PostgreSQL 11 新特性解讀 : Initdb/Pg_resetwal支援修改WAL檔案大小
PostgreSQL 11 版本的一個重要調整是支援 initdb 和 pg_resetwal 修改 WAL 檔案大小，而 11 版本之前只能在編譯安裝 PostgreSQL 時設定 WAL 檔案大小。這一特效能夠方便 WAL 檔案的管理。
Release 的說明
Allow the WAL file size to be set via initdb (Beena Emerson)
Previously the 16MB default could only be changed at compile time.
下面分別演示通過 initdb 和 pg_resetwal 修改 WAL 檔案大小。
使用 initdb 調整WAL檔案大小
initdb 命令關於修改 WAL 檔案大小選項，如下:
--wal-segsize=size
Set the WAL segment size, in megabytes. This is the size of each individual file in the WAL log. The default size is 16 megabytes. The value must be a power of 2 between 1 and 1024 (megabytes). This option can only be set during initialization, and cannot be changed later.
It may be useful to adjust this size to control the granularity of WAL log shipping or archiving. Also, in databases with a high volume of WAL, the sheer number of WAL files per directory can become a performance and management problem. Increasing the WAL file size will reduce the number of WAL files.
WAL 日誌檔案大小預設為16MB，該值必須是1到1024之間的2的次方，增大WAL檔案大小能夠減少WAL日誌檔案的產生。
初始化一個新的 PostgreSQL 資料庫例項，指定WAL檔案大小64MB，如下:
[[email protected] ~]$ initdb -E UTF8 --locale=C --wal-segsize=64 -D /home/pg11/data01 -U postgres -W The files belonging to this database system will be owned by user "pg11". This user must also own the server process. The database cluster will be initialized with locale "C". The default text search configuration will be set to "english". Data page checksums are disabled. Enter new superuser password: Enter it again: creating directory /home/pg11/data01 ... ok creating subdirectories ... ok selecting default max_connections ... 100 selecting default shared_buffers ... 128MB selecting dynamic shared memory implementation ... posix creating configuration files ... ok running bootstrap script ... ok performing post-bootstrap initialization ... ok syncing data to disk ... ok WARNING: enabling "trust" authentication for local connections You can change this by editing pg_hba.conf or using the option -A, or --auth-local and --auth-host, the next time you run initdb. Success. You can now start the database server using: pg_ctl -D /home/pg11/data01 -l logfile start
修改 postgresql.conf 相關配置，之後啟動資料庫。
[[email protected] data01]$ pg_ctl start -D /home/pg11/data01
waiting for server to start....2018-10-16 15:58:16.714 CST [10583] LOG: listening on IPv6 address "::1", port 1950
2018-10-16 15:58:16.714 CST [10583] LOG: listening on IPv4 address "127.0.0.1", port 1950
2018-10-16 15:58:16.731 CST [10583] LOG: listening on Unix socket "/tmp/.s.PGSQL.1950"
2018-10-16 15:58:16.762 CST [10584] LOG: database system was shut down at 2018-10-16 15:56:46 CST
2018-10-16 15:58:16.782 CST [10583] LOG: database system is ready to accept connections
done
server started驗證WAL檔案大小，如下:
[[email protected] ~]$ ll /home/pg11/data01/pg_wal
total 65M
-rw------- 1 pg11 pg11 64M Oct 16 16:03 000000010000000000000001
drwx------ 2 pg11 pg11 4.0K Oct 16 15:56 archive_status可見WAL檔案大小為64MB。
使用 pg_resetwal 調整WAL檔案大小
pg_resetwal 用來重置WAL日誌和一些控制資訊，常用於資料庫恢復場景，不到萬不得已不輕易使用，以下演示使用pg_resetwal命令調整WAL日誌檔案大小，僅供測試參考，生產環境慎用。
pg_resetwal 命令關於調整WAL檔案大小的選項，如下:
--wal-segsize=wal_segment_size
Set the new WAL segment size, in megabytes. The value must be set to a power of 2 between 1 and 1024 (megabytes). See the same option of initdb for more information.
以下演示在已有PostgreSQL例項基礎上調整WAL日誌檔案大小。
檢視當前資料庫的 pg_wal 目錄，如下:
[[email protected] pg_wal]$ ll /database/pg11/pg_root/pg_wal/
total 2.3G
-rw------- 1 pg11 pg11 16M Sep 30 14:45 000000010000001700000013
-rw------- 1 pg11 pg11 16M Sep 30 14:45 000000010000001700000014
-rw------- 1 pg11 pg11 16M Sep 30 14:45 000000010000001700000015
-rw------- 1 pg11 pg11 16M Sep 30 14:45 000000010000001700000016
-rw------- 1 pg11 pg11 16M Sep 30 14:45 000000010000001700000017
-rw------- 1 pg11 pg11 16M Sep 30 14:45 000000010000001700000018
-rw------- 1 pg11 pg11 16M Sep 30 14:45 000000010000001700000019
-rw------- 1 pg11 pg11 16M Sep 30 14:45 00000001000000170000001A
-rw------- 1 pg11 pg11 16M Sep 30 14:45 00000001000000170000001B
...
省略
drwx------ 2 pg11 pg11 16K Oct 16 08:38 archive_statuspg_wal 目錄中已有大量WAL日誌檔案，WAL檔案大小為16MB，計劃將WAL日誌檔案調整成64MB。
pg_resetwal 操作時需要關閉資料庫，如下。
[[email protected] ~]$ pg_ctl stop -m fast
waiting for server to shut down.... done
server stoppedpg_resetwal 命令調整WAL日誌檔案大小為 64MB:
[[email protected] ~]$ pg_resetwal --wal-segsize=64 -D /database/pg11/pg_root
Write-ahead log reset驗證WAL檔案大小,如下:
[[email protected] ~]$ ll /database/pg11/pg_root/pg_wal/
total 65M
-rw------- 1 pg11 pg11 64M Oct 16 08:55 000000010000001700000029
drwx------ 2 pg11 pg11 16K Oct 16 08:55 archive_status發現 pg_wal 目錄中原有的WAL日誌被清理，同時生成了大小為64MB新的WAL檔案。
啟動資料庫提示 min_wal_size 引數至少需設定成 wal_segment_size 大小為 2 倍。
[[email protected] ~]$ pg_ctl start
waiting for server to start....2018-10-16 09:01:26.096 CST [24318] FATAL: "min_wal_size" must be at least twice "wal_segment_size".
2018-10-16 09:01:26.096 CST [24318] LOG: database system is shut down
stopped waiting
pg_ctl: could not start server
Examine the log output.根據提示調整 postgresql.conf，設定如下：
min_wal_size = 128MB啟動資料庫正常，如下:
[[email protected] ~]$ pg_ctl start
waiting for server to start....2018-10-16 09:02:45.680 CST [24614] LOG: listening on IPv4 address "0.0.0.0", port 1930
2018-10-16 09:02:45.680 CST [24614] LOG: listening on IPv6 address "::", port 1930
2018-10-16 09:02:45.687 CST [24614] LOG: listening on Unix socket "/tmp/.s.PGSQL.1930"
2018-10-16 09:02:45.715 CST [24614] LOG: redirecting log output to logging collector process
2018-10-16 09:02:45.715 CST [24614] HINT: Future log output will appear in directory "log".總結
- 以上演示了 11 版本通過
initdb和pg_resetwal調整WAL檔案大小。 -
pg_resetwal會清除pg_wal目錄的WAL檔案，本部落格的測試樣例僅供參考，生產環境使用需慎重。
參考
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