Managing terabytes: When Postgres gets big
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This document summarizes the challenges and solutions for maintaining large PostgreSQL databases at Emma, including: - Maintaining terabytes of data across multiple clusters up to version 9.0 - Facing performance issues when the hardware load was pushed to its limits - Dealing with huge catalogs containing millions of data points that caused slow performance - Addressing problems like bloat, backups that took hours, system resource exhaustion, and transaction wraparound issues - Implementing solutions such as scripts to clean up bloat, sharding to a Linux filesystem, and increasing autovacuum thresholds
Managing terabytes: When Postgres gets big
- 1. Maintaining Terabytes with Postgres Selena Deckelmann Emma, Inc - http://myemma.com PostgreSQL Global Development Group
- 2. http://tech.myemma.com @emmaemailtech
- 3. Environment at Emma • 1.6 TB, 1 cluster,Version 8.2 (RAID10) • 1.1 TB, 2 clusters,Version 8.3 (RAID10) • 8.4, 9.0 Dev • Putting 9.0 into production (May 2011) • pgpool, Redis, RabbitMQ, NFS
- 4. Other stats • daily peaks: ~3000 commits per second • average writes: 4 MBps • average reads: 8 MBps • From benchmarks we’ve done, load is pushing the limits of our hardware.
- 5. I say all of this with love.
- 6. Huge catalogs • 409,994 tables • Minor mistake in parent table definitions • Parent table updates take 30+ minutes
- 7. not null default nextval('important_sequence'::regclass) vs not null default nextval('important_sequence'::text)
- 8. Huge catalogs • Bloat in the catalog • User-provoked ALTER TABLE • VACUUM FULL of catalog takes 2+ hrs
- 9. Huge catalogs suck • 9,019,868 total data points for table stats • 4,550,770 total data points for index stats • Stats collection is slow
- 10. Disk Management • $PGDATA: • pg_tblspc (TABLESPACES) • pg_xlog • global/pg_stats • wal for warm standby
- 11. Problems we worked through with big schemas Postgres • Bloat • Backups • System resource exhaustion • Minor upgrades • Major upgrades • Transaction wraparound
- 12. Bloat Causes • Frequent UPDATE patterns • Frequent DELETEs without VACUUM • a terabyte of dead tuples
- 13. SELECT BLOAT QUERY schemaname, tablename, reltuples::bigint, relpages::bigint, otta, ROUND(CASE WHEN otta=0 THEN 0.0 ELSE sml.relpages/otta::numeric END,1) AS tbloat, CASE WHEN relpages < otta THEN 0 ELSE relpages::bigint - otta END AS wastedpages, CASE WHEN relpages < otta THEN 0 ELSE bs*(sml.relpages-otta)::bigint END AS wastedbytes, CASE WHEN relpages < otta THEN '0 bytes'::text ELSE (bs*(relpages-otta))::bigint || ' bytes' END AS wastedsize, iname, ituples::bigint, ipages::bigint, iotta, ROUND(CASE WHEN iotta=0 OR ipages=0 THEN 0.0 ELSE ipages/iotta::numeric END,1) AS ibloat, CASE WHEN ipages < iotta THEN 0 ELSE ipages::bigint - iotta END AS wastedipages, CASE WHEN ipages < iotta THEN 0 ELSE bs*(ipages-iotta) END AS wastedibytes, CASE WHEN ipages < iotta THEN '0 bytes' ELSE (bs*(ipages-iotta))::bigint || ' bytes' END AS wastedisize FROM ( SELECT schemaname, tablename, cc.reltuples, cc.relpages, bs, CEIL((cc.reltuples*((datahdr+ma- (CASE WHEN datahdr%ma=0 THEN ma ELSE datahdr%ma END))+nullhdr2+4))/(bs-20::float)) AS otta, COALESCE(c2.relname,'?') AS iname, COALESCE(c2.reltuples,0) AS ituples, COALESCE(c2.relpages,0) AS ipages, COALESCE(CEIL((c2.reltuples*(datahdr-12))/(bs-20::float)),0) AS iotta FROM ( SELECT ma,bs,schemaname,tablename, (datawidth+(hdr+ma-(case when hdr%ma=0 THEN ma ELSE hdr%ma END)))::numeric AS datahdr, (maxfracsum*(nullhdr+ma-(case when nullhdr%ma=0 THEN ma ELSE nullhdr%ma END))) AS nullhdr2 FROM ( SELECT schemaname, tablename, hdr, ma, bs, SUM((1-null_frac)*avg_width) AS datawidth, MAX(null_frac) AS maxfracsum, hdr+( SELECT 1+count(*)/8 FROM pg_stats s2 WHERE null_frac<>0 AND s2.schemaname = s.schemaname AND s2.tablename = s.tablename ) AS nullhdr FROM pg_stats s, ( SELECT (SELECT current_setting('block_size')::numeric) AS bs, CASE WHEN substring(v,12,3) IN ('8.0','8.1','8.2') THEN 27 ELSE 23 END AS hdr, CASE WHEN v ~ 'mingw32' THEN 8 ELSE 4 END AS ma FROM (SELECT version() AS v) AS foo ) AS constants GROUP BY 1,2,3,4,5 ) AS foo ) AS rs JOIN pg_class cc ON cc.relname = rs.tablename JOIN pg_namespace nn ON cc.relnamespace = nn.oid AND nn.nspname = rs.schemaname AND nn.nspname <> 'information_schema' LEFT JOIN pg_index i ON indrelid = cc.oid LEFT JOIN pg_class c2 ON c2.oid = i.indexrelid ) AS sml WHERE tablename = 'addr' ORDER BY wastedbytes DESC LIMIT 1; Use check_postgres.pl https://github.com/bucardo/check_postgres/
- 14. Fixing bloat • Wrote scripts to clean things up • VACUUM (for small amounts) • CLUSTER • TRUNCATE (data loss!) • Or most extreme: DROP/CREATE • And then ran the scripts.
- 15. Backups • pg_dump takes longer and longer
- 16. backup | duration -‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐+-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐ 2009-‐11-‐22 | 02:44:36.821475 2009-‐11-‐23 | 02:46:20.003507 2009-‐11-‐24 | 02:47:06.260705 2009-‐12-‐06 | 07:13:04.174964 2009-‐12-‐13 | 05:00:01.082676 2009-‐12-‐20 | 06:24:49.433043 2009-‐12-‐27 | 05:35:20.551477 2010-‐01-‐03 | 07:36:49.651492 2010-‐01-‐10 | 05:55:02.396163 2010-‐01-‐17 | 07:32:33.277559 2010-‐01-‐24 | 06:22:46.522319 2010-‐01-‐31 | 10:48:13.060888 2010-‐02-‐07 | 21:21:47.77618 2010-‐02-‐14 | 14:32:04.638267 2010-‐02-‐21 | 11:34:42.353244 2010-‐02-‐28 | 11:13:02.102345
- 17. Backups • pg_dump fails • patching pg_dump for SELECT ... LIMIT • Crank down shared_buffers • or...
- 19. Install 32-bit Postgres and libraries on a 64-bit system. Install 64-bit Postgres/libs of the same version. Copy “hot backup” from 32-bit sys over to 64-bit sys. Run pg_dump from 64-bit version on 32-bit Postgres.
- 20. PSA • Warm standby is not a backup • Hot backup instances • “You don’t have valid backups, you have valid restores.” (thanks @sarahnovotny) • Necessity is the mother of invention...
- 21. Ship WAL from Solaris x86 -> Linux It did work!
- 22. Running out of inodes • UFS on Solaris “The only way to add more inodes to a UFS filesystem is: 1. destroy the filesystem and create a new filesystem with a higher inode density 2. enlarge the filesystem - growfs man page” • Solution 0: Delete files. • Solution 1: Sharding and bigger FS on Linux • Solution 2: ext4 (soon!)
- 23. Running out of available file descriptors • Too many open files by the database • Pooling - pgpool-II or pgbouncer?
- 24. Minor upgrades • Stop/start database • CHECKPOINT() before shutdown
- 25. Major Version upgrades • Too much downtime to dump/restore • Write tools to migrate data • Trigger-based replication • pg_upgrade
- 26. Transaction wraparound avoidance • autovacuum triggers are too small • 200,000 transactions (2 days) • Watch age(datfrozenxid) • Increase autovacuum_freeze_max_age
- 29. Thanks! • We’re hiring! - [email protected] • Emma’s Tech Blog: http://tech.myemma.com • My blog: http://chesnok.com • http://twitter.com/selenamarie