What is the maximum number of files allowed in a HDFS directory?

In modern Apache Hadoop versions, various HDFS limits are controlled by configuration properties with fs-limits in the name, all which have reasonable default values. This question specifically asked about number of children in a directory. That's defined by dfs.namenode.fs-limits.max-directory-items, and its default value is 1048576.

Refer to the Apache Hadoop documentation in hdfs-default.xml for the full list of fs-limits configuration properties and their default values. Copy-pasting here for convenience:

<property>  <name>dfs.namenode.fs-limits.max-component-length</name>  <value>255</value>  <description>Defines the maximum number of bytes in UTF-8 encoding in each      component of a path.  A value of 0 will disable the check.</description></property><property>  <name>dfs.namenode.fs-limits.max-directory-items</name>  <value>1048576</value>  <description>Defines the maximum number of items that a directory may      contain. Cannot set the property to a value less than 1 or more than      6400000.</description></property><property>  <name>dfs.namenode.fs-limits.min-block-size</name>  <value>1048576</value>  <description>Minimum block size in bytes, enforced by the Namenode at create      time. This prevents the accidental creation of files with tiny block      sizes (and thus many blocks), which can degrade      performance.</description></property><property>    <name>dfs.namenode.fs-limits.max-blocks-per-file</name>    <value>1048576</value>    <description>Maximum number of blocks per file, enforced by the Namenode on        write. This prevents the creation of extremely large files which can        degrade performance.</description></property><property>  <name>dfs.namenode.fs-limits.max-xattrs-per-inode</name>  <value>32</value>  <description>    Maximum number of extended attributes per inode.  </description></property><property>  <name>dfs.namenode.fs-limits.max-xattr-size</name>  <value>16384</value>  <description>    The maximum combined size of the name and value of an extended attribute    in bytes. It should be larger than 0, and less than or equal to maximum    size hard limit which is 32768.  </description></property>

All of these settings use reasonable default values as decided upon by the Apache Hadoop community. It is generally recommended that users do not tune these values except in very unusual circumstances.

From http://blog.cloudera.com/blog/2009/02/the-small-files-problem/:

Every file, directory and block in HDFS is represented as an object in the namenode’s memory, each of which occupies 150 bytes, as a rule of thumb. So 10 million files, each using a block, would use about 3 gigabytes of memory. Scaling up much beyond this level is a problem with current hardware. Certainly a billion files is not feasible.

The blocks and files are stored in a HashMap. So you are bound to Integer.MAX_VALUE.So a directory does not have any limitation, but the whole FileSystem.