RAPIDS Accelerator File Cache

The RAPIDS Accelerator for Apache Spark provides an optional file cache which may improve performance of Spark applications that access the same input files multiple times. It caches portions of remote files being accessed onto the local filesystem of executors to speedup access if that data is accessed again in the same application.

The file cache is only used by Parquet and ORC table scans that have been GPU-accelerated by the RAPIDS Accelerator. CPU table scans or scans of other data formats will not use the file cache.

The file cache does not perform well if the executor node’s local disks are relatively slow. The file cache performs best when the local disks are significantly faster than the distributed filesystem from which data is being cached. Enabling the file cache when the executor local disks are too slow can cause applications to run slower rather than faster.

File caching is disabled by default. It can be enabled by setting spark.rapids.filecache.enabled to true. The file cache stores data locally in the same local directories that have been configured for the Spark executor.

By default the file cache will use up to half of the available space in the Spark local directories. To specify an absolute limit, set spark.rapids.filecache.maxBytes to the maximum number of bytes to use for the file cache in a single executor. For example, setting spark.rapids.filecache.maxBytes=50g will limit the filecache to 50 gigabytes of local storage per executor.

Immutable Input Files

By default the file cache will detect when a local copy of data is stale with respect to the remote filesystem. If input files are immutable during the lifetime of the application then it is recommended to set spark.rapids.filecache.checkStale to false. Note that modern data lakehouse table formats have immutable files, so even if a data lakehouse table is overwritten/updated, individual files stored as part of the table data are not modified.

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