ssd_cache dictionary layout types

ssd_cache

Similar to cache, but stores data on SSD and index in RAM. All cache dictionary settings related to update queue can also be applied to SSD cache dictionaries. Cache contents are local to each server and are lost when a server restarts or is replaced. Do not use ssd_cache or complex_key_ssd_cache for reliable insert-time enrichment: an empty cache makes inserts depend on lookups to the dictionary source. In ClickHouse Cloud, autoscaling and managed node replacement make this condition more likely. Use a layout that loads the complete dictionary on each server instead; see Dictionary best practices. Like cache, this layout keeps no record of which keys exist, so reading it as a table with SELECT ... FROM <dictionary> returns only the cells currently held that hold a value: it cannot discover keys absent from the cache, and it skips the default cells that record keys which were looked up but not found at the source. Resident cells that have expired are still re-requested from the source. See cache. The dictionary key has the UInt64 type.

DDL
Configuration file

LAYOUT(SSD_CACHE(BLOCK_SIZE 4096 FILE_SIZE 16777216 READ_BUFFER_SIZE 1048576
    PATH '/var/lib/clickhouse/user_files/test_dict'))

<layout>
    <ssd_cache>
        <!-- Size of elementary read block in bytes. Recommended to be equal to SSD's page size. -->
        <block_size>4096</block_size>
        <!-- Max cache file size in bytes. -->
        <file_size>16777216</file_size>
        <!-- Size of RAM buffer in bytes for reading elements from SSD. -->
        <read_buffer_size>131072</read_buffer_size>
        <!-- Size of RAM buffer in bytes for aggregating elements before flushing to SSD. -->
        <write_buffer_size>1048576</write_buffer_size>
        <!-- Path where cache file will be stored. -->
        <path>/var/lib/clickhouse/user_files/test_dict</path>
    </ssd_cache>
</layout>

complex_key_ssd_cache

This type of storage is for use with composite keys. Similar to ssd_cache.

SQL Reference

Data Types

Engines

Functions

Formats

Settings

System Tables

Data Lakes

ssd_cache

complex_key_ssd_cache

​ssd_cache

​complex_key_ssd_cache

ssd_cache

complex_key_ssd_cache