The need for optimize write on Apache Spark

Analytical workloads on Big Data processing engines such as Apache Spark perform most efficiently when using standardized larger file sizes. The relation between the file size, the number of files, the number of Spark workers and its configurations, play a critical role on performance. Ingestion workloads into data lake tables may have the inherited characteristic of constantly writing lots of small files; this scenario is commonly known as the "small file problem".

Optimize Write is a Delta Lake on Synapse feature that reduces the number of files written and aims to increase individual file size of the written data. It dynamically optimizes partitions while generating files with a default 128 MB size. The target file size may be changed per workload requirements using configurations.

This feature achieves the file size by using an extra data shuffle phase over partitions, causing an extra processing cost while writing the data. The small write penalty should be outweighed by read efficiency on the tables.

Note

  • Optimize write is available as a Preview feature.
  • It is available on Synapse Pools for Apache Spark versions 3.1 and 3.2.

Benefits of Optimize Writes

  • It's available on Delta Lake tables for both Batch and Streaming write patterns.
  • There's no need to change the spark.write command pattern. The feature is enabled by a configuration setting or a table property.
  • It reduces the number of write transactions as compared to the OPTIMIZE command.
  • OPTIMIZE operations will be faster as it will operate on fewer files.
  • VACUUM command for deletion of old unreferenced files will also operate faster.
  • Queries will scan fewer files with more optimal file sizes, improving either read performance or resource usage.

Optimize write usage scenarios

When to use it

  • Delta lake partitioned tables subject to write patterns that generate suboptimal (less than 128 MB) or non-standardized files sizes (files with different sizes between itself).
  • Repartitioned data frames that will be written to disk with suboptimal files size.
  • Delta lake partitioned tables targeted by small batch SQL commands like UPDATE, DELETE, MERGE, CREATE TABLE AS SELECT, INSERT INTO, etc.
  • Streaming ingestion scenarios with append data patterns to Delta lake partitioned tables where the extra write latency is tolerable.

When to avoid it

  • Non partitioned tables.
  • Use cases where extra write latency isn't acceptable.
  • Large tables with well defined optimization schedules and read patterns.

How to enable and disable the optimize write feature

The optimize write feature is disabled by default.

Once the configuration is set for the pool or session, all Spark write patterns will use the functionality.

To use the optimize write feature, enable it using the following configuration:

  1. Scala and PySpark
spark.conf.set("spark.microsoft.delta.optimizeWrite.enabled", "true")
  1. Spark SQL
SET `spark.microsoft.delta.optimizeWrite.enabled` = true

To check the current configuration value, use the command as shown below:

  1. Scala and PySpark
spark.conf.get("spark.microsoft.delta.optimizeWrite.enabled")
  1. Spark SQL
SET `spark.microsoft.delta.optimizeWrite.enabled`

To disable the optimize write feature, change the following configuration as shown below:

  1. Scala and PySpark
spark.conf.set("spark.microsoft.delta.optimizeWrite.enabled", "false")
  1. Spark SQL
SET `spark.microsoft.delta.optimizeWrite.enabled` = false

Controlling optimize write using table properties

On new tables

  1. SQL
CREATE TABLE <table_name> TBLPROPERTIES (delta.autoOptimize.optimizeWrite = true)
  1. Scala

Using the DeltaTableBuilder API:

val table = DeltaTable.create()
  .tableName("<table_name>")
  .addColumnn("<colName>", <dataType>)
  .location("<table_location>")
  .property("delta.autoOptimize.optimizeWrite", "true") 
  .execute()

On existing tables

  1. SQL
ALTER TABLE <table_name> SET TBLPROPERTIES (delta.autoOptimize.optimizeWrite = true)
  1. Scala

Using the DeltaTableBuilder API

val table = DeltaTable.replace()
  .tableName("<table_name>")
  .location("<table_location>")
  .property("delta.autoOptimize.optimizeWrite", "true") 
  .execute()

How to get & change the current max file size configuration for Optimize Write

To get the current config value, use the bellow commands. The default is 128 MB.

  1. Scala and PySpark
spark.conf.get("spark.microsoft.delta.optimizeWrite.binSize")
  1. SQL
SET `spark.microsoft.delta.optimizeWrite.binSize`
  • To change the config value
  1. Scala and PySpark
spark.conf.set("spark.microsoft.delta.optimizeWrite.binSize", "134217728")
  1. SQL
SET `spark.microsoft.delta.optimizeWrite.binSize` = 134217728

Next steps