Local on-disk cache for remote files in Aiven for ClickHouse®'s tiered storage
Aiven for ClickHouse®'s tiered storage features local on-disk cache for remote files for improved query performance and reduced latency.
To manage data, Aiven for ClickHouse's tiered storage uses local storage and remote storage. When remote storage is used, Aiven for ClickHouse leverages a local on-disk cache to avoid repeated remote fetches.
How it works
When a query requires parts of a table stored in the remote tier, Aiven for ClickHouse fetches the required parts from the remote storage. The fetched parts are automatically stored in a local cache directory on the disk to avoid repeated downloads for subsequent queries. For future queries, Aiven for ClickHouse checks the local cache first:
- If the data is found in the cache, it is read directly from the local disk.
- If the data is not found in the cache, it is fetched from the remote storage and stored in the local cache.
Local on-disk cache for remote files is enabled by default for Aiven for ClickHouse's tiered storage. You can disable the cache or drop it to free up the space it occupies.
Prerequisites
- At least one Aiven for ClickHouse service using tiered storage
- Command line tool (ClickHouse client) installed
Disable the cache
To disable the local cache for a query, set the enable_filesystem_cache setting for the
query to false.
You can achieve this by appending SETTINGS enable_filesystem_cache = false to the end of
your query using an SQL client (for example, the
ClickHouse client):
SELECT 1
SETTINGS enable_filesystem_cache = false;
Free up space
To drop the local cache and free up the used space, use the following cache command:
SYSTEM DROP FILESYSTEM CACHE 'remote_cache'