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DAOS Hadoop Filesystem

Here, we describe the quick steps required to use the DAOS Hadoop filesystem to access DAOS from Hadoop and Spark.

Prerequisites

  1. Linux OS
  2. Java 8
  3. Hadoop 2.7 or later
  4. Spark 3.1 or later
  5. DAOS Readiness

We assume that the DAOS servers and agents have already been deployed in the environment. Otherwise, they can be deployed by following the DAOS Installation Guide.

Maven Download

There are two artifacts to download, daos-java and hadoop-daos, from maven. Here are maven dependencies.

You can download them with below commands if you have maven installed.

mvn dependency:get -Dartifact=io.daos:daos-java:<version> -Ddest=./
mvn dependency:get -Dartifact=io.daos:hadoop-daos:<version> -Ddest=./

Or search these artifacts from maven central(https://search.maven.org) and download them manually.

You can also build artifacts by yourself. see Build DAOS Hadoop Filesystem for details.

Deployment

JAR Files

daos-java-<version>.jar and hadoop-daos-<version>.jar need to be deployed on every compute node that runs Spark or Hadoop. Place them in a directory, e.g., $SPARK_HOME/jars for Spark and $HADOOP_HOME/share/hadoop/common/lib for Hadoop, which is accessible to all the nodes or copy them to every node.

daos-site-example.xml

Extract from hadoop-daos-<version>.jar and rename to daos-site.xml. Then copy it to your application config directory, e.g., $SPARK_HOME/conf for Spark and $HADOOP_HOME/etc/hadoop for Hadoop.

Configuring Hadoop

Environment Variable

Export all DAOS-specific env variables in your application, e.g., spark-env.sh for Spark and hadoop-env.sh for Hadoop. Or you can simply put env variables in your .bashrc.

Besides, you should have LD_LIBRARY_PATH include DAOS library path so that Java can link to DAOS libs, like below.

$ export LD_LIBRARY_PATH=$LD_LIBRARY_PATH:<DAOS_INSTALL>/lib64:<DAOS_INSTALL>/lib

DAOS URI

In daos-site.xml, we default the DAOS URI as simplest form, "daos:///". For other form of URIs, please check DAOS More URIs.

If the DAOS pool and container have not been created, we can use the following command to create them and get the pool UUID and container UUID.

$ dmg pool create --scm-size=<scm size> --nvme-size=<nvme size>
$ daos cont create --pool <pool UUID> --type POSIX

After that, configure daos-site.xml with the pool and container created.

<configuration>
...
  <property>
    <name>fs.daos.pool.uuid</name>
    <value>your pool UUID</value>
    <description>UUID of DAOS pool</description>
  </property>
  <property>
    <name>fs.daos.container.uuid</name>
    <value>your container UUID</value>
    <description>UUID of DAOS container created with "--type posix"</description>
  </property>
...
</configuration>

Please put daos-site.xml in right place, e.g., Java classpath, and loadable by Hadoop DAOS FileSystem.

Validating Hadoop Access

If everything goes well, you should see /user directory being listed after issuing below command.

$ hadoop fs -ls /

You can also play around with other Hadoop commands, like -copyFromLocal and -copyToLocal. You can also start Yarn and run some mapreduce jobs on Yarn. See Run Map-Reduce in Hadoop

Configuring Spark

To access DAOS Hadoop filesystem in Spark, add the jar files to the classpath of the Spark executor and driver. This can be configured in Spark's configuration file spark-defaults.conf.

spark.executor.extraClassPath   /path/to/daos-java-<version>.jar:/path/to/hadoop-daos-<version>.jar
spark.driver.extraClassPath     /path/to/daos-java-<version>.jar:/path/to/hadoop-daos-<version>.jar

Validatng Spark Access

All Spark APIs that work with the Hadoop filesystem will work with DAOS. We can use the daos:/// URI to access files stored in DAOS. For example, to read the people.json file from the root directory of DAOS filesystem, we can use the following pySpark code:

df = spark.read.json("daos:///people.json")

Appendix

Building DAOS Hadoop Filesystem

Below are the steps to build the Java jar files for the DAOS Java and DAOS Hadoop filesystem. Spark and Hadoop require these jars in their classpath. You can ignore this section if you already have the pre-built jars.

$ git clone https://github.com/daos-stack/daos.git
$ cd daos
$ git checkout <desired branch or commit>
## assume DAOS is built and installed to <daos_install> directory
$ cd src/client/java
## with-proto3-netty4-deps profile builds jars with protobuf 3 and netty-buffer 4 shaded
## It spares you potential third-party jar compatibility issue.
$ mvn -Pdistribute,with-proto3-netty4-deps clean package -DskipTests -Dgpg.skip -Ddaos.install.path=<daos_install>

After build, the package daos-java-<version>-assemble.tgz will be available under distribution/target.

DAOS More URIs

DAOS FileSystem binds to schema "daos". DAOS URIs are in the format of "daos://[authority]//[path]". Both authority and path are optional. There are three types of DAOS URIs, DAOS UNS path, DAOS Non-UNS path and Special UUID path depending on where you want the DAOS Filesystem to get initialized and configured.

DAOS UNS Path

The simple form of URI is "daos:///\<your uns path>[/sub path]". "\<your path>" is your OS file path created with the daos command or Java DAOS UNS method, DaosUns.create(). The "[sub path]" is optional. You can create the UNS path with below command.

$ daos cont create --pool <pool UUID> -path <your path> --type=POSIX

Or

$ java -Dpath="your path" -Dpool_id="your pool uuid" -cp ./daos-java-<version>-shaded.jar io.daos.dfs.DaosUns create

After creation, you can use below command to see what DAOS properties set to the path.

$ getfattr -d -m - <your path>

DAOS Non-UNS Path

Check Set DAOS URI and Pool/Container.

Special UUID Path

DAOS supports a specialized URI with pool/container UUIDs embedded. The format is "daos://pool UUID/container UUID". As you can see, we don't need to find the UUIDs from neither UNS path nor configuration like above two types of URIs.

You may want to connect to two DAOS servers or two DFS instances mounted to different containers in one DAOS server from same JVM. Then, you need to add authority to your URI to make it unique since Hadoop caches filesystem instance keyed by "schema + authority" in global (JVM). It applies to the both types of URIs described above.

Run Map-Reduce in Hadoop

Edit $HADOOP_HOME/etc/hadoop/core-site.xml to change fs.defaultFS to daos:///. It is not recommended to set fs.defaultFS to a DAOS UNS path. You may get an error complaining pool/container UUIDs cannot be found. It's because Hadoop considers the default filesystem is DAOS since you configured DAOS UNS URI. YARN has some working directories defaulting to local path without schema, like "/tmp/yarn", which is then constructed as "daos:///tmp/yarn". With this URI, Hadoop cannot connect to DAOS since no pool/container UUIDs can be found if daos-site.xml is not provided too. .

Then append below configuration to this file and $HADOOP_HOME/etc/hadoop/yarn-site.xml.

<property>
    <name>fs.AbstractFileSystem.daos.impl</name>
    <value>io.daos.fs.hadoop.DaosAbsFsImpl</value>
</property>

DAOS has no data locality since it is remote storage. You need to add below configuration to the scheduler configuration file, like capacity-scheduler.xml in yarn.

<property>
  <name>yarn.scheduler.capacity.node-locality-delay</name>
  <value>-1</value>
</property>

Then replicate daos-site.xml, core-site.xml, yarn-site.xml and capacity-scheduler.xml to other nodes.

Known Issues

If you use Omni-path PSM2 provider in DAOS, you'll get connection issue in Yarn container due to PSM2 resource not being released properly in time.

Tune More Configurations

If your DAOS URI is the non-UNS, you can follow descriptions of each config item to set your own values in loadable daos-site.xml.

If your DAOS URI is the UNS path, your configurations, except those set by DAOS UNS creation, in daos-site.xml can still be effective. To make configuration source consistent, an alternative to the configuration file daos-site.xml is to set all configurations to the UNS path. You put the configs to the same UNS path with below command.

# install attr package if get "command not found" error
$ setfattr -n user.daos.hadoop -v "fs.daos.server.group=daos_server" <your path>

Or

$ java -Dpath="your path" -Dattr=user.daos.hadoop -Dvalue="fs.daos.server.group=daos_server"
    -cp ./daos-java-<version>-shaded.jar io.daos.dfs.DaosUns setappinfo

For the "value" property, you need to follow pattern, key1=value1:key2=value2.. .. And key should be from daos-site-example.xml. If value contains characters of '=' or ':', you need to escape the value with below command.

$ java -Dop=escape-app-value -Dinput="daos_server:1=2" -cp ./daos-java-<version>-shaded.jar io.daos.dfs.DaosUns util

You'll get escaped value, "daos_server\u003a1\u003d2", for "daos_server:1=2".

If you configure the same property in both daos-site.xml and UNS path, the value in daos-site.xml takes priority. If user sets Hadoop configuration before initializing Hadoop DAOS FileSystem, the user's configuration takes priority.

PSM2 Issue

For some libfabric providers, like PSM2, signal chaining should be enabled to better interoperate with DAOS and its dependencies which may install its own signal handlers. It ensures that signal calls are intercepted so that they do not actually replace the JVM's signal handlers if the handlers conflict with those already installed by the JVM. Instead, these calls save the new signal handlers, or "chain" them behind the JVM-installed handlers. Later, when any of these signals are raised and found not to be targeted at the JVM, the DAOS's handlers are invoked.

$ export LD_PRELOAD=<YOUR JDK HOME>/jre/lib/amd64/libjsig.so
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