Data, processed in ClickHouse, is usually stored in the local file system — on the same machine with the ClickHouse server. That requires large-capacity disks, which can be expensive enough. To avoid that you can store the data remotely — on [Amazon S3](https://aws.amazon.com/s3/) disks or in the Hadoop Distributed File System ([HDFS](https://hadoop.apache.org/docs/current/hadoop-project-dist/hadoop-hdfs/HdfsDesign.html)).
To work with data stored on `Amazon S3` disks use [S3](/docs/en/engines/table-engines/integrations/s3.md) table engine, and to work with data in the Hadoop Distributed File System — [HDFS](/docs/en/engines/table-engines/integrations/hdfs.md) table engine.
[MergeTree](/docs/en/engines/table-engines/mergetree-family/mergetree.md) and [Log](/docs/en/engines/table-engines/log-family/log.md) family table engines can store data to HDFS using a disk with type `HDFS`.
You can encrypt the data stored on [S3](/docs/en/engines/table-engines/mergetree-family/mergetree.md/#table_engine-mergetree-s3), or [HDFS](#configuring-hdfs) external disks, or on a local disk. To turn on the encryption mode, in the configuration file you must define a disk with the type `encrypted` and choose a disk on which the data will be saved. An `encrypted` disk ciphers all written files on the fly, and when you read files from an `encrypted` disk it deciphers them automatically. So you can work with an `encrypted` disk like with a normal one.
For example, when ClickHouse writes data from some table to a file `store/all_1_1_0/data.bin` to `disk1`, then in fact this file will be written to the physical disk along the path `/path1/store/all_1_1_0/data.bin`.
When writing the same file to `disk2`, it will actually be written to the physical disk at the path `/path1/path2/store/all_1_1_0/data.bin` in encrypted mode.
-`key` — The key for encryption and decryption. Type: [Uint64](/docs/en/sql-reference/data-types/int-uint.md). You can use `key_hex` parameter to encode the key in hexadecimal form.
-`current_key_id` — The key used for encryption. All the specified keys can be used for decryption, and you can always switch to another key while maintaining access to previously encrypted data.
It is possible to configure local cache over disks in storage configuration starting from version 22.3. For versions 22.3 - 22.7 cache is supported only for `s3` disk type. For versions >= 22.8 cache is supported for any disk type: S3, Azure, Local, Encrypted, etc. Cache uses `LRU` cache policy.
-`max_size` - maximum size of the cache in bytes or in readable format, e.g. `ki, Mi, Gi, etc`, example `10Gi` (such format works starting from `22.10` version). When the limit is reached, cache files are evicted according to the cache eviction policy. Default: None, this setting is obligatory.
-`cache_on_write_operations` - allow to turn on `write-through` cache (caching data on any write operations: `INSERT` queries, background merges). Default: `false`. The `write-through` cache can be disabled per query using setting `enable_filesystem_cache_on_write_operations` (data is cached only if both cache config settings and corresponding query setting are enabled).
-`enable_filesystem_query_cache_limit` - allow to limit the size of cache which is downloaded within each query (depends on user setting `max_query_cache_size`). Default: `false`.
-`enable_cache_hits_threshold` - a number, which defines how many times some data needs to be read before it will be cached. Default: `0`, e.g. the data is cached at the first attempt to read it.
-`do_not_evict_index_and_mark_files` - do not evict small frequently used files according to cache policy. Default: `false`. This setting was added in version 22.8. If you used filesystem cache before this version, then it will not work on versions starting from 22.8 if this setting is set to `true`. If you want to use this setting, clear old cache created before version 22.8 before upgrading.
-`max_file_segment_size` - a maximum size of a single cache file in bytes or in readable format (`ki, Mi, Gi, etc`, example `10Gi`). Default: `104857600` (`100Mi`).
-`read_from_filesystem_cache_if_exists_otherwise_bypass_cache` - allows to use cache in query only if it already exists, otherwise query data will not be written to local cache storage. Default: `false`.
-`enable_filesystem_cache_on_write_operations` - turn on `write-through` cache. This setting works only if setting `cache_on_write_operations` in cache configuration is turned on.
-`enable_filesystem_cache_log` - turn on logging to `system.filesystem_cache_log` table. Gives a detailed view of cache usage per query. Default: `false`.
-`max_query_cache_size` - a limit for the cache size, which can be written to local cache storage. Requires enabled `enable_filesystem_query_cache_limit` in cache configuration. Default: `false`.
-`skip_download_if_exceeds_query_cache` - allows to change the behaviour of setting `max_query_cache_size`. Default: `true`. If this setting is turned on and cache download limit during query was reached, no more cache will be downloaded to cache storage. If this setting is turned off and cache download limit during query was reached, cache will still be written by cost of evicting previously downloaded (within current query) data, e.g. second behaviour allows to preserve `last recentltly used` behaviour while keeping query cache limit.
Cache configuration settings and cache query settings correspond to the latest ClickHouse version, for earlier versions something might not be supported.
-`SHOW FILESYSTEM CACHES` -- show list of filesystem caches which were configured on the server. (For versions <= `22.8` the command is named `SHOW CACHES`)
-`DESCRIBE CACHE '<cache_name>'` - show cache configuration and some general statistics for a specific cache. Cache name can be taken from `SHOW CACHES` command. (For versions <= `22.8` the command is named `DESCRIBE CACHE`)
There is a tool `clickhouse-static-files-uploader`, which prepares a data directory for a given table (`SELECT data_paths FROM system.tables WHERE name = 'table_name'`). For each table you need, you get a directory of files. These files can be uploaded to, for example, a web server with static files. After this preparation, you can load this table into any ClickHouse server via `DiskWeb`.
This is a read-only disk. Its data is only read and never modified. A new table is loaded to this disk via `ATTACH TABLE` query (see example below). Local disk is not actually used, each `SELECT` query will result in a `http` request to fetch required data. All modification of the table data will result in an exception, i.e. the following types of queries are not allowed: [CREATE TABLE](/docs/en/sql-reference/statements/create/table.md), [ALTER TABLE](/docs/en/sql-reference/statements/alter/index.md), [RENAME TABLE](/docs/en/sql-reference/statements/rename.md/#misc_operations-rename_table), [DETACH TABLE](/docs/en/sql-reference/statements/detach.md) and [TRUNCATE TABLE](/docs/en/sql-reference/statements/truncate.md).
Web server storage is supported only for the [MergeTree](/docs/en/engines/table-engines/mergetree-family/mergetree.md) and [Log](/docs/en/engines/table-engines/log-family/log.md) engine families. To access the data stored on a `web` disk, use the [storage_policy](/docs/en/engines/table-engines/mergetree-family/mergetree.md/#terms) setting when executing the query. For example, `ATTACH TABLE table_web UUID '{}' (id Int32) ENGINE = MergeTree() ORDER BY id SETTINGS storage_policy = 'web'`.
If a query fails with an exception `DB:Exception Unreachable URL`, then you can try to adjust the settings: [http_connection_timeout](/docs/en/operations/settings/settings.md/#http_connection_timeout), [http_receive_timeout](/docs/en/operations/settings/settings.md/#http_receive_timeout), [keep_alive_timeout](/docs/en/operations/server-configuration-parameters/settings.md/#keep-alive-timeout).
`clickhouse static-files-disk-uploader --metadata-path <path> --output-dir <dir>` (`--metadata-path` can be found in query `SELECT data_paths FROM system.tables WHERE name = 'table_name'`).
If URL is not reachable on disk load when the server is starting up tables, then all errors are caught. If in this case there were errors, tables can be reloaded (become visible) via `DETACH TABLE table_name` -> `ATTACH TABLE table_name`. If metadata was successfully loaded at server startup, then tables are available straight away.
Use [http_max_single_read_retries](/docs/en/operations/settings/settings.md/#http-max-single-read-retries) setting to limit the maximum number of retries during a single HTTP read.
Zero-copy replication is possible, but not recommended, with `S3` and `HDFS` disks. Zero-copy replication means that if the data is stored remotely on several machines and needs to be synchronized, then only the metadata is replicated (paths to the data parts), but not the data itself.
