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关于PostgreSQL流复制的延迟
问题
MySQL的复制存在一个回放延迟的问题，即备机回放的速度赶不上主机SQL执行的速度，导致延迟越来越厉害。原因在于MySQL的binlog回放是单线程，主机的SQL执行是多线程，单线程的回放速度跟不上。这个问题在新版本的MySQL上应该已经解决了，解决办法就是让备机回放也使用多线程。这个问题，BAT的工程师们在公开场合讲过N次了，印象很深刻。 PostgreSQL流复制的备机也是单线程(进程)的，那么有没有类似的问题呢？ 实际上不会，因为PostgreSQL的流复制是基于块的物理复制，回放的代价小。下面看看实际的测试结果。
测试环境
主机和备机都是以下配置的虚拟机
CPU：2 core
MEM: 2G
OS:CentOS release 6.5 (Final)
PostgreSQL:9.4.2(shared_buffers = 512MB)
测试数据
chenhj=# create table tb1(id serial,c1 int);
CREATE TABLE
[chenhj@node1 ~]$ cat test.sql
insert into tb1(c1) values(1);
测试结果
在主机上执行pgbench。
[chenhj@node1 ~]$ pgbench -n -f test.sql -T 50 -c 20
transaction type: Custom query
scaling factor: 1
query mode: simple
number of clients: 20
number of threads: 1
duration: 50 s
number of transactions actually processed: 95324
latency average: 10.491 ms
tps = 1905.733296 (including connections establishing)
tps = 1910.579339 (excluding connections establishing)
pgbench执行过程中，在备机不断执行下面的语句，发现偶尔会出现100个字节左右的回放延迟，但延迟没有积累，即没有出现备机跟不上主机的情况。
SELECT pg_last_xlog_receive_location() - pg_last_xlog_replay_location();
资源占用
根据下面的top资源占用情况可知，主机上WAL发送进程(18374)占用CPU最高为21.6%。备机上WAL接受进程（8181）最高为17.3%，其次是Startup进程，也即是恢复进程(5012)，占用CPU 8.0%。
主机
[root@node1 ~]# top
top - 12:37:28 up 3:18, 3 users, load average: 0.02, 0.01, 0.01
Tasks: 156 total, 5 running, 151 sleeping, 0 stopped, 0 zombie
Cpu(s): 24.9%us, 43.9%sy, 0.0%ni, 21.4%id, 5.4%wa, 3.1%hi, 1.3%si, 0.0%st
Mem: 1922268k total, 1718824k used, 203444k free, 10072k buffers
Swap: 2064376k total, 394032k used, 1670344k free, 463376k cached
PID USER PR NI VIRT RES SHR S %CPU %MEM TIME+ COMMAND
18374 chenhj 20 0 663m 2280 1432 R 21.6 0.1 0:10.99 postgres
18965 chenhj 20 0 13848 1176 812 R 10.0 0.1 0:00.74 pgbench
18967 chenhj 20 0 663m 5688 4688 S 8.3 0.3 0:00.59 postgres
18969 chenhj 20 0 663m 5660 4660 S 7.3 0.3 0:00.50 postgres
18968 chenhj 20 0 663m 5660 4660 S 7.0 0.3 0:00.50 postgres
18970 chenhj 20 0 663m 5664 4664 S 7.0 0.3 0:00.47 postgres
18971 chenhj 20 0 663m 5676 4676 S 6.6 0.3 0:00.47 postgres
18972 chenhj 20 0 663m 5648 4648 S 6.3 0.3 0:00.43 postgres
18975 chenhj 20 0 663m 5628 4636 S 6.3 0.3 0:00.37 postgres
18973 chenhj 20 0 663m 5636 4636 S 6.0 0.3 0:00.39 postgres
18982 chenhj 20 0 663m 5632 4636 S 6.0 0.3 0:00.38 postgres
18984 chenhj 20 0 663m 5652 4656 S 6.0 0.3 0:00.38 postgres
18974 chenhj 20 0 663m 5648 4648 R 5.6 0.3 0:00.40 postgres
18976 chenhj 20 0 663m 5632 4640 S 5.6 0.3 0:00.38 postgres
18978 chenhj 20 0 663m 5656 4664 S 5.6 0.3 0:00.37 postgres
18979 chenhj 20 0 663m 5644 4648 S 5.6 0.3 0:00.37 postgres
18983 chenhj 20 0 663m 5636 4640 S 5.6 0.3 0:00.37 postgres
18986 chenhj 20 0 663m 5624 4628 S 5.6 0.3 0:00.38 postgres
18977 chenhj 20 0 663m 5656 4664 S 5.3 0.3 0:00.36 postgres
18980 chenhj 20 0 663m 5636 4640 S 5.3 0.3 0:00.35 postgres
18981 chenhj 20 0 663m 5636 4640 S 5.3 0.3 0:00.36 postgres
18985 chenhj 20 0 663m 5656 4660 S 5.0 0.3 0:00.34 postgres
[root@node1 ~]# ps -ef|grep postgres
chenhj 18367 1 0 12:31 pts/1 00:00:00 /usr/local/pgsql/bin/postgres -D data
chenhj 18369 18367 0 12:31 ? 00:00:00 postgres: checkpointer process
chenhj 18370 18367 0 12:31 ? 00:00:00 postgres: writer process
chenhj 18371 18367 0 12:31 ? 00:00:00 postgres: wal writer process
chenhj 18372 18367 0 12:31 ? 00:00:00 postgres: autovacuum launcher process
chenhj 18373 18367 0 12:31 ? 00:00:00 postgres: stats collector process
chenhj 18374 18367 0 12:31 ? 00:00:00 postgres: wal sender process chenhj 192.168.150.201(34998) streaming 0/A5734050
备机
[root@node1 ~]# top
top - 12:37:36 up 3:11, 4 users, load average: 0.19, 0.06, 0.02
Tasks: 138 total, 1 running, 137 sleeping, 0 stopped, 0 zombie
Cpu(s): 0.7%us, 8.6%sy, 0.0%ni, 58.4%id, 32.0%wa, 0.2%hi, 0.2%si, 0.0%st
Mem: 1922268k total, 1851180k used, 71088k free, 4816k buffers
Swap: 2064376k total, 660616k used, 1403760k free, 459252k cached
PID USER PR NI VIRT RES SHR S %CPU %MEM TIME+ COMMAND
8181 chenhj 20 0 667m 2148 1332 D 17.3 0.1 0:15.65 postgres
5012 chenhj 20 0 662m 172m 171m S 8.0 9.2 0:30.94 postgres
23 root 20 0 0 0 0 S 1.0 0.0 0:06.64 kblockd/1
11 root 20 0 0 0 0 S 0.3 0.0 0:08.91 events/0
12 root 20 0 0 0 0 S 0.3 0.0 0:10.60 events/1
1263 root -2 0 2574m 1.2g 1540 S 0.3 65.5 0:52.81 corosync
2195 root -2 0 4900 452 420 S 0.3 0.0 0:00.56 iscsid
8188 root 20 0 15036 1216 908 R 0.3 0.1 0:00.32 top
[chenhj@node2 ~]$ ps -ef|grep postgres
chenhj 5011 1 0 11:08 ? 00:00:00 /usr/local/pgsql/bin/postgres -D data
chenhj 5012 5011 0 11:08 ? 00:00:35 postgres: startup process recovering 0000000100000000000000A6
chenhj 5014 5011 0 11:08 ? 00:00:04 postgres: checkpointer process
chenhj 5015 5011 0 11:08 ? 00:00:00 postgres: writer process
chenhj 5016 5011 0 11:08 ? 00:00:02 postgres: stats collector process
chenhj 8124 5011 0 12:13 ? 00:00:00 postgres: chenhj chenhj [local] idle
chenhj 8181 5011 4 12:31 ? 00:00:24 postgres: wal receiver process streaming 0/A644EE20
chenhj 9477 8239 0 12:40 pts/3 00:00:00 grep postgres
July 17, 2015