低 Downtime 將 4TB 的 PostgreSQL 9.6 資料庫升級到 13 的故事

前幾天在 Hacker News 首頁上看到的文章,講怎麼把一個 4TB 的 PostgreSQL 從 9.6 升級到 13 的故事:「How we upgraded our 4TB Postgres database (retool.com)」,原文在「How Retool upgraded our 4 TB main application PostgreSQL database」,翻了一下 LinkedIn,這篇文章的作者 Peter Johnston 在 Retool 掛的是 Security Software Engineer,另外他也有在 Hacker News 上的討論出現 (帳號是 mrbabbage),可以搜尋翻翻看他的回覆。

看完文章後發現方法的概念其實不難,主要是要找到對的工具來用。基本的想法是先生出一個 initial dump,然後架構 logical replication,接下來就是處理各種因為在 4TB data 這個 scale 下會遇到的問題。

主要用到的工具是 Citus Data 的 Warp:「Citus warp: Database migrations without the pain」,不過這個工具的限制是表格必須都有 single column primary key,所以他們為了這次轉移也有小改 database schema 配合 Warp 的要求:

We had to do a bit of finagling to coax Warp into processing our database. Warp expects all tables to have a single column primary key, so we had to convert compound primary keys into unique constraints and add scalar primary keys. Otherwise, Warp was very straightforward to use.

另外針對比較大的兩個 append-only 的表格 (分別是 2TB 與 x00 GB) 做處理,在 initial dump 的階段不對這兩個表格做 replication,而是透過自製的 Python script 搬移:

To handle the two massive tables we skipped in Warp, we wrote a Python script to bulk transfer data from the old database server to the new.

然後是 foreign key 相關的關閉與重啟,這算是 RDBMS 在大量資料的 dump & restore 的標準作業了:

As you can see from the runbook above, one of the steps we had to do was to turn off and then re-enable foreign key constraint checks.

最後執行下去,整個 downtime 只有十五分鐘:

We scheduled the maintenance window late on Saturday, October 23rd, at the lowest period of Retool cloud traffic. With the configuration described above, we were able to bring up a new database server at version 13 in around 15 minutes, subscribed to changes at our 9.6 primary with logical decoding.

另外也有提到有計畫要 sharding,之後 main database 就有機會被拆小:

We anticipate we’ll have sharded our database by the end of that support window, and be performing our next substantial version upgrades incrementally.

整個計畫的核心概念不難,主要是要怎麼順出來並且執行...

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