CHECKPOINT keyword

Checkpoint SQL toggles the database into and out of "checkpoint mode". In this mode the databases file system can be safely backed up using external tools, such as disk snapshots or copy utilities.

Looking for a detailed guide backup creation and restoration? Check out our Backup and Restore guide!

caution

QuestDB currently does not support creating checkpoints on Windows.

If you are a Windows user and require backup functionality, please comment on this issue.

CHECKPOINT syntax​

Flow chart showing the syntax of the CHECKPOINT keyword

CHECKPOINT overview​

To enable online backups, data in QuestDB is mutated via either file append or via copy-on-write. Checkpoint leverages these storage methods to achieve reliable and consistent restorations from your database backups.

What happens during CHECKPOINT CREATE?​

When initiatied, CHECKPOINT CREATE:

  • Disables background jobs that housekeep stale files and data blocks
  • Takes snapshot of table transactions across the whole database (all tables)
  • Creates a new on-disk data structure that captures append offsets and versions of files that represent data for the above transactions. Typically this data is stored in the /var/lib/questdb/.checkpoint directory.
    • Do not alter contents of this directory manually!
  • fsync - synchronously flushes page cache and buffers to disk

What happens after a checkpoint has been created?​

Once a checkpoint is created, QuestDB continues taking in writes. However, it will consume more disk space. How much more depends on the shape of the data that is being written. Data that is written via the append method will yeild almost no additional disk space consumption other that of the data itself. In contrast, the copy-on-write method will make data copies, which are usually copies of non-recent table partitions. This will lead to an increase in disk space consumption.

It is strongly recommended that you minimize the time database is in checkpoint mode and monitor the free disk space closely. The recommended way to achive this is to utilize file system SNAPSHOTS as described in our backup and restore guide.

Also note that QuestDB can only enter checkpoint mode once. After that period of time, the next checkpoint operation must be to exit checkpoint mode. Attempts to create a new checkpoint when once exists will fail with the appropriate message.

When in checkpoint mode, you can safely access the file system to take your snapshot.

What happens after my snapshot is complete?​

After your snapshot is complete, checkpoint mode must be exited via the CHECKPOINT RELEASE SQL. Once executed, QuestDB will reinstate the usual housekeeping and reclaim disk space.

The database restore is preformed semi-automatically on the database startup. This is done deliberately to avoid the restore procedure running accidentally on the source database instance. The database will attempt a restore when empty an file, typically /var/lib/questdb/_restore is present.

The restore procedure will use /var/lib/questdb/.checkpoint to adjust the database files and remove extra data copies. After the restore is successful the database is avaialble as normal with no extra intervantion required.

CHECKPOINT examples​

To enter checkpoint mode:

CHECKPOINT CREATE

To exit checkpoint mode:

CHECKPOINT RELEASE