DiskBlockManager

DiskBlockManager creates and maintains the logical mapping between logical blocks and physical on-disk locations.

By default, one block is mapped to one file with a name given by its BlockId. It is however possible to have a block map to only a segment of a file.

Block files are hashed among the local directories.

Note	`DiskBlockManager` is used exclusively by DiskStore and created when `BlockManager` is created (and passed to `DiskStore`).

Tip	Enable `INFO` or `DEBUG` logging levels for `org.apache.spark.storage.DiskBlockManager` logger to see what happens inside. Add the following line to `conf/log4j.properties`: `log4j.logger.org.apache.spark.storage.DiskBlockManager=DEBUG` Refer to Logging.

Finding File — `getFile` Method

Caution

FIXME

`createTempShuffleBlock` Method

createTempShuffleBlock(): (TempShuffleBlockId, File)

createTempShuffleBlock creates a temporary TempShuffleBlockId block.

Caution

FIXME

Collection of Locks for Local Directories — `subDirs` Internal Property

subDirs: Array[Array[File]]

subDirs is a collection of locks for every local directory where DiskBlockManager stores block data (with the columns being the number of local directories and the rows as collection of subDirsPerLocalDir size).

Note	`subDirs(n)` is to access `n`-th local directory.

`getAllFiles` Method

Caution

FIXME

Creating `DiskBlockManager` Instance

DiskBlockManager(conf: SparkConf, deleteFilesOnStop: Boolean)

When created, DiskBlockManager uses spark.diskStore.subDirectories to set subDirsPerLocalDir.

DiskBlockManager creates one or many local directories to store block data (as localDirs). When not successful, you should see the following ERROR message in the logs and DiskBlockManager exits with error code 53.

ERROR DiskBlockManager: Failed to create any local dir.

DiskBlockManager initializes the internal subDirs collection of locks for every local directory to store block data with an array of subDirsPerLocalDir size for files.

In the end, DiskBlockManager registers a shutdown hook to clean up the local directories for blocks.

Registering Shutdown Hook — `addShutdownHook` Internal Method

addShutdownHook(): AnyRef

addShutdownHook registers a shutdown hook to execute doStop at shutdown.

When executed, you should see the following DEBUG message in the logs:

DEBUG DiskBlockManager: Adding shutdown hook

addShutdownHook adds the shutdown hook so it prints the following INFO message and executes doStop.

INFO DiskBlockManager: Shutdown hook called

Removing Local Directories for Blocks — `doStop` Internal Method

doStop(): Unit

doStop deletes the local directories recursively (only when the constructor’s deleteFilesOnStop is enabled and the parent directories are not registered to be removed at shutdown).

Creating Directories for Blocks — `createLocalDirs` Internal Method

createLocalDirs(conf: SparkConf): Array[File]

createLocalDirs creates blockmgr-[random UUID] directory under local directories to store block data.

Internally, createLocalDirs reads local writable directories and creates a subdirectory blockmgr-[random UUID] under every configured parent directory.

If successful, you should see the following INFO message in the logs:

INFO DiskBlockManager: Created local directory at [localDir]

When failed to create a local directory, you should see the following ERROR message in the logs:

ERROR DiskBlockManager: Failed to create local dir in [rootDir]. Ignoring this directory.

Getting Local Directories for Spark to Write Files — `Utils.getConfiguredLocalDirs` Internal Method

getConfiguredLocalDirs(conf: SparkConf): Array[String]

getConfiguredLocalDirs returns the local directories where Spark can write files.

Internally, getConfiguredLocalDirs uses conf SparkConf to know if External Shuffle Service is enabled (using spark.shuffle.service.enabled).

getConfiguredLocalDirs checks if Spark runs on YARN and if so, returns LOCAL_DIRS-controlled local directories.

In non-YARN mode (or for the driver in yarn-client mode), getConfiguredLocalDirs checks the following environment variables (in the order) and returns the value of the first met:

SPARK_EXECUTOR_DIRS environment variable
SPARK_LOCAL_DIRS environment variable
MESOS_DIRECTORY environment variable (only when External Shuffle Service is not used)

In the end, when no earlier environment variables were found, getConfiguredLocalDirs uses spark.local.dir Spark property or eventually java.io.tmpdir System property.

Getting Writable Directories in YARN — `getYarnLocalDirs` Internal Method

getYarnLocalDirs(conf: SparkConf): String

getYarnLocalDirs uses conf SparkConf to read LOCAL_DIRS environment variable with comma-separated local directories (that have already been created and secured so that only the user has access to them).

getYarnLocalDirs throws an Exception with the message Yarn Local dirs can’t be empty if LOCAL_DIRS environment variable was not set.

Checking If Spark Runs on YARN — `isRunningInYarnContainer` Internal Method

isRunningInYarnContainer(conf: SparkConf): Boolean

isRunningInYarnContainer uses conf SparkConf to read Hadoop YARN’s CONTAINER_ID environment variable to find out if Spark runs in a YARN container.

Note	`CONTAINER_ID` environment variable is exported by YARN NodeManager.

`getAllBlocks` Method

getAllBlocks(): Seq[BlockId]

getAllBlocks lists all the blocks currently stored on disk.

Internally, getAllBlocks takes the block files and returns their names (as BlockId).

Note	`getAllBlocks` is used when `BlockManager` computes the ids of existing blocks (for a given filter).

Settings

Table 1. Spark Properties
Spark Property	Default Value	Description
`spark.diskStore.subDirectories`	`64`	The number of …FIXME

DiskBlockManager