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PostgreSQL WAL for DBAs
Devrim Gunduz gives a presentation on Write-Ahead Logging (WAL) in PostgreSQL. WAL logs all transactions to files called write-ahead logs (WAL files) before changes are written to data files. This allows for crash recovery by replaying WAL files. WAL files are used for replication, backup, and point-in-time recovery (PITR) by replaying WAL files to restore the database to a previous state. Checkpoints write all dirty shared buffers to disk and update the pg_control file with the checkpoint location.
PostgreSQL WAL for DBAs
- 1. © 2013 EnterpriseDBCorporation. All rights reserved. 1 WAL for DBAs – Everything you want to know Devrim Gündüz Principal Systems Engineer @ EnterpriseDB devrim.gunduz@EnterpriseDB.com Twitter : @DevrimGunduz
- 2. PGDay.Asia 2017 WAL forDBAs 2 ● Who is this guy? – Using Red Hat (and then Fedora) since 1996. – Using PostgreSQL since 1998. – Responsible for PostgreSQL YUM repository. ● Used to break website, but recently gave up. – Started some work on PostgreSQL Dockerfiles recently. https://www.pgdocker.org – Working at EnterpriseDB since 2011. – The Guy With The PostgreSQL Tattoo! (imitations may exist :) ) – Istanbul, Turkiye. About me
- 3. FOSDEM PGDAY 2017 WALfor DBAs 3 ● Please tweet! – #PostgreSQL – #pgday – #pgdayasia Social media
- 4. FOSDEM PGDAY 2017 WALfor DBAs 4 (Did you tweet? Thanks!) Social media
- 5. FOSDEM PGDAY 2017 WALfor DBAs 5 (Did you tweet? Thanks!) Alternative hashtag: #blamemagnus Social media
- 6. FOSDEM PGDAY 2017 WALfor DBAs 6 June 26-28, Boston We want to see great speakers who can talk to the technical aspects of using Postgres in the enterprise. http://postgresvision.com/ Postgres Vision 2017
- 7. PGDay.Asia 2017 WAL forDBAs 7 Agenda (in random order) • What is WAL? • What does it include? • How to read it? • What about wal_level ? • Replication and WAL • Backup and WAL • PITR and WAL • Other topics
- 8. FOSDEM PGDAY 2017 WALfor DBAs 8 Please do not delete WAL files manually. Please. Before we actually start:
- 9. FOSDEM PGDAY 2017 WALfor DBAs 9 Please do not delete WAL files manually. Please. Please. Before we actually start:
- 10. FOSDEM PGDAY 2017 WALfor DBAs 10 Please do not delete WAL files manually. Please. Please. PLEASE. Before we actually start:
- 11. PGDay.Asia 2017 WAL forDBAs 11 What is WAL? • Write Ahead Log: • Logging of transactions • a.k.a. xlog (transaction log), • 16 MB in most of the installations (can be configured, --with- wal-segsize) • 8 kB page size (can be configured, --with-wal-blocksize) • pg_xlog (<= 9.6)→ pg_wal (10+) • Because people deleted files under “log” directory.
- 12. FOSDEM PGDAY 2017 WALfor DBAs 12 MAKE WAL GREAT AGAIN! So:
- 13. PGDay.Asia 2017 WAL forDBAs 13 What is WAL? • Designed to prevent data loss in most of the situations • OS crash, hardware failure, PostgreSQL crash. • Write transactions are written to WAL • Before transaction result is sent to the client • Data files are not changed on each transaction • Performance benefit • Should be kept in a separate drive. • Initdb, or symlink
- 14. PGDay.Asia 2017 WAL forDBAs 14 What is WAL? • Built-in feature • Life before WAL (not before B.C., though): • All changes go to durable storage (eventually), but: • Data page is loaded to shared_buffers • Changes are made there • Dirty buffers! • But not timely! • Crash → Data loss!
- 15. PGDay.Asia 2017 WAL forDBAs 15 What is WAL? • Life after WAL: • Almost all “modifications” are “logged” to WAL files (xlog record) • Even if the transaction is aborted (ROLLBACK) • wal_buffers → WAL segments (files) • Ability to recover data after a crash • Checkpoint!
- 16. PGDay.Asia 2017 WAL forDBAs 16 Where is it used? • Transaction logging! • Replication • PITR • REDO • Sequentially availability is a must. • REDO vs UNDO • No REDO for temp tables and unlogged tables.
- 17. PGDay.Asia 2017 WAL forDBAs 17 WAL file naming • 24 chars, hex. • 1st 8 chars: timelineID • 00000001 is the timelineID created by initdb • 2nd 8 chars: logical WAL file • 3rd 8 chars: WAL segment name (physical WAL file) • 000000010000000000000001 → 000000010000000000000002 • … 0000000100000000000000FF → 000000010000000100000000 • Use PostgreSQL’s internal tools to manage them • pg_archivecleanup • pg_resetxlog • pg_xlogdump • ....
- 18. PGDay.Asia 2017 WAL forDBAs 18 pg_xlogdump • We are all human. • Use pg_xlogdump, if you want to see contents of WAL files • rmgr --help to get list of all resource names, -f for follow, -n for limit. -z for stats. • pg_xlogdump -n 20 -f 000000010000000700000033 • rmgr: Heap len (rec/tot): 3/ 59, tx: 389744, lsn: 7/33B66228, prev 7/33B661F0, desc: INSERT+INIT off 1, blkref #0: rel 1663/13326/190344 blk 0 • rmgr: Heap len (rec/tot): 3/ 59, tx: 389744, lsn: 7/33B66268, prev 7/33B66228, desc: INSERT off 2, blkref #0: rel 1663/13326/190344 blk 0 • rmgr: Transaction len (rec/tot): 8/ 34, tx: 389744, lsn: 7/33B662A8, prev 7/33B66268, desc: COMMIT 2017-02- 03 03:03:49.482223 +03 • rmgr: Heap len (rec/tot): 14/ 69, tx: 389745, lsn: 7/33B662D0, prev 7/33B662A8, desc: HOT_UPDATE off 1 xmax 389745 ; new off 3 xmax 0, blkref #0: rel 1663/13326/190344 blk 0 • rmgr: Transaction len (rec/tot): 8/ 34, tx: 389745, lsn: 7/33B66318, prev 7/33B662D0, desc: COMMIT 2017-02- 03 03:03:54.091645 +03 • rmgr: XLOG len (rec/tot): 80/ 106, tx: 0, lsn: 7/33B66340, prev 7/33B66318, desc: CHECKPOINT_ONLINE redo 7/33B66340; tli 1; prev tli 1; fpw true; xid 0/389746; oid 198532; multi 1; offset 0; oldest xid 1866 in DB 129795; oldest multi 1 in DB 90123; oldest/newest commit timestamp xid: 388437/389745; oldest running xid 0; online • rmgr: XLOG len (rec/tot): 0/ 24, tx: 0, lsn: 7/33B663B0, prev 7/33B66340, desc: SWITCH •
- 19. PGDay.Asia 2017 WAL forDBAs 19 Shared Buffers, Bgwriter and checkpointer • shared_buffers in PostgreSQL • Dirty buffers • This is where transactions are performed • Side effect: Causes inconsistency(?) on durable storage, due to dirty buffers. • Bgwriter: Background writer • LRU • Checkpointer • Pushing all dirty buffers to durable storage • Triggered automatically or manually • Backends may also write data to heap
- 20. PGDay.Asia 2017 WAL forDBAs 20 WAL: LSN • Log Sequence Number • Position of the record in WAL file. • Provides uniqueness for each xlog record. • Per docs: “Pointer to a location in WAL file” • During recovery, LSN on the page and LSN in the WAL file are compared. • The larger one wins.
- 21. PGDay.Asia 2017 WAL forDBAs 21 ● Probably not the last one in ls list! WAL: Finding current WAL file • postgres=# SELECT * from pg_current_xlog_location(); pg_current_xlog_location -------------------------- 40E6/2C85AC10 • postgres=# SELECT pg_xlogfile_name(‘40E6/2C85AC10'); pg_xlogfile_name -------------------------- 00000003000040E60000002C So: • postgres=# SELECT pg_xlogfile_name(pg_current_xlog_location()); pg_xlogfile_name -------------------------- 00000003000040E60000002C
- 22. PGDay.Asia 2017 WAL forDBAs 22 Checkpoint, and pg_control ● As soon as the checkpoint starts, REDO point is stored in shared buffers. ● A WAL record is created referencing checkpoint start, and it is first written to WAL buffers, and then eventually to pg_control. – pg_control is under $PGDATA/global ● Unlike bgwriter, checkpointer writes all of the data in the shared_buffers to durable storage. ● PostgreSQL knows the latest REDO point, by looking at pg_control file.
- 23. PGDay.Asia 2017 WAL forDBAs 23 Checkpoint, and pg_control ● pg_controldata: – Latest checkpoint location: 40E7/E43B16B8 – Prior checkpoint location: 40E7/D8689090 They are LSN. ● When checkpoint is completed, pg_control is updated with the position of checkpoint. ● After checkpoint, old WAL files are either recycled, or removed. ● An “estimation” is done while recycling (based on previous checkpoint cycles) ● 9.5+: In minimum, min_wal_size WAL files are always recycled for future usage
- 24. PGDay.Asia 2017 WAL forDBAs 24 pg_control and REDO ● postmaster reads pg_control on startup. /usr/pgsql-10/bin/pg_controldata -D /var/lib/pgsql/10/data | grep state – “Database cluster state”: ● starting up ● shut down ● shut down in recovery ● shutting down ● in crash recovery ● in archive recovery ● in production ● If pg_control says “in production”, but db server is not running, then this instance is eligible for a recovery!
- 25. PGDay.Asia 2017 WAL forDBAs 25 pg_control and REDO ● pg_control is the critical piece – Should not be corrupted – Per docs: “...theoretically a weak spot” ● REDO: All WAL files must be sequentially available for complete recovery.
- 26. PGDay.Asia 2017 WAL forDBAs 26 Moving to the new WAL ● A WAL segment may be full ● PostgreSQL archiver will switch to the new xlog, if PostgreSQL reaches archive_timeout value. ● DBA issues pg_switch_xlog() function.
- 27. PGDay.Asia 2017 WAL forDBAs 27 WAL: Archiving • Replication, backup, PITR • archive_mode • archive_command • archive_timeout
- 28. PGDay.Asia 2017 WAL forDBAs 28 WAL: Point-In-Time Recovery (PITR) • A base backup (pg_basebackup!) and the WAL files are needed. • WAL files must be sequentially complete – otherwise PITR won’t be finished. • “Roll-forward recovery”
- 29. PGDay.Asia 2017 WAL forDBAs 29 WAL: Point-In-Time Recovery (PITR) • PITR: Replaying WAL files on base backups, until recovery target. • recovery_target_{time, xid, name, lsn} • If not specified, all archived WAL files are replayed. • recovery.conf and backup_label: Enters recovery mode. • restore_command, recovery_target_XXX,recovery_target_inclusive • backup_label: Also includes checkpoint location (starting point of recovery) • Almost like regular recovery process (WAL replay) • Up to recovery_target_XXX is replayed.
- 30. PGDay.Asia 2017 WAL forDBAs 30 WAL: Point-In-Time Recovery (PITR) • After recovery process, timelineID is increased by 1 (also physical WAL file name is also increased by 1) • A .history file is created. • $ cat 00000003.history 1 403F/58000098 no recovery target specified 2 4048/43000098 before 2017-01-28 11:13:21.124512+03 “WAL files were replayed until the given time above, and theit replay location is 4048/43000098. • • •
- 31. PGDay.Asia 2017 WAL forDBAs 31 Full page writes ● A WAL record cannot be replayed on a page which is corrupted during bgwriter and/or checkpointer, because of hardware failure, OS crash, kernel failure, etc. ● Full page writes IYF. ● Enabled by default. – Please turn it off, if you want to throw a lot of money to PostgreSQL support companies. Otherwise, don’t do so ;) ● PostgreSQL writes header data + the entire page as XLOG record, when a page changes after every checkpoint. – Increasing checkpoint_timeout helps. – Full-page image, backup block. ● PostgreSQL can even recover itself from write failures (not hw failures, though)
- 32. PGDay.Asia 2017 WAL forDBAs 32 WAL parameters • wal_level: Minimal, replica or logical • Must be > minimal for archiver to be able to run • fsync : Always on, please. • synchronous_commit: May lose some of the latest transactions • Server returns success to the client • Server waits a bit to flush the data to durable storage. • Less risky than fsync • wal_sync_method : fdatasync is usually better. Use pg_test_fsync for testing.
- 33. PGDay.Asia 2017 WAL forDBAs 33 WAL parameters • wal_log_hints: When this value is set to on , the server writes the entire content of each disk page to WAL after a checkpoint and during the first modification of that page, even for non-critical modifications of so-called hint bits. • wal_compression: off by default. Less WAL files, more CPU overhead. • wal_buffers: -1: Automatic tuning of wal buffers: 1/32 of shared_buffers (not less than 64kB or no more than 16 MB (1 WAL file) • wal_writer_delay : Rounds between WAL writer flushes WAL. • wal_writer_flush_after: New in 9.6
- 34. FOSDEM PGDAY 2017 WALfor DBAs 34 Questions, comments?
- 35. © 2013 EnterpriseDBCorporation. All rights reserved. 35 WAL for DBAs – Everything you want to know Devrim Gündüz Principal Systems Engineer @ EnterpriseDB devrim.gunduz@EnterpriseDB.com Twitter : @DevrimGunduz