Backup and restore

You should back up QuestDB to be prepared for the case where your original database or data is lost, or if your database or table is corrupted. The backup & restore process also speeds up the creation of replica instances in QuestDB Enterprise.

Overview

To perform a backup, follow these steps:

1. Issue SQL: CHECKPOINT CREATE, which activates the special CHECKPOINT mode of QuestDB
2. Create a copy of the QuestDB root directory
3. Issue SQL: CHECKPOINT RELEASE, bringing QuestDB back to regular operation

When in the CHECKPOINT mode, QuestDB remains available for both reads and writes. However, some housekeeping tasks are paused. While this is safe in principle, database writes may consume more space than normal. When the database exits CHECKPOINT mode, it will resume the housekeeping tasks and quickly reclaim the disk space.

In the second step above, you must create a copy of the database using a tool of your choice. These are some suggestions:

• Cloud snapshot, e.g. EBS volume snapshot on AWS, Premium SSD Disk snapshot on Azure etc
• On-prem backup tools and software you typically use
• Basic command line tools, such as cp or rsync

To recover the database, follow these steps:

1. Restore the QuestDB root directory from the backup copy
2. Create an empty _restore trigger file in the QuestDB root directory
3. Start QuestDB as usual

If the trigger file is present in the root directory, QuestDB performs the recovery process on startup. If successful, the process deletes the trigger file, so it won't perform recovery in future restarts. Should recovery fail, QuestDB will exit with an error, and the trigger file will remain in place.

Data backup checklist

Before backing up QuestDB, consider these items:

Pick a good time

We recommend that teams take a database backup when the database write load is at its lowest. If the database is under constant write load, a helpful workaround is to ensure that the disk has at least 50% free space. The more free space, the safer it is to enter the checkpoint mode.

Determine backup frequency

We recommend daily backups.

If you are using QuestDB Enterprise, the frequency of backups impacts the time it takes to create a new replica instance. Creating replicas involves choosing a backup and having the replica replay WAL files until it has caught up. The older the backup, the more WAL files the replica will have to replay, and thus there is a longer time-frame. For these reasons, we recommend a daily backup schedule to keep the process rapid.

Choose your data copy method

When choosing the right copy method, consider the following goals:

• Minimize the time QuestDB spends in checkpoint mode
• Ensure that the copy time remains sustainable as the database grows

QuestDB backup lends itself relatively well to all types of differential data copying. Due to time partitioning, older data is often unmodified, at both block and file levels.

Cloud snapshots

If you're using cloud disks, such as EBS on AWS, SSD on Azure, or similar, we strongly recommend using their existing cloud snapshot infrastructure. The advantages of this approach are that:

• Cloud snapshots minimizes the time QuestDB spends in checkpoint mode
• Cloud snapshots are differential and can be restored cleanly

See the following guides for volume snapshot creation on the following cloud platforms:

• AWS - creating EBS snapshots
• Azure - creating snapshots of a virtual hard disk
• GCP - working with persistent disk snapshots

Cloud snapshot-based systems usually break down their backup process into two steps:

1. Take a snapshot
2. Back up the snapshot

Exit the CHECKPOINT mode as soon the snapshoting stage is complete.

Specifically, exit checkpoint mode at the following snapshot stage:

Cloud Provider	State	Exit checkpoint mode
Google Cloud (GCP)	RUNNING (UPLOADING)	When RUNNING substate changes from CREATING to UPLOADING
Amazon Web Services (AWS)	PENDING	When status is PENDING
Microsoft Azure	PENDING	Before the longer running "CREATING" stage

Filesystem or volume snapshots

When the database is on-prem, we recommend using the existing file system backup tools. Volume snapshots can be taken using either the filesystem (ZFS) or a volume manager (LVM).

File copy

If filesystem or volume snapshots are not available, consider using a file copy method to back up the QuestDB server root directory. We recommend using a copy tool that can skip copying files based on the modification date. One such popular tool to accomplish this is rsync.

Leaving this step, you should know:

• Whether your method is cloud or file-system snapshot-based, or file copy-based
• When to enter and exit checkpoint mode
• How to perform your snapshot/backup method

Steps in the backup procedure

While explaining the steps, we'll assume the database root directory is /var/lib/questdb.

Enter checkpoint mode

To enter the checkpoint mode:

CHECKPOINT CREATE

You can create only one checkpoint. Attempting to create a second checkpoint will fail.

Check checkpoint status

You can double-check at any time that the database is in the checkpoint mode:

SELECT * FROM checkpoint_status();

Having confirmed that QuestDB has entered the checkpoint mode, we now create the backup.

Take a snapshot or begin file copy

After a checkpoint is created and before it is released, you may safely access the file system using tools external to the database instance. In other words, you're now OK to begin your backup.

If your data copy method is a volume snapshot, you can exit the checkpoint mode as soon as the snapshot is taken (which takes a minute or two).

Make sure to back up the entire server root directory, including the db, snapshot, and all other directories.

File copy may take longer to back up files compared to snapshot. You will have to wait until the data transfer is fully complete before exiting checkpoint mode.

It is very important to exit the checkpoint mode regardless of whether the copy operation succeeded or failed!

Exit checkpoint mode

With your backup complete, exit checkpoint mode:

CHECKPOINT RELEASE

This concludes the backup process.

Now, with our additional copy, we're ready to restore QuestDB.

Restore to a saved checkpoint

Restoring to a checkpoint will restore the entire database.

Follow these steps:

• Ensure your QuestDB version matches the one that did the backup
• Restore QuestDB root directory contents (/var/lib/questdb/) from the backup
• Touch the _restore file
• Start the database using the restored root directory

Database versions

Restoring data is only possible if the backup and restore QuestDB versions have the same major version number, for example: 8.1.0 and 8.1.1 are compatible. 8.1.0 and 7.5.1 are not compatible.

Restore the root directory

When using cloud tools, create a new disk from the snapshot. The entire disk contents of the original database will be available when the compute instance starts.

If you are not using cloud tools, you have to make sure that you restore the root from the backup using your own tools of choice!

The trigger file

When you are starting the database from the backup for the first time, the database must perform a restore procedure. This ensures the data is consistent and can be read and written. It only takes place on startup, and requires a specific blank file to exist as the indication of user intent.

Touch the _restore file in the root directory. The following command will do the trick:

touch /var/lib/questdb/_restore

Start the database

Start the database using the root directory as usual. When the _restore file is present, the database will perform the restore procedure. There are two possible outcomes:

• Restore is successful: the database continues to run normally and is ready to use; the _restore file is removed to prevent the same procedure running twice
• Restore fails: the database exits and the _restore file remains in place. An error message appears in stderr. If it can be resolved, starting the database again will retry the restore procedure

Supported filesystems

QuestDB supports the following filesystems:

• APFS
• EXT4
• NTFS
• OVERLAYFS (used by Docker)
• XFS
• ZFS

Other file systems are untested and while they may work, we do not officially support them. See the filesystem compatibility section for more information.