File system cache#
Примечание
Ниже приведена оригинальная документация Trino. Скоро мы ее переведем на русский язык и дополним полезными примерами.
Trino accesses files directly on object storage and remote file system storage. This often involves the transfer of large amounts of data. The files are retrieved from HDFS, or any other supported object storage, by multiple workers and processed on these workers. Repeated queries with different parameters, or even different queries from different users, often access, and therefore transfer, the same objects.
Trino includes support for caching these files with the help of the open source Alluxio libraries with catalogs using the following connectors:
Distributed caching#
File system caching is distributed in Trino as part of the mechanism of any other query processing. Query processing, detailed more in Trino concepts is broken up into different stages, where tasks and splits are processed by different nodes in the cluster. The lowest level splits retrieve data from the data source with the help of the connector of the specific catalog. For file system caching, these splits result in the retrieval of files from object storage.
Different nodes process splits with data from objects storage randomly over time, but with preference for using a fixed set of nodes for a given file. If the preferred nodes are too busy, the split, and hence the caching, takes place on a non-preferred, less busy node. File system caching keeps copies of the retrieved files on a local cache storage, separate for each node. Over time the same files from object storage are cached on any nodes that require the data file for processing a specific task. Each cache on each node is managed separately, following the TTL and size configuration, and cached files are evicted from the cache.
You can limit the number of hosts that are preferred to process these tasks with
fs.cache.preferred-hosts-count. Query processing still uses all other nodes as
required for the parallel processing of tasks, and therefore potentially caches
files on more nodes than the preferred hosts only. A low setting, such as the
default 2, can reduce the overall size of the cache because it can reduce how
often the same file is cached on multiple nodes. A higher setting, up to the
number of nodes in the cluster, distributes the workload across more workers by
default, and leads to more resilience against node failures at the expense of
effective cache size.
Benefits#
Enabling caching can result in the following significant benefits:
Reduced load on storage
Every retrieved and cached file avoids repeated retrieval from the storage in subsequent queries on the same worker. As a result the storage system does not have to provide the file again and again.
For example, if your query accesses 100MB of files from the storage, the first time the query runs 100MB are downloaded and cached. Any following query uses these files. If your users run another 100 queries accessing the same files, your storage system does not have to provide all data repeatedly. Without caching it has to provide the same files again and again, resulting in up to 10GB of total files to serve.
Increased query performance
Caching can provide significant performance benefits, by avoiding the repeated network transfers and instead accessing copies of the files from a local cache. Performance gains are more significant if the performance of directly accessing the storage is low compared to accessing the local cache.
For example, if you access storage in a different network, different data center, or even different cloud-provider region query performance is slow. Adding caching using fast, local storage has a significant impact and makes your queries much faster.
On the other hand, if your storage is already running at very high performance for I/O and network access, and your local cache storage is at similar speeds, or even slower, performance benefits can be minimal.
Reduced query costs
A result of the reduced load on the storage, mentioned earlier, is significantly reduced network traffic and access to storage. Network traffic and access, often in the form of API access, are often a considerable cost factor, specifically also when hosted in public cloud provider systems.
Configuration#
Use the properties from the following table in your catalog properties files to enable and configure caching for the specific catalogs.
Property |
Description |
|---|---|
|
Enable object storage caching. Defaults to no caching with the value |
|
Required, comma-separated list of absolute paths to directories to use for
caching. All directories must exist on the coordinator and all workers.
Trino must have read and write permissions for files and nested directories.
A valid example with only one directory is Directories must be specific for each catalog with caching enabled. When
enabling caching in multiple catalogs, you must use different directories
and set the values for |
|
Comma-separated list of maximum data sizes for each
caching directory. Order of values must be identical to the directories
list. Configuring either |
|
Comma-separated list of maximum percentage values of the used disk for each
directory. Each value is an integer between 1 and 100. Order of values must
be identical to the directories list. If multiple directories use the same
disk, ensure that total percentages per drive remains below 100 percent.
Configuring either |
|
The maximum duration for objects to remain in the cache
before eviction. Defaults to |
|
The number of preferred nodes for caching files. Defaults to 2. Processing identifies and subsequently prefers using specific nodes. If the preferred nodes identified for caching a split are unavailable or too busy, then an available node is chosen at random from the cluster. More information in Distributed caching. |
|
The page data size used for caching data. Each transfer of files
uses at least this amount of data. Defaults to |
Monitoring#
The cache exposes the
Alluxio JMX client metrics
under the org.alluxio package, and metrics on external reads and cache reads under
io.trino.filesystem.alluxio.AlluxioCacheStats.
The cache code uses OpenTelemetry tracing.
Recommendations#
The speed of the local cache storage is crucial to the performance of the cache. The most common and cost efficient approach is to attach high performance SSD disk or equivalents. Fast cache performance can be also be achieved with a RAM disk used as in-memory cache.
In all cases, avoid using the root partition and disk of the node. Instead attach one or more dedicated storage devices for the cache on each node. Storage should be local, dedicated on each node, and not shared.
Your deployment method for Trino decides how to attach storage and create the directories for caching. Typically you need to connect a fast storage system, like an SSD drive, and ensure that is it mounted on the configured path.