Monitoring PostgreSQL at Scale

@LukasFittl Monitoring PostgreSQL At Scale

@LukasFittl @LukasFittl pganalyze

@LukasFittl Statistics That Matter Two Tables To Remember Breaking Down
High-Level Statistics Log Events Worth Knowing Fingerprinting & Tracing Queries

@LukasFittl Postgres Statistics Tables

@LukasFittl 1 “Block” = 8 kB (usually, check block_size to
confirm)

@LukasFittl Tuple = Row

@LukasFittl Statistics Are Often Counters * except when reset /
overrun Counts only go up*,  calculate diffs!

@LukasFittl Schema Statistics

@LukasFittl pg_stat_user_tables relname: name of the table seq_scan: # of
sequential scans idx_scan: # of index scans n_tup_(ins/del/upd): # of rows modified n_live_tup: live rows n_dead_tup: dead rows last_(auto)vacuum: last VACUUM last_(auto)analyze: last ANALYZE …

@LukasFittl SELECT relname, n_live_tup, seq_scan + idx_scan, 100 * idx_scan
/ (seq_scan + idx_scan) FROM pg_stat_user_tables ORDER BY n_live_tup DESC Index Hit Rate Target: >= 95% on large, active tables

@LukasFittl pg_statio_user_tables relname: name of the table heap_blks_read: blocks from
disk / OS cache heap_blks_hit: blocks from buffer cache idx_blks_read: index blks from disk idx_blks_hit: index blks from buffer cache …

@LukasFittl SELECT sum(heap_blks_hit) / nullif(sum(heap_blks_hit + heap_blks_read),0) FROM pg_statio_user_tables Table
Cache Hit Rate Target: >= 99%

@LukasFittl Query Workload

@LukasFittl pg_stat_activity pid: process ID backend_type: “client backend” vs internal
processes state: idle/active/idle in transaction state_change: time of state change query: current/last running query backend_start: process start time xact_start: TX start time query_start: query start time wait_event: what backend is waiting for (e.g. Lock, I/O, etc) …

@LukasFittl # of Connections By State SELECT state, backend_type, COUNT(*)
FROM pg_stat_activity  GROUP BY 1, 2

@LukasFittl Longest Running Query SELECT now() - query_start, query FROM
pg_stat_activity  WHERE state = ‘active’ ORDER BY 1 LIMIT 1

@LukasFittl Age Of Oldest Transaction SELECT MAX(now() - xact_start)  FROM
pg_stat_activity  WHERE state <> ‘idle’

@LukasFittl pg_stat_activity lock information https://github.com/postgrespro/pg_wait_sampling

@LukasFittl 1. Install postgresql contrib package (if not installed) 2.
Enable in postgresql.conf  shared_preload_libraries = ‘pg_stat_statements’ 3. Restart your database 4. Create the extension  CREATE EXTENSION pg_stat_statements; pg_stat_statements

@LukasFittl Supported on cloud platforms

@LukasFittl queryid | 1720234670 query | SELECT * FROM x
WHERE y = ? calls | 5 total_time | 15.249 Query + No. of Calls + Avg Time

@LukasFittl shared_blks_hit | 2447215 shared_blks_read | 55335 Avg. Shared Buffer
Hit Rate hit_rate = shared_blks_hit / (shared_blks_hit + shared_blks_read) 97.78% Cache Hit Rate

@LukasFittl blk_read_time | 14.594 blk_write_time | 465.661 Time spent reading/writing
to disk track_io_timing = on

pg_qtop Simple top-like tool that shows pg_stat_statements data https://github.com/lfittl/pg_qtop

AVG | QUERY -------------------------------------------------------------------------------- 10.7ms | SELECT oid, typname, typelem,
typdelim, typinput FROM pg_type 3.0ms | SET time zone 'UTC' 0.4ms | SELECT a.attname, format_type(a.atttypid, a.atttypmod), pg_get_expr(d.adbin, d.adrelid), a.attnotnull, a.atttypid, a.atttypmod FROM pg_attribute a LEFT JOIN pg_attrdef d ON a.attrelid = d.adrelid AND a.attnum = d.adnum WHERE a.attrelid = ?::regclass AND a.attnum > ? AND NOT a.attisdropped ORDER BY a.attnum 0.2ms | SELECT pg_stat_statements_reset() 0.1ms | SELECT query, calls, total_time FROM pg_stat_statements 0.1ms | SELECT attr.attname FROM pg_attribute attr INNER JOIN pg_constraint cons ON attr.attrelid = cons.conrelid AND attr.attnum = cons.conkey[?] WHERE cons.contype = ? AND cons.conrelid = ?: :regclass 0.0ms | SELECT COUNT(*) FROM pg_class c LEFT JOIN pg_namespace n ON n.oid = c.relnamespace WHERE c.relkind in (?,?) AND c.relname = ? AND n.nspname = ANY (current_schemas(?)) 0.0ms | SELECT * FROM posts JOIN users ON (posts.author_id = users.id) WHERE users.login = ?; 0.0ms | SET client_min_messages TO 'panic' 0.0ms | set client_encoding to 'UTF8' 0.0ms | SHOW client_min_messages 0.0ms | SELECT * FROM ad_reels WHERE id = ?; 0.0ms | SELECT * FROM posts WHERE guid = ?; 0.0ms | SELECT ? 0.0ms | SET client_min_messages TO 'warning' 0.0ms | SET standard_conforming_strings = on 0.0ms | SELECT "posts".* FROM "posts" ORDER BY "posts"."id" DESC LIMIT ? 0.0ms | SHOW TIME ZONE pg_qtop -d testdb

AVG | QUERY -------------------------------------------------------------------------------- 0.0ms | SELECT * FROM posts
JOIN users ON (posts.author_id = users.id) WHERE users.login = ?; 0.0ms | SELECT * FROM posts WHERE guid = ?; 0.0ms | SELECT "posts".* FROM "posts" ORDER BY "posts"."id" DESC LIMIT ? pg_qtop -d testdb -t posts

AVG | CALLS | HIT RATE | QUERY -------------------------------------------------------------------------------- 0.1ms
| 1 | 100.0 | SELECT * FROM users; 0.1ms | 1 | - | SELECT * FROM databases; 0.0ms | 1 | - | SELECT * FROM invoices; 0.0ms | 1 | - | SELECT * FROM query_snapshots; pg_qtop -d testdb -s select

pganalyze.com

@LukasFittl Lock Statistics pg_locks pid: process ID (JOIN to pg_stat_activity.pid!)
locktype: type of object being locked mode: locking type (e.g. AccessExclusive) granted: Lock Granted vs Being Waited For …

@LukasFittl Lock Statistics pg_locks SELECT * FROM pg_locks WHERE NOT
granted

@LukasFittl Lock Statistics pg_locks SELECT locktype, mode, COUNT(*) FROM pg_locks
WHERE granted GROUP BY 1, 2

@LukasFittl Checkpoint Statistics pg_stat_bgwriter checkpoints_timed: # of scheduled checkpoints checkpoints_req:
# of requested checkpoints 1. Time Between Checkpoints 2. % of Timed Checkpoints

@LukasFittl autovacuum => SELECT pid, query FROM pg_stat_activity WHERE query
LIKE 'autovacuum: %'; 10469 | autovacuum: VACUUM ANALYZE public.schema_columns 12848 | autovacuum: VACUUM public.replication_follower_stats 28626 | autovacuum: VACUUM public.schema_index_stats | (to prevent wraparound) (3 rows) pg_stat_activity

@LukasFittl autovacuum pg_stat_activity

@LukasFittl autovacuum pg_stat_progress_vacuum relid: OID of the table phase: current
VACUUM phase heap_blks_total: Heap Blocks Total heap_blks_scanned: Heap Blocks Scanned heap_blks_vacuumed: Heap Blocks Vacuumed …

@LukasFittl autovacuum pg_stat_progress_vacuum

@LukasFittl pg_stat_replication client_addr: ip address of the follower backend_start: replication
start time state: replication state (ideally = streaming) replay_location: WAL location

@LukasFittl pg_stat_replication SELECT client_addr, pg_wal_lsn_diff( pg_current_wal_lsn(), replay_location) FROM pg_stat_replication Replication
Lag in Bytes, Per Follower

@LukasFittl pg_stat_replication Replication Lag in Bytes, Per Follower

@LukasFittl CPU & I/O Utilization

@LukasFittl “We had an outage yesterday at 10am - what
happened?”

@LukasFittl Keeping Historic Statistics Data Is Essential

@LukasFittl DIY Monitoring Hack: Save pg_stat_activity and pg_stat_database every 10
seconds into a separate monitoring database

@LukasFittl pg_stat_activity - Number & State of Connections - Oldest
Query Still Running - Oldest Transaction Still Open - Blocked Queries

@LukasFittl pg_stat_database - Transactions Per Second - Data Read Per
Second - Rows Updated/etc Per Second - Deadlocks Per Second - …

@LukasFittl Ability to Drill Down From “High CPU Utilization” To
Specific Set of Queries

@LukasFittl

@LukasFittl CPU Utilization pg_stat_statements.total_runtime

@LukasFittl I/O Utilization pg_stat_statements.blk_read_time pg_stat_statements.blk_write_time

@LukasFittl Cache Hit Ratio % pg_stat_statements.shared_blks_hit pg_stat_statements.shared_blks_read pg_stat_database.blks_hit pg_stat_database.blks_read

@LukasFittl Temporary Files Written pg_stat_statements.temp_blks_written pg_stat_database.temp_bytes

@LukasFittl LOG: duration: 4079.697 ms execute <unnamed>: SELECT * FROM
x WHERE y = $1 LIMIT $2 DETAIL: parameters: $1 = 'long string', $2 = ‘1' Slow Queries log_min_duration_statement = 1000 ms

@LukasFittl

@LukasFittl auto_explain logs the query plan  for specific slow queries

@LukasFittl

@LukasFittl Cancelled Queries ERROR: canceling statement due to statement timeout
STATEMENT: SELECT 1 ERROR: canceling statement due to  user request STATEMENT: SELECT 1 …

@LukasFittl log_lock_waits = on LOG: process 20679 still waiting for
ExclusiveLock on tuple (566,1) of relation 16421 after 1000.115 ms LOG: process 20678 still waiting for ExclusiveLock on tuple (566,1) of relation 16421 after 1000.126 ms LOG: process 15533 still waiting for ExclusiveLock on tuple (566,1) of relation 16421 1000.129 ms LOG: process 20663 still waiting for ExclusiveLock on tuple (566,1) of relation 16421 1000.100 ms LOG: process 15537 still waiting for ExclusiveLock on tuple (566,1) of relation 16421 1000.130 ms LOG: process 15536 still waiting for ExclusiveLock on tuple (566,1) of relation 16421 1000.222 ms LOG: process 20734 still waiting for ExclusiveLock on tuple (566,1) of relation 16421 1000.130 ms LOG: process 15538 still waiting for ExclusiveLock on tuple (566,1) of relation 16421 1000.136 ms LOG: process 15758 still waiting for ShareLock on transaction 250175899 after 1000.073 ms Lock Waits

@LukasFittl archive_command Failures LOG: archive command failed with exit code
1 DETAIL: The failed archive command was: /my_backup_script.sh pg_xlog/ 0000000100025DFA00000023

@LukasFittl Out of Memory ERROR: out of memory DETAIL: Failed
on request of size 408028. QUERY: SELECT 1 ...

@LukasFittl Out of Connections FATAL: remaining connection slots are reserved
for non-replication superuser connections

@LukasFittl Server Crash / Segfault LOG: server process (PID 660)
was terminated by signal 6: Aborted DETAIL: Failed process was running: SELECT pg_advisory_lock(1, 2); LOG: terminating any other active server processes WARNING: terminating connection because of crash of another server process …

@LukasFittl TXID Wraparound WARNING: database “mydb" must be vacuumed within
938860 transactions HINT: To avoid a database shutdown, execute a full- database VACUUM in “mydb".

@LukasFittl TXID Wraparound ERROR: database is not accepting commands to
avoid wraparound data loss in database “mydb” HINT: Stop the postmaster and use a standalone backend to vacuum that database. You might also need to commit or roll back old prepared transactions.

@LukasFittl Fingerprinting Identifying & Grouping Queries

@LukasFittl SELECT a, b FROM public.test WHERE col = ‘value’
A

@LukasFittl SELECT a, b FROM public.test WHERE col = ‘value’
SELECT a, b FROM public.test WHERE col = ‘other_value’ A B

@LukasFittl SELECT a, b FROM public.test WHERE col = ?
SELECT a, b FROM public.test WHERE col = ? A A

SELECT a, b —— COMMENT FROM public.test WHERE col = ? A B

@LukasFittl # SELECT queryid, query FROM pg_stat_statements; queryid | query
------------+---------------------------------------- 1115711211 | SELECT a, b FROM public.test WHERE col = $1 (1 row) pg_stat_statements

SELECT a, b —— COMMENT FROM public.test WHERE col = ? A queryid = 1115711211 queryid = 1115711211 A

SELECT b, a —— COMMENT FROM public.test WHERE col = ? A queryid = 1115711211 queryid = 2511327719 B

@LukasFittl irb> PgQuery.fingerprint( ‘SELECT a, b FROM public.test WHERE col
= $1’) => 0254f1e78f2d47b258d7b022f3dfa5794351a75128 pg_query

SELECT b, a /* COMMENT */ FROM public.test WHERE col = ? A fingerprint = 0254f1e78f2d47b258d7b022f3dfa5794351a75128 fingerprint = 0254f1e78f2d47b258d7b022f3dfa5794351a75128 A

@LukasFittl PgQuery.fingerprint - Based on Postgres Parsetree - Table names,
not OIDs - Identical across servers  & Postgres versions https://github.com/lfittl/libpg_query/wiki/Fingerprinting

@LukasFittl Tracing Queries Based On Their Query Origin

@LukasFittl

@LukasFittl application: pganalyze controller: graphql action: graphql line: /app/graphql/organization_type.rb …
graphql: getOrganizationDetails.logVolume24h request_id: 44bd562e-0f53-453f-831f-498e61ab6db5

@LukasFittl github.com/basecamp/marginalia Automatic Query Annotations For Ruby on Rails

@LukasFittl 3 Take-Aways 1. Collect Historic Metrics 2. Focus on
Drill-Down To Query Level 3. Annotate Your Queries With Their Origin

@LukasFittl Monitor Your Postgres: pganalyze.com    Scale Your Postgres: citusdata.com
Thanks!

Monitoring PostgreSQL at Scale

Monitoring PostgreSQL at Scale

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