使用pg_repack實現在線vacuum

  • 2019 年 12 月 18 日
  • 筆記

Postgresql通過數據多版本實現mvcc，刪除數據並不會真正刪除數據，而是修改標識，更新是通過刪除+插入的方式進行，所以在頻繁更新的系統，數據膨脹是個頭疼的問題，如果不進行處理，數據膨脹倍數可能達到十幾倍。

為了處理膨脹問題，pg提供了vacuum工具，vacuum分為普通vacuum和vacuum full，普通vacuum會清理死元組，但是不會進行空間重組，磁碟上的空間不會釋放，但是會釋放死元組的空間，後續插入的元組會根據空閑空間管理fsm優先插入空閑空間。Vacuum full清理會釋放磁碟空間，但是會獲取八級鎖，因為vacuum full的原理是新建一個表數據文件，然後從老表中拷貝數據到新文件中，這個過程會阻塞select。

因為影響業務，pg社區開發了pg_repack工具，老版本叫pg_reorg。Pg_repack以extension的方式存在，用戶可以自己安裝該插件。本文簡單介紹一下pg_repack的使用。

Pg_repack的源碼在github或者pgxn上都可以下載，這裡編譯安裝、創建插件的過程就不再贅述。

在安裝完後，就可以在作業系統命令行使用pg_repack命令了，下面列舉了pg_repack的一些命令用法：

pg_repack --no-order --table test_1 test  pg_repack --wait-timeout 3600 --jobs 10 --no-order -d test  pg_repack --wait-timeout 3600 --jobs 10 --no-order --schema=test -d test  pg_repack --wait-timeout 3600 --jobs 10 --only-indexes --table test.test_1 --no-order -d test  pg_repack --wait-timeout 3600 --jobs 10 --index test.idx1 --no-order -d test

經過測試，在執行pg_repack的同時對錶進行並發查詢，性能下降大概只有10%到20%，讀取操作可以正常進行。並且表的oid沒有發生變化，repack執行完成後，通過pg_relation_filepath()函數查詢發現表的數據文件發生了改變，同時會刪除原來的數據文件，其實執行vacuum full數據文件也會發生改變。

test=# select pg_relation_filepath(16475);   pg_relation_filepath  ----------------------   base/16387/16580  (1 row)    test=# select pg_relation_filepath(16475);   pg_relation_filepath  ----------------------   base/16387/16601  (1 row)

下面聊聊repack的原理吧，原理其實和vacuum full是類似的，都是新建一個文件，然後將老文件數據拷貝過來，然後進行文件切換，它不阻塞讀寫的秘訣就是新建文件和拷貝的過程是在線做的，在沒有完成拷貝之前，原來的文件還是可以讀寫的，只有在切表那一瞬間可能會有影響。

那麼它是怎麼做到在線拷貝的呢？源庫的數據文件一直在變，所以表文件其實分為兩部分，一部分是基礎數據，一部分是增量數據，基礎數據的拷貝就是正常的拷貝，增量數據是通過創建觸發器來捕獲在該表上的讀寫操作來實現的，待基礎數據拷貝完後再將trigger捕獲的增量sql進行應用，達到最終結果。

我們其實可以發現，pg_repack會在庫里創建名為repack的schema，裡面有兩張表：primary_keys和tables。Primarys分為兩列，第一列indrelid代表表的oid，第二列indexrelid代表主鍵或唯一索引的oid。Tables表記錄了創建trigger以及捕獲的相關語句，語句按一條條的record進行記錄。如下所示：

test=# select * from tables;  -[ RECORD 1 ]-----+-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------  relname           | public.products  relid             | 16388  reltoastrelid     | 16391  reltoastidxid     | 16393  schemaname        | public  pkid              |  ckid              |  create_pktype     |  create_log        | CREATE TABLE repack.log_16388 (id bigserial PRIMARY KEY, pk repack.pk_16388, row public.products)  create_trigger    |  enable_trigger    | ALTER TABLE public.products ENABLE ALWAYS TRIGGER repack_trigger  create_table_1    | CREATE TABLE repack.table_16388 WITH (oids = false) TABLESPACE  tablespace_orig   | pg_default  create_table_2    |  AS SELECT product_no,name,price FROM ONLY public.products  copy_data         | INSERT INTO repack.table_16388 SELECT product_no,name,price FROM ONLY public.products  alter_col_storage |  drop_columns      |  delete_log        | DELETE FROM repack.log_16388  lock_table        | LOCK TABLE public.products IN ACCESS EXCLUSIVE MODE  ckey              |  sql_peek          | SELECT * FROM repack.log_16388 ORDER BY id LIMIT $1  sql_insert        | INSERT INTO repack.table_16388 VALUES ($1.*)  sql_delete        |  sql_update        |  sql_pop           | DELETE FROM repack.log_16388 WHERE id IN (  -[ RECORD 2 ]-----+-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------  relname           | public.test  relid             | 16400  reltoastrelid     | 0  reltoastidxid     | 0  schemaname        | public  pkid              |  ckid              |  create_pktype     |  create_log        | CREATE TABLE repack.log_16400 (id bigserial PRIMARY KEY, pk repack.pk_16400, row public.test)  create_trigger    |  enable_trigger    | ALTER TABLE public.test ENABLE ALWAYS TRIGGER repack_trigger  create_table_1    | CREATE TABLE repack.table_16400 WITH (oids = false) TABLESPACE  tablespace_orig   | pg_default  create_table_2    |  AS SELECT id FROM ONLY public.test  copy_data         | INSERT INTO repack.table_16400 SELECT id FROM ONLY public.test  alter_col_storage |  drop_columns      |  delete_log        | DELETE FROM repack.log_16400  lock_table        | LOCK TABLE public.test IN ACCESS EXCLUSIVE MODE  ckey              |  sql_peek          | SELECT * FROM repack.log_16400 ORDER BY id LIMIT $1  sql_insert        | INSERT INTO repack.table_16400 VALUES ($1.*)  sql_delete        |  sql_update        |  sql_pop           | DELETE FROM repack.log_16400 WHERE id IN (  -[ RECORD 3 ]-----+-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------  relname           | public.fruits  relid             | 16394  reltoastrelid     | 16397  reltoastidxid     | 16399  schemaname        | public  pkid              |  ckid              |  create_pktype     |  create_log        | CREATE TABLE repack.log_16394 (id bigserial PRIMARY KEY, pk repack.pk_16394, row public.fruits)  create_trigger    |  enable_trigger    | ALTER TABLE public.fruits ENABLE ALWAYS TRIGGER repack_trigger  create_table_1    | CREATE TABLE repack.table_16394 WITH (oids = false) TABLESPACE  tablespace_orig   | pg_default  create_table_2    |  AS SELECT number,name,price FROM ONLY public.fruits  copy_data         | INSERT INTO repack.table_16394 SELECT number,name,price FROM ONLY public.fruits  alter_col_storage |  drop_columns      |  delete_log        | DELETE FROM repack.log_16394  lock_table        | LOCK TABLE public.fruits IN ACCESS EXCLUSIVE MODE  ckey              |  sql_peek          | SELECT * FROM repack.log_16394 ORDER BY id LIMIT $1  sql_insert        | INSERT INTO repack.table_16394 VALUES ($1.*)  sql_delete        |  sql_update        |  sql_pop           | DELETE FROM repack.log_16394 WHERE id IN (  -[ RECORD 4 ]-----+-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------  relname           | public.test_1  relid             | 16475  reltoastrelid     | 0  reltoastidxid     | 0  schemaname        | public  pkid              | 16493  ckid              |  create_pktype     | CREATE TYPE repack.pk_16475 AS (c1 integer)  create_log        | CREATE TABLE repack.log_16475 (id bigserial PRIMARY KEY, pk repack.pk_16475, row public.test_1)  create_trigger    | CREATE TRIGGER repack_trigger AFTER INSERT OR DELETE OR UPDATE ON public.test_1 FOR EACH ROW EXECUTE PROCEDURE repack.repack_trigger('INSERT INTO repack.log_16475(pk, row) VALUES( CASE WHEN $1 IS NULL THEN NULL ELSE (ROW($1.c1)::repack.pk_16475) END, $2)')  enable_trigger    | ALTER TABLE public.test_1 ENABLE ALWAYS TRIGGER repack_trigger  create_table_1    | CREATE TABLE repack.table_16475 WITH (oids = false) TABLESPACE  tablespace_orig   | pg_default  create_table_2    |  AS SELECT c1,c2,c3,c4,c5 FROM ONLY public.test_1  copy_data         | INSERT INTO repack.table_16475 SELECT c1,c2,c3,c4,c5 FROM ONLY public.test_1  alter_col_storage |  drop_columns      |  delete_log        | DELETE FROM repack.log_16475  lock_table        | LOCK TABLE public.test_1 IN ACCESS EXCLUSIVE MODE  ckey              |  sql_peek          | SELECT * FROM repack.log_16475 ORDER BY id LIMIT $1  sql_insert        | INSERT INTO repack.table_16475 VALUES ($1.*)  sql_delete        | DELETE FROM repack.table_16475 WHERE (c1) = ($1.c1)  sql_update        | UPDATE repack.table_16475 SET (c1, c2, c3, c4, c5) = ($2.c1, $2.c2, $2.c3, $2.c4, $2.c5) WHERE (c1) = ($1.c1)  sql_pop           | DELETE FROM repack.log_16475 WHERE id IN (

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