在pg中,由于事务id采用32位实现,所以是采用循环复用的,如下:
虽然最大支持4 billion个事务(32位无符号数,xid最大可以到40亿),但是新老事务相差2 billion是上限,当达到2 billion的时候,PostgreSQL就会强制关闭。 在此之前,pg会有一系列的提示(以下为默认值):
- 当小于10 million的时候,日志中会告警。
- 当小于1 million的时候,PostgreSQL 会进入只读状态。
只要保证日常维护freeze,确保当前xid-oldestxid不超过20亿这个边界,是能够完全正常运行的。
实际和冻结相关的参数如下:
zjh@lt_test=# show %freez%; name | setting | description -------------------------------------+-----------+------------------------------------------------------------------------------- autovacuum_freeze_max_age | 200000000 | Age at which to autovacuum a table to prevent transaction ID wraparound. autovacuum_multixact_freeze_max_age | 400000000 | Multixact age at which to autovacuum a table to prevent multixact wraparound. vacuum_freeze_min_age | 50000000 | Minimum age at which VACUUM should freeze a table row. --接近很新的记录不用处理,可以降低不必要的冻结 vacuum_freeze_table_age | 150000000 | Age at which VACUUM should scan whole table to freeze tuples. --因为正常可能会被关闭全局autovacuum,而是人工调度,所以这个参数是这个用途 vacuum_multixact_freeze_min_age | 5000000 | Minimum age at which VACUUM should freeze a MultiXactId in a table row. vacuum_multixact_freeze_table_age | 150000000 | Multixact age at which VACUUM should scan whole table to freeze tuples. (6 rows) zjh@lt_test=# select * from pg_xid_assignment(); next_xid | oldest_xid | xid_vac_limit | xid_warn_limit | xid_stop_limit | xid_wrap_limit | oldest_xid_db ----------+------------+---------------+----------------+----------------+----------------+--------------- 41119082 | 480 | 200000480 | 2136484127 | 2146484127 | 2147484127 | 1 (1 row)
zjh@lt_test=# select * from pg_xid_assignment(); next_xid | oldest_xid | xid_vac_limit | xid_warn_limit | xid_stop_limit | xid_wrap_limit | oldest_xid_db ------------+------------+---------------+----------------+----------------+----------------+--------------- 2941154270 | 2891144668 | 3091144668 | 732661019 | 742661019 | 743661019 | 13604 (1 row) -- 40 billion 取模之后,相减和没有回卷是一样的。 zjh@lt_test=# SELECT datname, datfrozenxid, age(datfrozenxid), (age(datfrozenxid)::numeric/1000000000*100)::numeric(4,2) as "% WRAPAROUND RISK" FROM pg_database ORDER BY 3 DESC; datname | datfrozenxid | age | % WRAPAROUND RISK -----------+--------------+----------+------------------- postgres | 2891144668 | 50009950 | 5.00 test | 2891144737 | 50009881 | 5.00 lt_test | 2891144799 | 50009819 | 5.00 template1 | 2941144860 | 9758 | 0.00 template0 | 2941144921 | 9697 | 0.00 (5 rows)
xid_wrap_limit:发生回卷的上限,减去oldest_xid=2147483647。
xid_stop_limit:进入只读状态的上线,减去oldest_xid=2146483647。也就是10 million。
xid_warn_limit:log里面打告警,提示需要做freeze操作了。也就是1 million。
下列查询可以每个库的已冻结事务id:
zjh@lt_test=# SELECT datname, datfrozenxid, age(datfrozenxid), // 32位整型 (age(datfrozenxid)::numeric/1000000000*100)::numeric(4,2) as "% WRAPAROUND RISK" /*这里的基数到底是10亿还是20亿?下同*/ FROM pg_database ORDER BY 3 DESC; datname | datfrozenxid | age | % WRAPAROUND RISK -----------+--------------+-----------+------------------- postgres | 2091130736 | 150012652 | 15.00 test | 2091130804 | 150012584 | 15.00 lt_test | 2091130862 | 150012526 | 15.00 template1 | 2141130946 | 100012442 | 10.00 template0 | 2141131007 | 100012381 | 10.00 (5 rows) zjh@lt_test=# SELECT zjh@lt_test-# c.oid::regclass as table_name, zjh@lt_test-# greatest(age(c.relfrozenxid),age(t.relfrozenxid)) as "TXID age", zjh@lt_test-# (greatest(age(c.relfrozenxid),age(t.relfrozenxid))::numeric/1000000000*100)::numeric(4,2) as "% WRAPAROUND RISK" zjh@lt_test-# FROM pg_class c zjh@lt_test-# LEFT JOIN pg_class t ON c.reltoastrelid = t.oid zjh@lt_test-# WHERE c.relkind IN ('r', 'm') zjh@lt_test-# ORDER BY 2 DESC; table_name | TXID age | % WRAPAROUND RISK --------------------------------------------+-----------+------------------- utl_file.utl_file_dir | 150012644 | 15.00 pg_statistic | 150012644 | 15.00 pg_type | 150012644 | 15.00 pg_foreign_table | 150012644 | 15.00 pg_authid | 150012644 | 15.00 pg_statistic_ext_data | 150012644 | 15.00
通常是有几张表的冻结事务id特别小(因为vacuum正常不会做freeze,只有超过参数后才会做freeze,所以持续累计),导致整个库的事务冻结事务id无法推进、进而年龄越来越大直至超过为止。
所以,当vacuum进程无法跟上数据库DML活动的速度时,当到达一个瓶颈时,就可能发生事务ID回卷(Transaction ID Wraparound),此时pg数据库会强制重启。原因是此时已经可以确定mvcc无法在保证正确性。通常来说事务ID回卷会在下列情况下可能会发生:
- autovacuum被关闭
- 长时间未提交事务,如OLAP
- 复制槽保留了事务id
- DML活动导致autovacuum操作被频繁取消
1、3、4是比较容易出问题,需要注意。4要特别注意监控,不要该告警。
当当前的事务id - 表冻结事务id>=autovacuum_freeze_max_age时,会触发防止事务ID回卷的激进vacuum。如下:
2022-11-26 20:36:09.283575T,,,,,autovacuum worker,,00000,2022-11-26 20:35:59 CST,0,118166,LOG: automatic aggressive vacuum to prevent wraparound of table "lt_test.pg_toast.pg_toast_100508": index scans: 0 pages: 0 removed, 0 remain, 0 skipped due to pins, 0 skipped frozen tuples: 0 removed, 0 remain, 0 are dead but not yet removable, oldest xmin: 241134359 buffer usage: 25 hits, 1 misses, 0 dirtied avg read rate: 108.507 MB/s, avg write rate: 0.000 MB/s system usage: CPU: user: 0.00 s, system: 0.00 s, elapsed: 0.00 s WAL usage: 1 records, 0 full page images, 188 bytes
这是一个非常消耗io和cpu的操作,务必要安排在低峰期调度。
https://github.com/MasaoFujii/pg_cheat_funcs
https://developer.aliyun.com/article/560513
https://www.postgresql.org/docs/15/routine-vacuuming.html#VACUUM-FOR-WRAPAROUND
https://www.percona.com/blog/overcoming-vacuum-wraparound/
标签:xid,lightdb,age,回卷,lt,pg,test,postgresql,freeze From: https://www.cnblogs.com/zhjh256/p/16928126.html