can occur with Hot Standby. - These conflicts are hard conflicts in the sense that we may - need to cancel queries and in some cases disconnect sessions to resolve them. - The user is provided with a number of optional ways to handle these - conflicts, though we must first understand the possible reasons behind a conflict. + These conflicts are hard conflicts in the sense that queries + might need to be cancelled and, in some cases, sessions disconnected to resolve them. + The user is provided with several ways to handle these + conflicts, though it is important to first understand the possible causes + of conflicts: Access Exclusive Locks from primary node, including both explicit - LOCK commands and various kinds of DDL action + LOCK commands and various DDL actions Dropping tablespaces on the primary while standby queries are using - those tablespaces for temporary work files (work_mem overflow) + those tablespaces for temporary work files (work_mem overflow) @@ -1341,40 +1350,40 @@ primary_conninfo = 'host=192.168.1.50 port=5432 user=foo password=foopass' - Some WAL redo actions will be for DDL actions. These DDL actions are + Some WAL redo actions will be for DDL actions. These DDL actions are repeating actions that have already committed on the primary node, so they must not fail on the standby node. These DDL locks take priority and will automatically *cancel* any read-only transactions that get in their way, after a grace period. This is similar to the possibility of being canceled by the deadlock detector, but in this case the standby process always wins, since the replayed actions must not fail. This - also ensures that replication doesn't fall behind while we wait for a - query to complete. Again, we assume that the standby is there for high - availability purposes primarily. + also ensures that replication does not fall behind while waiting for a + query to complete. Again, the assumption is that the standby is + primarily for high availability. An example of the above would be an Administrator on Primary server - runs a DROP TABLE on a table that's currently being queried - in the standby server. - Clearly the query cannot continue if we let the DROP TABLE - proceed. If this situation occurred on the primary, the DROP TABLE - would wait until the query has finished. When the query is on the standby - and the DROP TABLE is on the primary, the primary doesn't have - information about which queries are running on the standby and so the query - does not wait on the primary. The WAL change records come through to the + running DROP TABLE on a table that is currently being queried + on the standby server. + Clearly the query cannot continue if DROP TABLE + proceeds. If this situation occurred on the primary, the DROP TABLE + would wait until the query had finished. When DROP TABLE is + run on the primary, the primary doesn't have + information about which queries are running on the standby and so + cannot wait for any of the standby queries. The WAL change records come through to the standby while the standby query is still running, causing a conflict. The most common reason for conflict between standby queries and WAL redo is "early cleanup". Normally, PostgreSQL allows cleanup of old - row versions when there are no users who may need to see them to ensure correct + row versions when there are no users who need to see them to ensure correct visibility of data (the heart of MVCC). If there is a standby query that has been running for longer than any query on the primary then it is possible for old row versions to be removed by either a vacuum or HOT. This will then generate WAL records that, if applied, would remove data on the - standby that might *potentially* be required by the standby query. + standby that might potentially be required by the standby query. In more technical language, the primary's xmin horizon is later than the standby's xmin horizon, allowing dead rows to be removed. @@ -1387,37 +1396,37 @@ primary_conninfo = 'host=192.168.1.50 port=5432 user=foo password=foopass' - We have a number of choices for resolving query conflicts. The default - is that we wait and hope the query completes. The server will wait + There are a number of choices for resolving query conflicts. The default + is to wait and hope the query finishes. The server will wait automatically until the lag between primary and standby is at most max_standby_delay seconds. Once that grace period expires, - we take one of the following actions: + one of the following actions is taken: - If the conflict is caused by a lock, we cancel the conflicting standby - transaction immediately. If the transaction is idle-in-transaction - then currently we abort the session instead, though this may change - in the future. + If the conflict is caused by a lock, the conflicting standby + transaction is cancelled immediately. If the transaction is + idle-in-transaction then the session is aborted + instead, though this might change in the future. - If the conflict is caused by cleanup records we tell the standby query - that a conflict has occurred and that it must cancel itself to avoid the + If the conflict is caused by cleanup records, the standby query is informed + a conflict has occurred and that it must cancel itself to avoid the risk that it silently fails to read relevant data because - that data has been removed. (This is regrettably very similar to the + that data has been removed. (This is regrettably similar to the much feared and iconic error message "snapshot too old"). Some cleanup - records only cause conflict with older queries, though some types of - cleanup record affect all queries. + records only conflict with older queries, while others + can affect all queries. If cancellation does occur, the query and/or transaction can always - be re-executed. The error is dynamic and will not necessarily occur - the same way if the query is executed again. + be re-executed. The error is dynamic and will not necessarily reoccur + if the query is executed again. @@ -1425,48 +1434,49 @@ primary_conninfo = 'host=192.168.1.50 port=5432 user=foo password=foopass' max_standby_delay is set in postgresql.conf. - The parameter applies to the server as a whole so if the delay is all used - up by a single query then there may be little or no waiting for queries that - follow immediately, though they will have benefited equally from the initial + The parameter applies to the server as a whole, so if the delay is consumed + by a single query then there may be little or no waiting for queries that + follow, though they will have benefited equally from the initial waiting period. The server may take time to catch up again before the grace period is available again, though if there is a heavy and constant stream of conflicts it may seldom catch up fully. - Users should be clear that tables that are regularly and heavily updated on + Users should be clear that tables that are regularly and heavily updated on the primary server will quickly cause cancellation of longer running queries on the standby. In those cases max_standby_delay can be - considered somewhat but not exactly the same as setting + considered similar to setting statement_timeout. Other remedial actions exist if the number of cancellations is unacceptable. - The first option is to connect to primary server and keep a query active - for as long as we need to run queries on the standby. This guarantees that - a WAL cleanup record is never generated and we don't ever get query - conflicts as described above. This could be done using contrib/dblink - and pg_sleep(), or via other mechanisms. If you do this, you should note - that this will delay cleanup of dead rows by vacuum or HOT and many - people may find this undesirable. However, we should remember that + The first option is to connect to the primary server and keep a query active + for as long as needed to run queries on the standby. This guarantees that + a WAL cleanup record is never generated and query conflicts do not occur, + as described above. This could be done using contrib/dblink + and pg_sleep(), or via other mechanisms. If you do this, you should note + that this will delay cleanup of dead rows by vacuum or HOT and + people might find this undesirable. However, remember that the primary and standby nodes are linked via the WAL, so this situation is no - different to the case where we ran the query on the primary node itself - except we have the benefit of off-loading the execution onto the standby. + different from the case where the query ran on the primary node itself + except for the benefit of off-loading the execution onto the standby. It is also possible to set vacuum_defer_cleanup_age on the primary - to defer the cleanup of records by autovacuum, vacuum and HOT. This may allow + to defer the cleanup of records by autovacuum, VACUUM + and HOT. This might allow more time for queries to execute before they are cancelled on the standby, without the need for setting a high max_standby_delay. - Three-way deadlocks are possible between AccessExclusiveLocks arriving from - the primary, cleanup WAL records that require buffer cleanup locks and - user requests that are waiting behind replayed AccessExclusiveLocks. Deadlocks - are resolved by time-out when we exceed max_standby_delay. + Three-way deadlocks are possible between AccessExclusiveLocks arriving from + the primary, cleanup WAL records that require buffer cleanup locks, and + user requests that are waiting behind replayed AccessExclusiveLocks. Deadlocks + are resolved by time-out when they exceed max_standby_delay. @@ -1479,13 +1489,13 @@ primary_conninfo = 'host=192.168.1.50 port=5432 user=foo password=foopass' Administrator's Overview - If there is a recovery.conf file present the server will start + If there is a recovery.conf file present, the server will start in Hot Standby mode by default, though recovery_connections can - be disabled via postgresql.conf, if required. The server may take + be disabled via postgresql.conf. The server might take some time to enable recovery connections since the server must first complete sufficient recovery to provide a consistent state against which queries can run before enabling read only connections. Look for these messages - in the server logs + in the server logs: LOG: entering standby mode @@ -1497,36 +1507,38 @@ LOG: database system is ready to accept read only connections Consistency information is recorded once per checkpoint on the primary, as long - as recovery_connections is enabled (on the primary). If this parameter - is disabled, it will not be possible to enable recovery connections on the standby. - The consistent state can also be delayed in the presence of both of these conditions + as recovery_connections is enabled on the primary. If this parameter + is disabled, it is not possible to enable recovery connections on the standby. + Reaching a consistent state can also be delayed in the presence + of both of these conditions: - a write transaction has more than 64 subtransactions + A write transaction has more than 64 subtransactions - very long-lived write transactions + Very long-lived write transactions - If you are running file-based log shipping ("warm standby"), you may need + If you are running file-based log shipping ("warm standby"), you might need to wait until the next WAL file arrives, which could be as long as the archive_timeout setting on the primary. The setting of some parameters on the standby will need reconfiguration - if they have been changed on the primary. The value on the standby must + if they have been changed on the primary. For these parameters, + the value on the standby must be equal to or greater than the value on the primary. If these parameters - are not set high enough then the standby will not be able to track work - correctly from recovering transactions. If these values are set too low the + are not set high enough then the standby will not be able to process + recovering transactions properly. If these values are set too low the server will halt. Higher values can then be supplied and the server - restarted to begin recovery again. + restarted to begin recovery again. The parameters are: @@ -1550,109 +1562,114 @@ LOG: database system is ready to accept read only connections It is important that the administrator consider the appropriate setting of max_standby_delay, set in postgresql.conf. - There is no optimal setting and should be set according to business + There is no optimal setting, so it should be set according to business priorities. For example if the server is primarily tasked as a High Availability server, then you may wish to lower max_standby_delay or even set it to zero, though that is a very aggressive setting. If the standby server is tasked as an additional - server for decision support queries then it may be acceptable to set this + server for decision support queries then it might be acceptable to set this to a value of many hours (in seconds). - Transaction status "hint bits" written on primary are not WAL-logged, - so data on standby will likely re-write the hints again on the standby. - Thus the main database blocks will produce write I/Os even though - all users are read-only; no changes have occurred to the data values - themselves. Users will be able to write large sort temp files and - re-generate relcache info files, so there is no part of the database - that is truly read-only during hot standby mode. There is no restriction - on the use of set returning functions, or other users of tuplestore/tuplesort + Transaction status "hint bits" written on the primary are not WAL-logged, + so data on the standby will likely re-write the hints again on the standby. + Thus, the standby server will still perform disk writes even though + all users are read-only; no changes occur to the data values + themselves. Users will still write large sort temporary files and + re-generate relcache info files, so no part of the database + is truly read-only during hot standby mode. There is no restriction + on the use of set returning functions, or other users of + tuplestore/tuplesort code. Note also that writes to remote databases will still be possible, even though the transaction is read-only locally. - The following types of administrator command are not accepted - during recovery mode + The following types of administration commands are not accepted + during recovery mode: - Data Definition Language (DDL) - e.g. CREATE INDEX + Data Definition Language (DDL) - e.g. CREATE INDEX - Privilege and Ownership - GRANT, REVOKE, REASSIGN + Privilege and Ownership - GRANT, REVOKE, + REASSIGN - Maintenance commands - ANALYZE, VACUUM, CLUSTER, REINDEX + Maintenance commands - ANALYZE, VACUUM, + CLUSTER, REINDEX - Note again that some of these commands are actually allowed during + Again, note that some of these commands are actually allowed during "read only" mode transactions on the primary. As a result, you cannot create additional indexes that exist solely - on the standby, nor can statistics that exist solely on the standby. - If these administrator commands are needed they should be executed - on the primary so that the changes will propagate through to the + on the standby, nor can statistics exist solely on the standby. + If these administration commands are needed they should be executed + on the primary so that the changes will propagate to the standby. pg_cancel_backend() will work on user backends, but not the - Startup process, which performs recovery. pg_stat_activity does not + Startup process, which performs recovery. pg_stat_activity does not show an entry for the Startup process, nor do recovering transactions - show as active. As a result, pg_prepared_xacts is always empty during - recovery. If you wish to resolve in-doubt prepared transactions - then look at pg_prepared_xacts on the primary and issue commands to - resolve those transactions there. + show as active. As a result, pg_prepared_xacts is always empty during + recovery. If you wish to resolve in-doubt prepared transactions, + view pg_prepared_xacts on the primary and issue commands to + resolve transactions there. - pg_locks will show locks held by backends as normal. pg_locks also shows + pg_locks will show locks held by backends, + as normal. pg_locks also shows a virtual transaction managed by the Startup process that owns all - AccessExclusiveLocks held by transactions being replayed by recovery. - Note that Startup process does not acquire locks to - make database changes and thus locks other than AccessExclusiveLocks - do not show in pg_locks for the Startup process, they are just presumed - to exist. + AccessExclusiveLocks held by transactions being replayed by recovery. + Note that the Startup process does not acquire locks to + make database changes, and thus locks other than AccessExclusiveLocks + do not show in pg_locks for the Startup + process; they are just presumed to exist. - check_pgsql will work, but it is very simple. - check_postgres will also work, though many some actions + The Nagios plugin check_pgsql will + work, but it is very simple. + check_postgres will also work, though some actions could give different or confusing results. - e.g. last vacuum time will not be maintained for example, since no + For example, last vacuum time will not be maintained, since no vacuum occurs on the standby (though vacuums running on the primary do send their changes to the standby). - WAL file control commands will not work during recovery - e.g. pg_start_backup, pg_switch_xlog etc.. + WAL file control commands will not work during recovery, + e.g. pg_start_backup, pg_switch_xlog etc. - Dynamically loadable modules work, including pg_stat_statements. + Dynamically loadable modules work, including pg_stat_statements. Advisory locks work normally in recovery, including deadlock detection. - Note that advisory locks are never WAL logged, so it is not possible for + Note that advisory locks are never WAL logged, so it is impossible for an advisory lock on either the primary or the standby to conflict with WAL replay. Nor is it possible to acquire an advisory lock on the primary and have it initiate a similar advisory lock on the standby. Advisory - locks relate only to a single server on which they are acquired. + locks relate only to the server on which they are acquired. @@ -1671,19 +1688,19 @@ LOG: database system is ready to accept read only connections - Currently, temp table creation is not allowed during read only + Currently, temporary table creation is not allowed during read only transactions, so in some cases existing scripts will not run correctly. - It is possible we may relax that restriction in a later release. This is + This restriction might be relaxed in a later release. This is both a SQL Standard compliance issue and a technical issue. DROP TABLESPACE can only succeed if the tablespace is empty. Some standby users may be actively using the tablespace via their - temp_tablespaces parameter. If there are temp files in the - tablespace we currently cancel all active queries to ensure that temp - files are removed, so that we can remove the tablespace and continue with - WAL replay. + temp_tablespaces parameter. If there are temporary files in the + tablespace, all active queries are cancelled to ensure that temporary + files are removed, so the tablespace can be removed and WAL replay + can continue. @@ -1695,7 +1712,7 @@ LOG: database system is ready to accept read only connections - In normal running, if you issue DROP USER or DROP ROLE + In normal (non-recovery) mode, if you issue DROP USER or DROP ROLE for a role with login capability while that user is still connected then nothing happens to the connected user - they remain connected. The user cannot reconnect however. This behaviour applies in recovery also, so a @@ -1703,24 +1720,26 @@ LOG: database system is ready to accept read only connections - Stats collector is active during recovery. All scans, reads, blocks, - index usage etc will all be recorded normally on the standby. Replayed + The statististics collector is active during recovery. All scans, reads, blocks, + index usage, etc., will be recorded normally on the standby. Replayed actions will not duplicate their effects on primary, so replaying an insert will not increment the Inserts column of pg_stat_user_tables. - The stats file is deleted at start of recovery, so stats from primary - and standby will differ; this is considered a feature not a bug. + The stats file is deleted at the start of recovery, so stats from primary + and standby will differ; this is considered a feature, not a bug. - Autovacuum is not active during recovery, though will start normally + Autovacuum is not active during recovery, though it will start normally at the end of recovery. - Background writer is active during recovery and will perform - restartpoints (similar to checkpoints on primary) and normal block - cleaning activities. The CHECKPOINT command is accepted during recovery, - though performs a restartpoint rather than a new checkpoint. + The background writer is active during recovery and will perform + restartpoints (similar to checkpoints on the primary) and normal block + cleaning activities. (Remember, hint bits will cause blocks to + be modified on the standby server.) + The CHECKPOINT command is accepted during recovery, + though it performs a restartpoint rather than a new checkpoint. @@ -1728,21 +1747,19 @@ LOG: database system is ready to accept read only connections Hot Standby Parameter Reference - Various parameters have been mentioned above in the - and sections. + Various parameters have been mentioned above in + and . On the primary, parameters recovery_connections and - vacuum_defer_cleanup_age can be used to enable and control the - primary server to assist the successful configuration of Hot Standby servers. + vacuum_defer_cleanup_age can be used. max_standby_delay has no effect if set on the primary. On the standby, parameters recovery_connections and - max_standby_delay can be used to enable and control Hot Standby. - standby server to assist the successful configuration of Hot Standby servers. + max_standby_delay can be used. vacuum_defer_cleanup_age has no effect during recovery. @@ -1751,7 +1768,7 @@ LOG: database system is ready to accept read only connections Caveats - At this writing, there are several limitations of Hot Standby. + There are several limitations of Hot Standby. These can and probably will be fixed in future releases: @@ -1765,33 +1782,33 @@ LOG: database system is ready to accept read only connections Full knowledge of running transactions is required before snapshots - may be taken. Transactions that take use large numbers of subtransactions + can be taken. Transactions that use large numbers of subtransactions (currently greater than 64) will delay the start of read only connections until the completion of the longest running write transaction. - If this situation occurs explanatory messages will be sent to server log. + If this situation occurs, explanatory messages will be sent to the server log. Valid starting points for recovery connections are generated at each - checkpoint on the master. If the standby is shutdown while the master - is in a shutdown state it may not be possible to re-enter Hot Standby + checkpoint on the master. If the standby is shut down while the master + is in a shutdown state it might not be possible to re-enter Hot Standby until the primary is started up so that it generates further starting points in the WAL logs. This is not considered a serious issue - because the standby is usually switched into the primary role while + because the standby is usually switched to act as primary when the first node is taken down. - At the end of recovery, AccessExclusiveLocks held by prepared transactions + At the end of recovery, AccessExclusiveLocks held by prepared transactions will require twice the normal number of lock table entries. If you plan on running either a large number of concurrent prepared transactions - that normally take AccessExclusiveLocks, or you plan on having one - large transaction that takes many AccessExclusiveLocks then you are + that normally take AccessExclusiveLocks, or you plan on having one + large transaction that takes many AccessExclusiveLocks, you are advised to select a larger value of max_locks_per_transaction, up to, but never more than twice the value of the parameter setting on - the primary server in rare extremes. You need not consider this at all if + the primary server. You need not consider this at all if your setting of max_prepared_transactions is 0. @@ -1828,7 +1845,7 @@ LOG: database system is ready to accept read only connections logs shipped from the primary, we will be able to reload that backup and restart the standby's recovery process from the last restart point. We no longer need to keep WAL files from before the standby's restart point. - If we need to recover, it will be faster to recover from the incrementally + If recovery is needed, it will be faster to recover from the incrementally updated backup than from the original base backup.