Vitess inherits the same issues that MySQL has with DROP TABLE. Doing a direct
DROP TABLE my_table in production can be a risky operation. In busy environments
this can lead to a complete lockdown of the database for the duration of seconds,
to minutes and more. This is typically less of a problem in Vitess than it might
be in normal MySQL, if you are keeping your shard instances (and thus shard
table instances) small, but could still be a problem.
There are two locking aspects to dropping tables:
Purging dropped table’s pages from the InnoDB buffer pool(s)
Removing table’s data file (.ibd) from the filesystem.
The exact locking behavior and duration can vary depending on
Which filesystem is used
Whether the MySQL adaptive hash index is used
Whether you are attempting to hack around some of the MySQL DROP TABLE
performance problems using hard links
It is common practice to avoid direct DROP TABLE statements and to follow
a more elaborate table lifecycle.
The lifecycle offered by Vitess consists of the following stages or some subset:
in use -> hold -> purge -> evac -> drop -> removed
To understand the flow better, consider the following breakdown:
In use: the table is serving traffic, like a normal table.
hold: the table is renamed to some arbitrary new name. The application cannot see it, and considers it as gone. However, the table still exists, with all of its data intact. It is possible to reinstate it (e.g. in case we realize some application still requires it) by renaming it back to its original name.
purge: the table is in the process of being purged (i.e. rows are being deleted). The purge process completes when the table is completely empty. At the end of the purge process the table no longer has any pages in the buffer pool(s). However, the purge process itself loads the table pages to cache in order to delete rows.
Vitess purges the table a few rows at a time, and uses a throttling mechanism to reduce load.
Vitess disables binary logging for the purge. The deletes are not written to the binary logs and are not replicated. This reduces load from disk IO, network, and replication lag. Data is not purged on the replicas.
Experience shows that dropping a table populated with data on a replica has lower performance impact than on the primary, and the tradeoff is worthwhile.
evac: a waiting period during which we expect normal production traffic to slowly evacuate the (now inactive) table’s pages from the buffer pool. Vitess hard codes this period for 72 hours. The time is heuristic, there is no tracking of table pages in the buffer pool.
drop: an actual DROP TABLE is imminent
removed: table is dropped. When using InnoDB and innodb_file_per_table this means the .ibd data file backing the table is removed, and disk space is reclaimed.
Different environments and users have different requirements and workflows. For example:
Some wish to immediately start purging the table, wait for pages to evacuate, then drop it.
Some want to keep the table’s data for a few days, then directly drop it.
Some just wish to directly drop the table, they see no locking issues (e.g. smaller table).
Vitess supports all subsets via -table_gc_lifecycle flag to vttablet. The default is "hold,purge,evac,drop" (the complete cycle). Users may configure any subset, e.g. "purge,drop", "hold,drop", "hold,evac,drop" or even just "drop".
Vitess will always work the steps in this order: hold -> purge -> evac -> drop. For example, setting -table_gc_lifecycle "drop,hold" still first holds, then drops
All subsets end with a drop, even if not explicitly mentioned. Thus, "purge" is interpreted as "purge,drop".
Vitess does not track the state of the table lifecycle. The process is stateless thanks to an encoding scheme in the table names. Examples:
The table _vt_HOLD_6ace8bcef73211ea87e9f875a4d24e90_20210915120000 is held until 2021-09-15 12:00:00. The data remains intact.
The table _vt_PURGE_6ace8bcef73211ea87e9f875a4d24e90_20210915123000 is at the state where it is being purged, or queued to be purged. Once it’s fully purged (zero rows remain), it transitions to the next stage.
The table _vt_EVAC_6ace8bcef73211ea87e9f875a4d24e90_20210918093000 is held until 2021-09-18 09:30:00
The table _vt_DROP_6ace8bcef73211ea87e9f875a4d24e90_20210921170000 is eligible to be dropped on 2021-09-21 17:00:00
Vitess internally uses the above table lifecycle for online, managed schema migrations. Online schema migration tools gh-ost and pt-online-schema-change create artifact tables or end with leftover tables: Vitess automatically collects those tables. The artifact or leftover tables are immediate moved to purge state. Depending on -table_gc_lifecycle, they may spend time in this state, getting purged, or immediately transitioned to the next state.
When using an online ddl_strategy, a DROP TABLE is a managed schema migration. It is internally replaced by a RENAME TABLE statement, renaming it into a HOLD state (e.g. _vt_HOLD_6ace8bcef73211ea87e9f875a4d24e90_20210915120000). It will then participate in the table lifecycle mechanism. If table_gc_lifecycle does not include the hold state, the table proceeds to transition to next included state.
A multi-table DROP TABLE statement is converted to multiple single-table DROP TABLE statements, each to then convert to a RENAME TABLE statement.