PostgreSQL DBA(27) - MVCC#7(避免长事务)
对于Update/Delete操作,PostgreSQL的MVCC机制仍会保留先前版本的数据,这些数据在VACUUM时将被清除.但如果在执行VACUUM时,存在先于VACUUM操作的活动事务,假定这些活动事务中最小的事务ID为OldestXmin,那么VACUUM不会清理删除事务ID(即xmax) > OldestXmin的元组,如果业务繁忙并且OldestXmin事务一直不提交,会导致存储空间一直膨胀,直至耗尽空间.
实验验证
数据准备
创建一张普通表,插入一行数据
drop table if exists t_page;create table t_page (id int,c1 char(8),c2 varchar(16));insert into t_page values(1,'1','a');获取该表对应的数据文件
10:26:31 (xdb@[local]:5432)testdb=# select pg_relation_filepath('t_page'); pg_relation_filepath ---------------------- base/16402/50824(1 row)查询该数据表占用的空间
10:42:43 (xdb@[local]:5432)testdb=# \set v_tablename t_page10:43:09 (xdb@[local]:5432)testdb=# SELECT pg_size_pretty( pg_total_relation_size(:'v_tablename') ); pg_size_pretty ---------------- 8192 bytes(1 row)过程
session 1启动事务,session 2执行pg_bench进行测试(在session 1后启动),session 3监控数据表的空间增长/执行vacuum
session 1
10:46:48 (xdb@[local]:5432)testdb=# begin;BEGIN10:46:50 (xdb@[local]:5432)testdb=#* select txid_current(); txid_current -------------- 397083(1 row)10:46:54 (xdb@[local]:5432)testdb=#*session 2执行pg_bench
[xdb@localhost script]$ cat test.sql \set id random(1,10000)begin;update t_page set c1='c1'||:id;commit;session 3监控数据表的空间增长/执行vacuum
-- 空间10:49:00 (xdb@[local]:5432)testdb=# SELECT pg_size_pretty( pg_total_relation_size(:'v_tablename') ); pg_size_pretty ---------------- 192 kB --> 在执行压力测试过程中从8K增长到192KB(1 row)-- 执行vacuum10:49:16 (xdb@[local]:5432)testdb=# vacuum verbose t_page;INFO: vacuuming "public.t_page"INFO: "t_page": found 0 removable, 10825 nonremovable row versions in 59 out of 59 pagesDETAIL: 10824 dead row versions cannot be removed yet, oldest xmin: 397083There were 0 unused item pointers.Skipped 0 pages due to buffer pins, 0 frozen pages.0 pages are entirely empty.CPU: user: 0.00 s, system: 0.00 s, elapsed: 0.00 s.VACUUMvacuum命令的输出信息:10824 dead row versions cannot be removed yet, oldest xmin: 397083
因为删除的xmax ＞ OldestXmin,因此这些元组不能被清除.
源码分析
元组对VACUUM的可见性判断与元组对SELECT操作的可见性判断类似,SELECT查询调用的可见性判断函数是HeapTupleSatisfiesMVCC,而VACUUM的可见性判断函数是HeapTupleSatisfiesVacuum,该函数由lazy_scan_heap调用.
lazy_scan_heap
lazy_scan_heap函数的逻辑先前已介绍过,这里不再详述,下面简单梳理与元组清理的相关逻辑
static voidlazy_scan_heap(Relation onerel, int options, LVRelStats *vacrelstats, Relation *Irel, int nindexes, bool aggressive){ ... for (blkno = 0; blkno < nblocks; blkno++) { //遍历每个block ... //遍历block中的每个元组 for (offnum = FirstOffsetNumber; offnum <= maxoff; offnum = OffsetNumberNext(offnum)) { ... if (ItemIdIsDead(itemid)) { //记录需删除的tuple //vacrelstats->dead_tuples[vacrelstats->num_dead_tuples] = *itemptr; //vacrelstats->num_dead_tuples++; lazy_record_dead_tuple(vacrelstats, &(tuple.t_self)); all_visible = false; continue; } ... //在这里,主要目的是一个元组是否可能对所有正在运行中的事务可见. switch (HeapTupleSatisfiesVacuum(&tuple, OldestXmin, buf)) { case HEAPTUPLE_DEAD: ... case HEAPTUPLE_LIVE: ... case HEAPTUPLE_RECENTLY_DEAD: //这些元组不能被清除! nkeep += 1; all_visible = false; break; ... } } } ...}如果ItemIdIsDead,则记录该元组,继续下一元组;如ItemIdIsNormal,调用HeapTupleSatisfiesVacuum函数,判断元组可见性.
ItemIdIsDead的判断很简单,判断ItemId的lp_flags标记是否为LP_DEAD
((itemId)->lp_flags == LP_DEAD)实际上,在执行update的时候,ItemId的lp_flags仍然是0x01,即Normal状态.
11:20:49 (xdb@[local]:5432)testdb=# select lp,lp_off,lp_flags,t_xmin,t_xmax,t_field3 as t_cid,t_ctid,t_infomask2,t_infomask from heap_page_items(get_raw_page('t_page',0)); lp | lp_off | lp_flags | t_xmin | t_xmax | t_cid | t_ctid | t_infomask2 | t_infomask -----+--------+----------+--------+--------+-------+---------+-------------+------------ 1 | 8152 | 1 | 457343 | 457343 | 0 | (0,1) | 3 | 10642 2 | 8112 | 1 | 457342 | 457343 | 0 | (0,1) | 3 | 9474 3 | 8072 | 1 | 457341 | 457342 | 0 | (0,2) | 3 | 9474...查看该block中3号元组的信息
[xdb@localhost pg111db]$ hexdump -C base/16402/50831 -s 32 -n 200000020 88 9f |..|00000022[xdb@localhost pg111db]$ hexdump -C base/16402/50831 -s 34 -n 200000022 4e 00 |N.|00000024计算偏移/大小/标记
[xdb@localhost pg111db]$ #偏移[xdb@localhost pg111db]$ echo $((0x9f88 & ~$((1<<15))))8072[xdb@localhost pg111db]$ #大小[xdb@localhost pg111db]$ echo $((0x004e >> 1))39[xdb@localhost pg111db]$ #lp_flags[xdb@localhost pg111db]$ echo $((0x004e & 0x0001))[xdb@localhost pg111db]$ echo $((0x9f88 >> 15))1lp_flags=0x01,即LP_NORMAL
下面,简述HeapTupleSatisfiesVacuum的实现逻辑,该函数判断元组对于VACUUM操作的可见性.
HeapTupleSatisfiesVacuum
HTSV_ResultHeapTupleSatisfiesVacuum(HeapTuple htup, TransactionId OldestXmin, Buffer buffer){ ... if (!HeapTupleHeaderXminCommitted(tuple)) { //xmin事务未提交 ... } //不符合以上条件,则可确定xmin事务已提交 if (tuple->t_infomask & HEAP_XMAX_INVALID) //xmax为无效事务ID return HEAPTUPLE_LIVE; ... if (!(tuple->t_infomask & HEAP_XMAX_COMMITTED)) { //xmax事务未提交 ... } ... //不符合以上条件,则可确定xmax事务已提交 if (!TransactionIdPrecedes(HeapTupleHeaderGetRawXmax(tuple), OldestXmin)) //6.元组xmax ≥ OldestXmin,标记为最近删除 return HEAPTUPLE_RECENTLY_DEAD; ...元组xmax ≥ OldestXmin,标记为HEAPTUPLE_RECENTLY_DEAD,这些元组不能被清理!
之所以需要避免长事务是因为如果OldestXmin事务一直不提交,那么后续被删除的元组都一直保留,无法通过VACUUM清除.
上述案例,压力测试完成后,空间是原来的330倍,而且通过VACUUM(包括VACUUM FULL)无法清理!
10:50:13 (xdb@[local]:5432)testdb=# SELECT pg_size_pretty( pg_total_relation_size(:'v_tablename') ); pg_size_pretty ---------------- 2640 kB(1 row)11:01:28 (xdb@[local]:5432)testdb=# vacuum verbose t_page;INFO: vacuuming "public.t_page"INFO: "t_page": found 0 removable, 60255 nonremovable row versions in 326 out of 326 pagesDETAIL: 60254 dead row versions cannot be removed yet, oldest xmin: 397083There were 0 unused item pointers.Skipped 0 pages due to buffer pins, 0 frozen pages.0 pages are entirely empty.CPU: user: 0.02 s, system: 0.00 s, elapsed: 0.02 s.VACUUM11:01:31 (xdb@[local]:5432)testdb=# 11:10:14 (xdb@[local]:5432)testdb=# vacuum full;VACUUM11:17:56 (xdb@[local]:5432)testdb=# SELECT pg_size_pretty( pg_total_relation_size(:'v_tablename') ); pg_size_pretty ---------------- 2640 kB(1 row)应用建议
在实际应用中,应尽可能避免长事务,跑批操作尽可能安排在非繁忙时段执行.如确定为只读事务,建议开启自动提交.
对于Java应用并且启用了连接池,如JDBC设置自动提交为false,就算是select操作,也务必在执行完毕后执行commit(Oracle不需要,但PG需要!).
参考资料
PostgreSQL Source Code
PostgreSQL 源码解读（134）- MVCC#18（vacuum过程-HeapTupleSatisfiesVacuum函数）
PostgreSQL 源码解读（130）- MVCC#14（vacuum过程-lazy_scan_heap函数）
PostgreSQL 源码解读（118）- MVCC#3(Tuple可见性判断)
