我们知道Postgresql数据库中有两大块buffer：wal buffer和shared buffer。两者作用分别是：
- wal buffer：预写日志缓冲区，wal writer进程负责将wal buffer缓冲区的数据写入WAL FILE中。
- shared buffer：数据页缓冲区，background writer进程负责将shared buffer缓冲区的数据写入DATA
FILE中。
当写入量非常大时，wal writer和background writer进程不足以满足系统负载时，用户进程（backend process）也会参与将BUFFER写到FILE的工作。
如何监控数据库的磁盘使用情况呢？我们可以通过操作系统的监控程序，如iotop、iostat等。除此之外，pg中提供了系统视图统计bgwriter,walwriter,backend process 写磁盘的实时指标。
pg_stat_bgwriter 视图的介绍:
@H_404_22@bill=# \d pg_stat_bgwriter View "pg_catalog.pg_stat_bgwriter" Column | Type | Collation | Nullable | Default -----------------------+--------------------------+-----------+----------+--------- checkpoints_timed | bigint | | | checkpoints_req | bigint | | | checkpoint_write_time | double precision | | | checkpoint_sync_time | double precision | | | buffers_checkpoint | bigint | | | buffers_clean | bigint | | | maxwritten_clean | bigint | | | buffers_backend | bigint | | | buffers_backend_fsync | bigint | | | buffers_alloc | bigint | | | stats_reset | timestamp with time zone | | |
1、pg_stat_bgwriter.buffers_clean bgwriter 每秒write多少shared buffer到disk(write-异步系统调用) , 说明了脏页产生较快，但是不影响业务。
2、pg_stat_bgwriter.buffers_backend backend process 每秒wirte多少shared buffer到disk(write-异步系统调用) , 说明产生脏页较快，并且bgwriter或checkpointer写脏页已经赶不上产生脏页的速度了，对业务开始有影响。
3、walwriter 每秒write多少wal buffer到disk(write-异步系统调用)
4、pg_stat_bgwriter.buffers_alloc 每秒分配多少新的shared buffer，说明了从磁盘读的频繁程度。
例子：
构建压测环境：
@H_404_22@pgbench -i -s 100 pgbench -M prepared -n -r -P 1 -c 16 -j 16 -T 10000
监测：
1、bgwriter 每秒write多少shared buffer到disk(write-异步系统调用)
@H_404_22@postgres@postgres=>select buffers_clean*8/1024||' MB' bg from pg_stat_bgwriter; bg ---------- 25911 MB (1 row) postgres@postgres=>\watch 1 Sun 19 Apr 2020 05:16:03 PM CST (every 1s) bg ---------- 26120 MB (1 row) Sun 19 Apr 2020 05:16:04 PM CST (every 1s) bg ---------- 26184 MB (1 row) Sun 19 Apr 2020 05:16:05 PM CST (every 1s) bg ---------- 26234 MB (1 row) Sun 19 Apr 2020 05:16:06 PM CST (every 1s) bg ---------- 26289 MB (1 row)
2、backend process 每秒wirte多少shared buffer到disk(write-异步系统调用)
@H_404_22@postgres@postgres=>select buffers_clean*8/1024||' MB' bg from pg_stat_bgwriter; bg ---------- 26533 MB (1 row) postgres@postgres=>\watch 1 Sun 19 Apr 2020 05:16:13 PM CST (every 1s) bg ---------- 26756 MB (1 row) Sun 19 Apr 2020 05:16:14 PM CST (every 1s) bg ---------- 26825 MB (1 row) Sun 19 Apr 2020 05:16:15 PM CST (every 1s) bg ---------- 26883 MB (1 row) Sun 19 Apr 2020 05:16:16 PM CST (every 1s) bg ---------- 26946 MB (1 row)
3、walwriter 每秒write多少wal buffer到disk(write-异步系统调用)
@H_404_22@postgres@postgres=>with a as (select pg_current_wal_lsn() lsn) select pg_size_pretty(pg_wal_lsn_diff(pg_current_wal_lsn(), lsn)) from a, pg_sleep(1); pg_size_pretty ---------------- 12 MB (1 row) postgres@postgres=>\watch 0.001 Sun 19 Apr 2020 05:16:40 PM CST (every 0.001s) pg_size_pretty ---------------- 112 MB (1 row) Sun 19 Apr 2020 05:16:41 PM CST (every 0.001s) pg_size_pretty ---------------- 96 MB (1 row) Sun 19 Apr 2020 05:16:42 PM CST (every 0.001s) pg_size_pretty ---------------- 104 MB (1 row)
4、每秒分配多少新的shared buffer
@H_404_22@postgres@postgres=>select buffers_alloc*8/1024||' MB' bg from pg_stat_bgwriter; bg ---------- 43668 MB (1 row) postgres@postgres=>\watch 1 Sun 19 Apr 2020 05:16:55 PM CST (every 1s) bg ---------- 43676 MB (1 row) Sun 19 Apr 2020 05:16:56 PM CST (every 1s) bg ---------- 43676 MB (1 row) Sun 19 Apr 2020 05:16:57 PM CST (every 1s) bg ---------- 43676 MB (1 row)
iotop监控情况：
经验证和前面的方法基本差不多。
@H_404_22@[root@oracle ~]# iotop Total disK READ : 0.00 B/s | Total disK WRITE : 60.69 M/s Actual disK READ: 0.00 B/s | Actual disK WRITE: 21.32 M/s TID PRIO USER disK READ disK WRITE SWAPIN IO> COMMAND 27511 be/4 pg12 0.00 B/s 21.46 M/s 0.00 % 26.74 % postgres: walwriter 9773 be/4 root 0.00 B/s 0.00 B/s 0.00 % 0.09 % [kworker/0:0] 28619 be/4 pg12 0.00 B/s 7.51 K/s 0.00 % 0.01 % postgres: postgres postgres [local] UPDATE 28622 be/4 pg12 0.00 B/s 165.11 K/s 0.00 % 0.00 % postgres: postgres postgres [local] idle in transaction 28628 be/4 pg12 0.00 B/s 22.52 K/s 0.00 % 0.00 % postgres: postgres postgres [local] idle in transaction 28620 be/4 pg12 0.00 B/s 15.01 K/s 0.00 % 0.00 % postgres: postgres postgres [local] BIND 27510 be/4 pg12 0.00 B/s 37.39 M/s 0.00 % 0.00 % postgres: background writer 28607 be/4 pg12 0.00 B/s 15.01 K/s 0.00 % 0.00 % postgres: postgres postgres [local] UPDATE 28611 be/4 pg12 0.00 B/s 315.21 K/s 0.00 % 0.00 % postgres: postgres postgres [local] idle in transaction 28612 be/4 pg12 0.00 B/s 172.62 K/s 0.00 % 0.00 % postgres: postgres postgres [local] idle in transaction 28615 be/4 pg12 0.00 B/s 7.51 K/s 0.00 % 0.00 % postgres: postgres postgres [local] idle in transaction 28624 be/4 pg12 0.00 B/s 15.01 K/s 0.00 % 0.00 % postgres: postgres postgres [local] idle in transaction 28625 be/4 pg12 0.00 B/s 315.21 K/s 0.00 % 0.00 % postgres: postgres postgres [local] UPDATE 28626 be/4 pg12 0.00 B/s 30.02 K/s 0.00 % 0.00 % postgres: postgres postgres [local] INSERT 28627 be/4 pg12 0.00 B/s 22.52 K/s 0.00 % 0.00 % postgres: postgres postgres [local] idle in transaction 28629 be/4 pg12 0.00 B/s 780.53 K/s 0.00 % 0.00 % postgres: postgres postgres [local] UPDATE
除此之外，还可以自定义监测某个时间段内的wal写入量。
@H_404_22@postgres@postgres=>select pg_current_wal_lsn(); pg_current_wal_lsn -------------------- 18/D6430000 (1 row) postgres@postgres=>select pg_current_wal_lsn(); pg_current_wal_lsn -------------------- 18/EA57A6D0 (1 row) postgres@postgres=>select pg_size_pretty(pg_wal_lsn_diff('18/D6430000','18/EA57A6D0')); pg_size_pretty ---------------- -321 MB (1 row)
参考链接：
https://www.postgresql.org/docs/12/monitoring-stats.html#MONITORING-STATS-VIEWS
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